Dissociation Lifetimes of Molecular Ions Produced by Charge

5
C f . A . KLEMM, Z . N a t u r f o r s c h . 1 , 2 5 2
6
A.
7
A . LUNDEN
LUNDEN
and
A.
EKHED,
and
A . EKHED,
Z.
[1946].
Naturforsch.
2 4 a,
16
Equation (9) is derived either from Equation (8) or easily
from the equation presented by A. KLEMM, Z. Naturforsch.
3 a, 127 [1948].
17
S. JORDAN
18
[1966].
The activation energy is evaluated from the data of B. DE
NOOIJER, Thesis, University of Amsterdam 1965.
19
A . S . D W O R K I N , R . B . ESCUE, a n d E . R . V A N A R T S D A L E N ,
20
A . LUNDEN, A . FLOBERG, a n d R . MATSSON, t o b e
892
[1969].
Z.
Naturforsch.
2 3 a,
1779
[1968].
8
9
10
11
I. OKADA, Thesis, Tokyo University 1966.
A. LUNDEN, Z. Naturforsch. 21 a, 1510 [ 1 9 6 6 ] .
A. LUNDEN, Z. Naturforsch. 14 a, 801 [1959].
F . R . DUKE a n d B . OWENS, J. E l e c t r o c h e m . S o c . 1 0 5 ,
12
E. P . HONIG a n d J. A . A . KETELAAR, T r a n s . F a r a d a y
13
S.C.WAIT and A.T.WARD, J. Chem. Phys. 44, 448 [1966];
62, 190
Soc.
21
[1966].
S. C. W A I T , A . T . W A R D , a n d G . J. JANZ, J. C h e m . 4 5 ,
133
[1966].
14
K . WILLIAMSON, P . L i , a n d J. P . DEVLIN, J. C h e m .
Phys.
48, 3891 [1968].
15
G . H . W E G D A M , R . B O N N , a n d J . V A N DER ELSKEN,
A . KLEMM,
Z.
Naturforsch.
21a,
1584
J. Phys. Chem. 64, 872 [ I 9 6 0 ] .
548
[1958].
and
publish-
ed.
For L i N 0 3 , the temperature dependence of ju-mt has been
measured in two laboratories, see Refs. 6 and 8 , the result
being that the mass effect either is independent of the temperature
or that it decreases slightly when the temperature increases 8 . Possible causes of the discrepancy are discussed in Ref. 6 .
Chem.
Phys. Letters 2, 182 [1968].
Dissociation Lifetimes of Molecular Ions Produced by Charge Exchange *
B.ANDLAUER**
a n d C H . OTTINGER * * *
Physikalisches Institut der Universität Freiburg, Germany
(Z. Naturforsch. 27 a, 293—309 [1972] ; received 27 November 1971)
Ions of the benzonitrile, benzene and thiophene molecules were produced in well-defined states
of excitation by charge exchange with X e + , Kr + , Ar + , CO + , and N 2 + . Their subsequent unimolecular
decomposition was followed, as a function of time, by allowing the dissociations to take place
within a strong homogeneous draw-out field and measuring the kinetic energy of the product ions.
The decompositions were found to be purely exponential within experimental error, and the corresponding decay rate constants K (for 2 - 1 0 5 s e c - 1
K ^ 5 - 1 0 8 sec - " 1 ) proved to be monotonically
increasing functions of the excitation energy. These are the first unambiguous measurements of
this function K(E) for any molecule. In the case of benzene, the reactions C 6 H 6 + - > C 6 H 5 + + H and
C 6 H 8 + - > C 4 H4 + + C 2 H2 were found to be definitely not in competition with one another. Furthermore the dependence of the excitation energy on the impact energy was measured. A variation of
the impact energy between 10 and 200 eV c .m. only changes the internal energy E by about 0.1 eV.
This value appears to be in qualitative agreement with calculations on near-resonant charge transfer by Gurnee and Magee.
I.
ciple
Introduction
the
excited
poly-
a t o m i c ions play an i m p o r t a n t role in m a n y
Unimolecular
decompositions
of
radio-
the
function
framework
henceforth
great
of
can
k(E)
the
abbreviated
number
of
be
calculated
quasi-equilibrium
as
QET
assumptions
However,
necessary
chemical and radiobiological
s y s t e m s as w e l l a s
in
such calculation makes an experimental
organic
An
understanding
of
tion
the host of p o s s i b l e c o n c u r r e n t a n d c o n s e c u t i v e
re-
mass
spectrometry.
actions has to b e f o u n d e d
of
the t i m e scale
unimolecular
as w e l l
processes.
on
a consistent
picture
as t h e e n e r g e t i c s
Today
still
very
of
the
little
k n o w n a b o u t the d e c o m p o s i t i o n
rate c o n s t a n t k
isolated
less a b o u t
excited
ions,
and
even
p e n d e n c e o f k o n t h e e x c i t a t i o n e n e r g y E.
the
In
of
highly
k(E)
munications 2'
3
desirable.
In
for
rate
of
com-
we have reported on a method
ions
produced
by
the
any
determina-
previous
measuring, in a well-defined manner, the
sition
within
theory,
for
decompo-
electron
impact.
is
Subsequently w e have been able, b y means of vary-
of
i n g the electron e n e r g y , to extract f r o m these m e a -
de-
prin-
* This work is a summary of the Diplomarbeit of B. ANDLAUER, Universität Freiburg 1970.
** Present address: Institut für Angewandte Festkörperphysik, Freiburg.
s u r e m e n t s k(E)
f o r a few decompositions of
benzo-
n i t r i l e , b u t a n e a n d h e p t a n e i o n s 4> 5 . H o w e v e r ,
this
* * * Present address: Max-Planck-Institut für Strömungsforschung, Göttingen.
Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung
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Creative Commons Namensnennung-Keine Bearbeitung 3.0 Deutschland
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derivation of k(E)
was not quite u n a m b i g u o u s due
to the fact that electron
produces
^
ions possessing a w i d e and largely u n k n o w n spec-
impact always
\
trum of excitation energies. T h i s disadvantage c a n
11
b e o v e r c o m e b y using ' c h a r g e exchange
ionization.
producing
ions in m o n o e n e r g e t i c
levels
Mo
IV
L i n d h o l m has s h o w n that charge exchange is capable of
Parent ions
0
Kinetic energy loss
of
excitation 6 . C o m b i n i n g L i n d h o l m ' s m e t h o d of welldefined excitation with o u r technique of measuring
decomposition
rates should therefore give definite
information on k
(E).
In the present w o r k w e report o n experiments of
this type
on
the
benzonitrile,
benzene
and
thio-
p h e n e i o n s . T h e y constitute, to o u r k n o w l e d g e , the
first
direct measurements of k(E)
f o r any
ions7.
[ F o r d e c o m p o s i t i o n of neutrals, RABINOVITCH has
determined k(E)
in a completely different w a y 8 . ]
A t the same time v e r y
accurate i n f o r m a t i o n
was
o b t a i n e d o n the a m o u n t of kinetic e n e r g y that is
c o n v e r t e d into excitation of the target in charge exchange processes. It is f o u n d to b e small
(though
m e a s u r a b l e ) , which a posteriori p r o v i d e s a s o u n d
basis f o r the " m o n o e n e r g e t i c "
excitation
method.
In a f e w cases the variation of the charge exchange
c r o s s section with initial kinetic e n e r g y was
also
measured.
F i g . 1. Schematic kinetic energy distributions of parent and
fragment ions f o r the unimolecular process M 0 + - > M ^ + z f M .
assuming f o r m a t i o n of parent ions M 0 + at a well-defined potential only (zero point, upper figure). Mj"1" ions appear at lower
kinetic energies, the energy loss b e i n g due to AM split off in
flight.
center and the d e v e l o p m e n t of a " t a i l " . A f r a g m e n t
ion
which
is
formed
from
a parent
ion
with
a
delay t after the initial i o n i z a t i o n , o n l y retains the
fraction
parent
of
m1/m0
ion
had
the
kinetic
at the instant
energy
of
which
the
decomposition.
T h i s e n e r g y deficit increases p r o p o r t i o n a l l y
to
t2.
T h u s , the l o w e n e r g y c o m p o n e n t of the e n e r g y dist r i b u t i o n of mass m j ions is related to the distribution of lifetimes t and h e n c e to the desired distrib u t i o n IJ (k).
II. Principle of M e a s u r e m e n t
Of great practical i m p o r t a n c e is the
d r a w - o u t field strength, F. F o r small F, o n l y small
T h e d e c o m p o s i t i o n rate constant k was measured
b y relating it, o r rather the mean parent i o n lifetime r = l/k,
to the f r a g m e n t i o n kinetic e n e r g y in
the f o l l o w i n g
way
(for
details see
3).
Ionization
takes place within a strong h o m o g e n e o u s
field.
The
nitrile)
gas
under
investigation
draw-out
(e. g .
benzo-
traverses this field in a very n a r r o w m o l e -
cular b e a m which c o i n c i d e s with an
equipotential
s u r f a c e . T h u s all ions are initially f o r m e d at the
potential of this particular surface. A n e n e r g y selector placed b e h i n d the exit of the i o n s o u r c e then
records
a kinetic e n e r g y
distribution
of
the
ions
which s i m p l y reflects the gas density p r o f i l e of the
beam
measured
equipotential
perpendicular
surface,
with
to
the
the
ionization
draw-out
field
strength as a scale f a c t o r . H o w e v e r , this is
true f o r the parent ions M0+
( o f mass m 0 )
values of
k p r o d u c e a n o t i c e a b l e peak
distortion,
while f o r the detection of c o n t r i b u t i o n s f r o m large
k h i g h values of F are necessary. W i t h o u r apparatus values of k r a n g i n g f r o m 2 - 1 0 5 to 2 - 1 0 8 s e c - 1
can be measured.
In a d d i t i o n to the l o w e n e r g y tail o n the b e a m
p r o f i l e peak,
the e n e r g y
distribution
exhibits
an-
other, so-called satellite peak at still l o w e r energy.
It results f r o m d e c o m p o s i t i o n s in the
field-free
re-
g i o n outside the i o n s o u r c e and is the a n a l o g o n the
e n e r g y scale of the classical " m e t a s t a b l e p e a k s " on
the mass scale. B o t h
"main peak"
distortion
and
relative satellite peak intensities w e r e used in the
analysis (see Sect. V ) .
only
of the
III.
Experimental
target gas. F o r any f r a g m e n t mass m t the kinetic
e n e r g y distribution is distorted f r o m that p r o f i l e to
the l o w - e n e r g y side. F i g u r e 1 illustrates
schemati-
cally this change of peak shape, shift of the peak
The
apparatus
has
been
described
in
detail
in
F i g u r e 2 gives a schematic of the ion source. T h e
3.
es-
s e n t i a l e l e m e n t is a p l a n a r d i s c - s h a p e d m o l e c u l a r b e a m
o f t h e t a r g e t g a s . It is d e f i n e d b y t h e c o n c e n t r i c
circu-
is applied. After retardation by the grid R the emerging ions are energy and mass analyzed. The experiments consist of scanning the kinetic energy distribution at a fixed mass.
A.N
AN•
potential
beam
Typical operating parameters were: electron current
30 mA, electron energy 120 eV. At a pressure of
5 - 1 0 " 4 Torr in the primary ionization region a primary ion current of 0.5 juA was obtained through the
charge exchange region, at an ion energy of 90 eV. The
energy spread was between 1.5 and 5 eV. The target
gas pressure in the center of the molecular beam disc
was about 5 - 1 0 ~ 5 Torr. The background pressure in
the apparatus resulting from primary gas effusing
through the grid AC+ amounted to 10~ 5 Torr. Great
care was taken to keep the target gas background pressure in the primary ionization region as small as possible. Under the intense electron bombardment, even
traces of target gas will yield considerable fragment
ion intensities. Because the primary ionization region
is at a more positive potential than the molecular beam
region, fragments resulting from delayed decompositions can appear at and around the molecular beam
potential, by virtue of an appropriate energy deficit as
described above. These ions may seriously interfere
with the measurement of the very small ion currents
resulting from charge exchange. Efficient differential
pumping of the primary ionization region is therefore
extremely important.
The energy and mass selected ion current at the
multiplier was, at the maximum of the beam profile,
on the order of 1 0 - 1 6 A. This means an intensity loss
of about two orders of magnitude compared with the
earlier electron impact experiments.
IV. Possible Sources of Error
Proper operation of the apparatus was ascertained by the following tests.
Fig. 2. Ion source (schematic) and potential diagram. S x , S 2
= b e a m slits; N = primary gas i n l e t ; F = f i l a m e n t ; A N , A N '
= a n o d e s ; E = electron barrier e l e c t r o d e ; A C + , A C _ , R = ion
acceleration and retardation electrodes.
lar slits Sj and S 2 . Its width on the center-line can be
made as narrow as 10 /u, but for the present work
70 — 90 /J, was used. The elements to the left of AC +
form the auxiliary ion source to produce the primary
ions and replace the electron gun used in the earlier
experiments. The gas inlet into the auxiliary source is
through the conical nozzle N. Electrons from the filament F ionize this gas between the anode AN and the
electrode AN' which is a few volts negative with respect to AN. The potential difference AN —AN' is
dictated by a compromise between draw-out efficiency
and energy spread of the primary ions. Electrode E
prevents electrons from reaching the molecular beam.
Charge exchange takes place in the hatched region.
Between the electrodes AC+ and A C - an acceleration
field of F = 100, 200, 400, 1000, 2000 or 4000 V/cm
a) As a check f o r possible collision-induced contributions
to
the
decomposition
the
background
pressure was raised f r o m 10"" 5 to 1 0 ~ 4 T o r r . For
all processes studied
(except Xe + -benzonitrile, see
below) the ratio of satellite to main peak remained
constant within \%. This remarkable insensitivity
against pressure merely results f r o m our choice of
processes. In this work only decompositions were
studied which have a very intense metastable peak
to begin with, so that collisions, assuming reasonable cross sections, can make only minor contributions 9 .
b)
Possible distortion of the draw-out field by
the space charge of the primary ions was checked
using the process
X e + + C4H4S - > CoH2S+ + CoH2 + X e .
Decreasing
5
10~8
the
Xe+
current
from
5-1CP7
to
A did not change the satellite to main peak
ion species. The mass spectra also served to check
the purity of the gases
15.
ratio within the error limits of ± 10%.
c)
For the experiments with varying
ion energy e U-w11 to be valid, the collection efficiency of the ions resulting f r o m the charge transfer has to be independent of E/j on - This was ascertained f o r the reaction A r + + C 2 H 2 —> C 2 H 2 + + A r
f o r which Maier has made reliable measurements
of the cross section as a function of impact energy
up to
50
CYLAB
10-
In the present
apparatus,
the
variation of the C 2 H 2 + yield with the A r + lab energy agreed with Maier's data within
tween 5 0 and 2 0 0
CVLAB
Table 1. Recombination energies R E of primary ions X + a .
primary
± 1%. Be-
similarly g o o d agreement
was f o u n d f o r the reaction N 2 + + 0 2 - > 0 2 + + N 2
with the results of STEBBINGS et al. n > 1 2 . This indicates that any transverse momentum transfer associated with these particular charge exchange re-
x+
R E in e V (relative a b u n d a n c e in % )
Xe+b
Xe++C
Kr+d
Kr++
Ar+
Ar++C
Ne+
Ne++
He+
He++
12.13 (74-66); 13.44 (25-33); 12.5-16.5 (1)
12.5(15); 1 8 - 2 0 ( 8 5 )
1 4 . 0 ( 7 2 - 5 0 ) ; 14.67 ( 2 7 - 4 9 ) ; 1 6 - 1 8 (1)
21 (almost 100)
15.67, 15.94 (together 9 9 ) ; 1 8 - 2 0 ( 1 )
1 1 . 5 - 1 2 . 5 (20); 24 (80)
21.56, 21.66 (together 100)
1 0 . 5 - 1 2 . 0 (almost 100)
24.58 (almost 100)
11.0-12.5 (?)
1 5 . 3 - 1 5 . 5 7 (almost 100)
10.97, 12.16. 14.54 (together 9 0 ) ; 15.03 (10)
14.0 (almost 100)
8.58, 10.0, 11.26 (together 7 5 - 6 0 ) ; 12.40, 16.58
(together 2 4 — 4 0 )
13.62 ( 3 3 ) ; 1 4 . 4 5 , 1 4 . 9 8 (together 4 2 ) ;
16.67,16.94. 18.64 (together 25)
N2+
N+
CO+
C+e
O+f
actions is within the range that can be accepted by
our apparatus. It is a plausible assumption that the
a
same is true f o r all reactions studied here, which
are also exothermic (see also Sect. V I I , H ) .
d) Finally processes other than charge exchange
which also could cause dissociative ionization had
to be excluded. Ionization b y secondary electrons
Most of the data are taken from E. LINDHOLM, in: Ion Molecule Reactions in the Gas Phase, Adv. in Chemistry Ser.
No. 58, Amer. Chem. Soc., Washington 1966, p. 1. Data
given there pertain to 100 eV electron energy and a time
lapse on the order of 1 jusec after ionization. Additional
Refs. see
B B. TURNER, R . MATHIS, a n d J. RUTHERFORD, i n :
Proc.
of
produced at the electrodes AC+ and A C _ or ioniza-
the Conf. on Heavy Particle Coll. 1968, Univ. of Belfast,
p. 126.
tion by the impact of the primary ions could be
C W . A . CHUPKA a n d E . LINDHOLM, A r k . F y s . 2 5 , 3 5 5 [ 1 9 6 4 ] .
discarded on the basis of the observed mass spectra
(see below, Tables 3 and 4 ) . These differ radically
f r o m the usual electron impact spectra and are,
moreover, characteristically different f o r each pri-
D
J. T. SCOTT and J. B. HASTED, Advan. Mass Spectrom. 3.
389
[1966].
E R. TAO,
R . ROZETTI,
and
W . KOSKI,
J. C h e m . P h y s . 4 9 .
4202 [1968].
F P . WILMENIUS a n d E . L I N D H O L M , A r k . F y s . 2 1 , 1 0 6
mary ion species. Penning-Ionization by metastable
[1962].
Table 2. Mass spectra of primary ions X + A.
neutral particles X * effusing f r o m the ion source
certainly o c c u r s 1 3 . However, the energy available
f r o m a metastable X *
14
is always several volts less
than R E ( X + ) , the energy liberated in charge exchange with X + 6 . Therefore, if the latter lies in the
range of
interest just above some
fragmentation
threshold, the former will be insufficient to produce this fragmentation.
Table 1 gives the recombination energies RE of
the primary
ions used. The existence of
can often usefully be
p r i m a r y inso relative a b u n d a n c e in % )
Xe
Kr
Ar
Ne
He
X e + (77.8), X e + + (16.7), Xe+++ (5.5)
K r + (86.6), K r + + (13.0), Kr+++ (0.5)
A r + (89.6), Ar++ (10.4)
N e + (97.5), Ne++ (2.5)
H e + (99.9), He++ (0.1)
N2+ ( 6 5 - 8 5 ) , N+ ( 3 5 - 1 5 ) , N2++ ( 0 . 5 - 1 . 0 )
C 0 + ( 6 5 - 8 5 ) , C+ ( 1 8 - 8 ) , 0 + ( 1 6 - 6 ) ,
C0++ ( 0 . 8 - 1 . 5 )
N2
CO
several
values of R E f o r the same ion is a complication
which, however,
Primary
gasV
exploited
a
b
(see b e l o w ) . A further complication concerns the
identity of the primary ions. Unlike in Lindholm's
work, a primary mass spectrometer could not be
used
for
intensity
reasons.
This
determined
the
choice of gases X . The mass spectra given in Table 2
show the relative abundances of the contributing
c
Electron energy 120 eV.
For the rare gases see: R. E. F o x , i n : Advan. Mass Spectrom. 1, 397 [ 1 9 5 9 ] ; W . BLEAKNEY, Phys. Rev. 36, 1303
[ 1 9 3 0 ] ; P. T. SMITH, Phys. Rev. 46, 773 [ 1 9 3 6 ] .
For N 2 and CO the uncertainties of fragment ion abundance
result from partial instrumental discrimination, cf. D. RAPP.
P.
ENGLANDER-GOLDEN,
and
D.
BRIGLIA, J. C h e m .
Phys.
42, 4081 [1965] ; C. E. BERRY, Phys. Rev. 78, 600 [1950].
In the present experiment the fragment abundance in the
charge exchange region is probably in the upper part of the
range given.
V. Method of Analysis
intensity A
Let us c o n s i d e r charge e x c h a n g e b e t w e e n an
X
and a target molecule o f
+
X + M
0+
+
+ M 0 —>
, w h i c h is f o l l o w e d b y d i s s o c i a t i o n
yielding
a fragment
M-^
of mass
m a s s m0:
mx.
X
ion
Let the o v e r a l l
rate
o f d i s s o c i a t i o n b e d e s c r i b e d b y a d i s t r i b u t i o n 7 7 (k)
of
individual
rate constants
Our
k.
aim
is to
d u c e 7 7 (k)
f r o m the e x p e r i m e n t a l d a t a . O f
w e h o p e to
find
a f a i r l y n a r r o w d i s t r i b u t i o n IF
c e n t e r e d a r o u n d s o m e m e a n km
associate
with
de-
course
(k)
which w e will then
the r e c o m b i n a t i o n
energy
RE(X+).
N o t e t h a t i n t h i s a p p r o a c h w e d o not a p r i o r y p o s t u late the existence of s o m e s m a l l n u m b e r o f
discrete
rate
0
constants.
analysis
With
has to
a given
be performed
target
gas
M
,
this
f o r each species
X
mental
information
on
7 7 (k)
the
consisted
experi-
mainly
planimetri-
variation
due
to
w i t h c h a n g i n g F,
malized
III.
to
the
The
a
change
in
focusing
intensity
conditions
a l l p e a k a r e a s A n + (F)
corresponding
ratio
of
satellite
were
nor-
areas
to
main
peak
areas,
S P / M P . T h e satellite p e a k results f r o m
decomposi-
tions
M0+
in the
field-free
region.
a n d leave this r e g i o n
The
at t i m e s T1
ions
and r 2 ,
enter
respecti-
vely. T h e difference r2 — rx does not depend o n
as in o u r a p p a r a t u s the i o n e n e r g y in the
region
was
always
k e p t at 5 7 0
eV.
l a r g e r F i s , t h e s m a l l e r a r e b o t h r1
F,
field-free
However,
and r 2 .
the
There-
f o r e , an i n c r e a s e i n F leads to an i n c r e a s e in satellite p e a k intensity.
A t the s a m e time, the M 1 +
[see
impact work,
was measured
c o m p e n s a t e f o r the concomitant
+
separately.
In the earlier electron
decreases. A
cally. T o
of
(II)].
For
the
main peak
evaluation
of
decreases
the
change
of
S P / M P w i t h F it i s a s s u m e d t h a t c h a n g e s i n f o c u s i n g c o n d i t i o n s affect S P and M P to the same extent.
t h e l o n g tail t h a t j o i n e d o n t o t h e M j * p e a k o n t h e
T h i s is j u s t i f i e d b e c a u s e o f the g e n e r a l
e n e r g y scale t o w a r d s the l o w - e n e r g y s i d e . T h e
of o u r apparatus to changes in F (the absolute inten-
peak
had
no
tail
on
the
low-energy
side
M0+
because
the e l e c t r o n e n e r g y w a s c h o s e n such that the
elec-
sity v a r i a t i o n b e t w e e n F = 1 0 0 a n d 7, =
background
beyond
the
beam.
Unfortunately,
this is i m p o s s i b l e with i o n i m p a c t . T h e i o n s X
not
be
prevented
producing
M0+
from
by
penetrating
charge
the
+
can-
beam
and
exchange with
g r o u n d all t h e w a y d o w n t h e d r a w - o u t
M0
back-
field.
There-
f o r e , even the M 0 + peak n o w has a l o w - e n e r g y
tail.
A t a n y p o i n t t h e Mj"1" tail i n t e n s i t y is n o w t h e c u m u lative result f r o m M 0 + d e c o m p o s i t i o n s
in the
beam
as well as in the l o w , b u t b r o a d w i n g s o f t h e b e a m .
W o r s e than that, the c h a r g e e x c h a n g e c r o s s
section
is e n e r g y d e p e n d e n t a n d c h a n g e s a l o n g t h e tail.
is t h e r e f o r e
impossible
measurements.
Instead
to
obtain
we
based
7 7 (k)
the
from
It
tail
analysis
on
three other kinds of data:
I. T h e
distortion
compared
appears
of
M]*
to the M 0 + p e a k .
as a
rounding-off
is really the b e g i n n i n g
of
main
peak
shape,
This distortion,
to
which
a
function
increasing
F,
of
draw-out
2
(II)
them
(III)
on
field
individual
relative
of
con-
values
of
weights,
equations with IF (ki)
(I),
linear
as u n k n o w n s . T h e c h o i c e
t h e ki a n d t h e t o t a l n u m b e r o f u n k n o w n s a r e
trary to s o m e extent; we w o r k e d with 2 0
strength
peak
F.
A,
With
rather
for
on
the
gives
a
type
a mixed
reaction
set o f
of
equations.
(III)
were
(II)
or (III) alone, but
equations. F o r
benzonitrile
well-developed
satellite
used,
for
with
peak,
example,
Kr+,
5
F = 100,
which
equations
200,
400,
1 0 0 0 , 2 0 0 0 V / c m ; furthermore 3 equations of
( I I ) , n a m e l y f o r t h e t h r e e p a i r s F t , F2 = 2 0 0 ,
4000;
200,
4000
V/cm.
The
each f o r f o u r values of
type
1000;
remaining
( I ) , based on, f o r
t h r e e p o i n t s o n t h e b e a m p r o f i l e (AU,
TI(k),
12
example,
2 AU,
3
AU),
F.
b u t o v e r a b r o a d r a n g e o f k.
used
which, based
were
chosen
peak
In
of
subsequent
r e f i n e m e n t s d i f f e r e n t sets o f 2 0 e q u a t i o n s e a c h w e r e
potential
( i . e . i n t h e t a i l ) , s o that t h e a b s o l u t e i n t e g r a l
of
arbi-
T h e p r o c e d u r e w e f o u n d m o s t s a t i s f a c t o r y is n o t
to rely o n data of type ( I ) ,
at
appear
k
IJ(k1),
T h i s initial step gives o n l y the g r o s s features
ions Mj4"
the M t +
the
can therefore be described b y
1000,
AU,
quantities
are calculable. T h e
T h e e x p e r i m e n t a l results o f t y p e
),... .
or
of
with
equations were of type
and m o r e
energies appreciably below
IF(k
to
additive,
measurable
(III)
side,
a l o n g the e n e r g y axis.
more
all
the
k,
and
It i s
I I . T h e c h a n g e in M i * integral peak intensity
as
tributions
are
fixed
(II)
the l o w - e n e r g y
t h e r e c o r d e r c h a r t at s e l e c t e d s a m p l i n g p o i n t s
n AU
any
(I),
t h e tail f o r m a t i o n .
numerically described b y b e a m profile readings
2 AU,...
For
type
of
the
4000V/cm
was only a factor of 3 ) .
t r o n s c o u l d o n l y ionize M 0 i n the b e a m , b u t n o t the
M0
insensitivity
so
on
the results o f the
first
as to e x p l o r e the details o f
in various limited regions.
step,
FI(k)
An
important computational
a least s q u a r e s m e t h o d
pense
of
trebling
detail is the u s e
described
the
number
in
of
16.
A t the
equations
(i. e.
w e actually solved 6 0 equations, o n an I B M
it g u a r a n t e e s a s m o o t h s o l u t i o n II
of
ex-
7040)
(k).
b)
content
of
were
II (k)
the backcalculation
a full width
tor
of
at h a l f
backcalculated.
For
2.8
therefore
on
the
A:-scale.
the u p p e r
limit
o u r measurements of
The complete
the
was a broadened
maximum
on
ex-
of
factor
with
a
fac-
represents
the r e s o l u t i o n
c)
of
all
as
de-
benzonitrile
carbon
simpler
in t e r m s
with krypton,
ions.
analysis
was
of
H{k)
of
proach
the existence of
Kr+
and
Their values
complete
sely
(see
carried
two
one for A r +
out
a
for
and Ar+.
much
the
re-
In this
ap-
discrete values
was
and
a priori
kx,
/II (k2)
the
]
could
then
correspondingly
mentioned
above.
and simplified
below),
so
k2
postulated.
[ a n d , f o r K r + , also their relative
II (kt)
from
equations
comparison,
C 6 H 5 C N with K r +
tributions
easily
argon, nitrogen,
For
also
actions
for
be
simplified
con-
found
results
from
the
methods
agreed
quite
clo-
that
for
all
other
reactions
In o r d e r f o r a d e c o m p o s i t i o n
following
a)
ion
would
not be
fluctuation
ac-
effects
18.
too
decompositions
pyridine
and H 2 S
little i n t e n s i t y
or
(H2S)
in
were
had
cyclopentane.
tried, b u t
too
gave
small
ratios
A m/m1.
A) Charge Exchange Mass Spectra
our method,
The
fragmentation
benzene
resulting
patterns
from
of
benzonitrile
dissociative
and
ionization
c h a r g e e x c h a n g e w e r e m e a s u r e d at the " m a i n
by
peak"
p o s i t i o n o n the e n e r g y scale. A w e a k d r a w - o u t
(F = 1 0 0 V / c m )
mize
was
effects d u e
F = 100
V/cm,
employed
to
order
the f r a g m e n t a t i o n
the
ment ions f o r m e d
in
main
peak
to
field
mini-
kinetics.
comprises
At
all
frag-
w i t h a d e l a y u p to 1 / / s e c
after
the initial i o n i z a t i o n .
T h e mass spectra obtained with various
ions X
ion
+
primary
are given in Tables 3 a n d 4. The
kinetic
energy
Uion = 2 0 V ,
for
in
the
benzene
laboratory
three
spectra
primary
system
was
taken
with
T h e striking differences b e t w e e n charge
to b e suitable
it h a s t o
meet
spectra
taken
with
different
ions
exchange
X+
are
al-
for
ready familiar f r o m Lindholm's work on many
mo-
the
l e c u l e s 6 . T h e p r e s e n t t w o e x a m p l e s are s h o w n
here
b e c a u s e , taken t o g e t h e r with the R E ' s f r o m T a b l e 1.
requirements:
sense
the d e c o m p o s i n g
Further
butadiene,
mass
It s h o u l d h a v e a n i n t e n s e m e t a s t a b l e p e a k
the usual
of
t/j0n = 2 0 0 V are also given.
VI. Results
with
parent
T h e s e c r i t e r i a l e d t o the p r o c e s s e s s t u d i e d i n t h i s
linear
The
o n l y the simplified m e t h o d w a s used.
investigation
the
k.
analysis
monoxide
of
T h e t h r e s h o l d e n e r g y of the p r o c e s s has to lie
work.
scribed a b o v e was o n l y p e r f o r m e d f o r the reactions
of
a decomposition
in t h e r a n g e o f R E ' s o f T a b l e 1 .
result
peak
(FWHM)
This
be
d u r i n g the p r e c e d i n g d i s s o c i a t i o n
w i t h a ( 5 - f u n c t i o n as t r u e II (k),
ample,
to
curately k n o w n b e c a u s e of e n e r g y
Test runs were p e r f o r m e d in which several k n o w n
distributions
It h a s
i o n itself. F o r s e c o n d a r y d e c o m p o s i t i o n s the e n e r g v
(e. g . as t a b u l a t e d
in electron
in
im-
p a c t m a s s s p e c t r a 171
they
provide
a
good
qualitative
insight
fragmentation energetics. The progressing
into
the
fragmen-
t a t i o n w i t h i n c r e a s i n g R E is e v i d e n t . N o t e , in
par-
Table 3. Charge exchange mass spectra of Benzonitrile
m/e
103
102
77
76
75
52
51
50
a
b
Xe
Kr
CO
100.0
1.2
0.5
1.0
0.3
0.5
0.5
1.3
100.0
0.7
5.9
29.9
0.5
0.6
1.0
0.8
100.0
0.8
4.1
19.6
0.6
0.8
1.0
1.1
Primary
N2
75.0
3.0
16.6
100.0
0.5
1.8
1.2
1.5
Gas
Ar
4.2
2.7
17.8
100.0
0.4
0.8
3.0
0.9
All intensities, uncorrected for isotopic contributions, are normalized to
were recorded on the "main p e a k " (intersection of primary ion beam and
F = 100 V / c m and primary ion energy e C/iOn = 20 e V ; the electron energy
Mass spectrum for electron impact ionization (Mass Spectral Data, Amer.
Ne
22
6
5
—
52
15
100
100
He
12
?
i
8
64
24
73
100
e- n
100.0
1.7
5.8
32.9
8.3
5.9
10.3
18.1
100 for the most abundant fragment ion. They
molecular beam, see text) with a drawout field
was 120 eV.
Petrol. Res. Inst., Res. P r o j . No. 4 4 ) .
Table 4. Charge exchange mass spectra of Benzene a .
m/e
Primary X e
Gas
[/ion (V) 20
100.0
0.5
0.2
1.1
1.2
0.5
78
77
76
52
51
50
39
a
b
Kr
CO
N2
N2
Ar
Ar
Ne
20
200
20
20
200
20
200
20
16.4
56.6
11.5
100.0
2.3
2.5
34.9
17.7
54.0
12.3
100.0
7.2
3.7
36.8
2
(45)
(13)
1
100
38
20
(100.0)
36.2
19.9
20.6
2.0
0.6
5.1
(100.0)
36.1
18.2
9.5
1.0
0.4
2.9
?
(100.0)
14.8
11.3
7.1
0.7
0.3
2.2
(100.0)
43.5
9.5
77.0
3.6
2.2
27.2
(100.0)
38.5
7.5
63.6
5.9
2.9
20.4
b
100.0
13.8
6.2
18.9
20.1
16.8
13.5
For legend, see Table 3. Here data for f/i O n = 200 eV have been included. Some relative intensities, marked by parentheses,
are at variance with results by B.-Ö. JONSSON and E. LINDHOLM, Ark. Fys. 39, 65 [1969]. They probably contain contributions from Penning ionization (see Sect. IV, d ) .
Mass spectrum for electron impact ionization (Mass Spectral Data. Amer. Petrol. Res. Inst., Res. Proj. No. 4 4 ) .
t i c u l a r , that b o t h i n C 6 H 5 C N a n d i n C 6 H 6 t h e o v e r -
tions,
all
c o m p l e t e l y m i s s i n g f o r the f o u r t h .
fragmentation
With
Kr,
the
14.67 eV,
is
33%
raises
less
Kr+
with
in
the m e a n
C+
and 0
CO
the
than
2P1/2
RE
1 4 . 0 e V , the R E of K r + ( 2 P 3 / 2 ) ,
level,
Kr.
RE =
appreciably
above
while with C O
admixtures are of such l o w
+
with
that, i n s p i t e o f t h e i r h i g h R E
16.8 e V ) ,
the
R E ( C O + ) = 14.0 eV
the
abundance
(Table 2)
(around
remains
domi-
nant.
1)
Rate Constants
B e n z o n i t r i l e
C 6 H 5 C N + —> C 6 H 4 + + H C N
of the p r o -
were
measured
ing the p r i m a r y ions K r + , A r + , C O +
by
C+
and
C6H5CN+
by
0+)
charge
collision with X e +
be
an
and
N2+
(with
exchange.
N+)
to
produce
Fragmentation
was also o b s e r v e d , but
endothermic
process
us-
(accompanied
(see
by
appears
below).
As
qualitative illustration, F i g . 3 s h o w s the M 0 +
beam
p r o f i l e , i n this c a s e f o r X + = C O + i m p a c t , a n d
various M j + b e a m profiles. T h e steadily
a
the
increasing
d i s t o r t i o n in g o i n g f r o m A r + to K r + is o b v i o u s .
It
are
almost
absent
for
T h e s e qualitative expectations
the t h i r d
are b o r n e out
F i g . 3 w a s s c a n n e d at least 1 0 t i m e s a n d
The
entire
amount
of
experimental
data
and
any
one p r im a r y ion species X + was simultaneously
pro-
by
the analytical
method
described
above.
It y i e l d e d a u t o m a t i c a l l y t h e c o m p l e t e r a t e
constant
distribution
Fl (k)
between
about k = 5 •
104
Kr,
and
resulting
CO,
for
each
109
four
No and A r
sec
-1
X+
species
.
distributions
are given
for X
II (k)
in F i g u r e 4 .
ture
can
Table
II (k),
1:
be
closely
Each
whose
correlated
RE
is
location
tonic function of R E
with
represented
on
the
the
by
k-axis
a
is
RE's
in
peak
in
a
mono-
( c f . the p e a k l a b e l i n g in F i g -
ure 4 ) .
T h e three peaks in F i g . 4 a c a n b e attributed
to
three different ionic species. T h e b i g C O + peak
at
low
k
explains
the p r e v i o u s l y
mentioned
satellite
p e a k f o r this r e a c t i o n . I n F i g . 4 b t h e t w o p e a k s a r e
ed f r o m many runs under varying
accurate than an
indivi-
=
They
differ dramatically in shape. M o r e o v e r , their struc-
d u e to the
are therefore much m o r e
in
gathered
f o r charge exchange between benzonitrile
t h e s a m e t w o states o f A r + a r e n o t r e s o l v e d .
and
by
averaged.
m u s t b e e m p h a s i z e d that t h e final r e s u l t s a r e d e r i v conditions
and
detailed analysis. Each c u r v e o f the type s h o w n
The
T h e rate c o n s t a n t d i s t r i b u t i o n s H(k)
cess
which
cessed
B) Measurements of Dissociation
to
e~
Kr
2 P3/ 2
and
2PI/2
states o f K r + . I n F i g . 4 d ,
t h e p o s i t i o n o f t h e A r + p e a k o n t h e k-axis:
Note
in c o n -
d u a l e x a m p l e such as i n F i g . 3 w o u l d s e e m t o w a r -
trast t o t h e K r + p e a k , it c o v e r s t h e r a n g e o f l a r g e k
r a n t . T h e A r + , N 2 + a n d C O + c u r v e s i n F i g . 3 a r e all
o n l y , w h i c h e x p l a i n s t h e a b s e n c e o f a satellite p e a k
rather similar. Nevertheless, the c o r r e s p o n d i n g
i n t h e A r + e x p e r i m e n t s . I n F i g . 4 c t h e N 2 + p e a k is
sa-
tellite p e a k s h a v e v e r y d i f f e r e n t i n t e n s i t y ; t h e r a t i o
again composite.
o f satellite to m a i n p e a k
c o r r e s o n d i n g to r e c o m b i n a t i o n of N 2 + into N o X 1
90 V)
(at F = 1 0 0 V / c m ,
is i n the o r d e r K r + , C O + , N 2 + , A r + :
= 0.47; 0.24; 0.028;
ponents
o f FI (k)
Uion
=
SP/MP
< 0 . 0 0 3 . This indicates c o m -
at l o w k f o r t h e
first
two
reac-
It c o m p r i s e s s e v e r a l v a l u e s o f
k,
(v' - 1 and v = 0 ) .
The
desired
dependence
tained b y c o m b i n i n g
k(E)
can
now
Figs. 4 a — d into o n e
be
ob-
figure.
'Kr'
140 eV
A
CO
o)
CO* 1
140 eV\
.
\
0*
1498 eV
9 c
/
C.O'
1658.1667.1694 M
Kr
(b)
c
\
°>
Kr *
!4C7eV
/
*
10s
*
106
(c)
to7
N2
°
10s
LA
109
10!
14.54 eV
1
10 V
Unimoiecuiar
Fig. 3. T y p i c a l results f o r C 6 H 5 C N + X + - >
C 6 H 5 C N + + X (top trace) and C 6 H 5 C N + X +
C ß H 4 + + H C N + X (with X + = s u m of all
ions f o r m e d f r o m C O in top trace and f r o m
A r , N 2 , C O , K r in the lower t r a c e s ) . Drawout field F = 1 0 0 0 V / c m . C o m p a r e the schematic Fig. 1 : T h e progressing peak distortion indicates an increase of mean parent
ion lifetime. Satellite peaks are not shown.
be
fixed.
exchange
According
ionization,
to
the
principle
represents
AE
mal
energy
the s u m
and
the collision.
0.1 eV
the
The
(taken
similar number
of
of
of
the initial
average
former
over
of
I
4.
j
energy
butane19,
degrees
the
bombarding
periment.
Results
ion
presented
average
o f a b o u t 0 . 1 5 e V . T h u s AE
to b e
TL ( & )
from
which
has
decomposition rate constant k (sec
lead
4 a — d
a
change
our
to
a
are
in
latter
in
is t a k e n as 0 . 2 5
Fig.
+
109
about
ex-
value
eV.
I n F i g . 5 t h e p o s i t i o n s o f all m a x i m a o f t h e
distributions
108
ther-
transferred
eU;on
below
I07
+
where
of f r e e d o m ) . The
energy
*
has
c a n b e assessed b y o b s e r v i n g the effect of a
of
109
x
benzonitrile.
is estimated
from
10s
\
charge
E = R E — I . P . + AE,
L P . = adiabatic ionization potential
I07
Ar
Fig. 4 . P r o b a b i l i t y distributions H(k)
of rate constants k f o r unimoiecuiar d e c o m p o s i t i o n of C 6 H 5 C N + , p r o d u c e d by charge exchange with ions
f r o m f o u r different primary gases. U (k) was obtained f r o m curves like
those s h o w n in F i g . 3 by a mathematical p r o c e d u r e explained in the text.
T h e pairs of s y m b o l s ( o , x ) and ( • , + ) give a measure of the uncertainty of the c a l c u l a t i o n ; f o r o and • the set of selected values ki (see
Sect. V ) was displaced as a whole towards lower k, as c o m p a r e d to X
and + b y a factor of 2. Negative values of 12 (k) are also a computational artifact.
A s a p r e r e q u i s i t e , t h e z e r o p o i n t o n t h e E-scale
to
io°
id)
L 1576 1
0
10s
107
10°
\
1
1 15.3 \
1:5.57
•A
'
four
plotted
Fig. 5. D e p e n d e n c e of unimoiecuiar decomposition rate c o n stant k on internal energy E of C 6 H 5 C N + . T h e solid curve was
obtained by plotting the peak positions f r o m F i g . 4 versus
£ = R E — L P . + 0.25 e V (see t e x t ) . T h e dashed curve gives
k(E)
as obtained f r o m earlier experiments with electron impact ionization.
Excitation
energy of C6 H5 CN',
E(eV)
v e r s u s E,
which was
calculated
using
I.P. = 9 . 7 0 5
e V 2 0 a n d the R E ' s f r o m T a b l e 1. T h e s m o o t h
vari-
suggests
transverse
ion
at l a r g e
loss
momentum
(see
U-10n
transfer leading
).
23
The
to
intensity
of
a t i o n o f l o g k w i t h E w h i c h is o b t a i n e d is e x t r e m e l y
C 6 H 5 C N + f r o m charge exchange with X e + was
very
g r a t i f y i n g . It g i v e s s t r o n g s u p p o r t t o o u r
m u c h l o w e r than with the other gases X . T h i s
also
of
analysis
procedure
and to the thesis that this c u r v e
represents the f u n c t i o n
truly
k(E).
because
4 b
of
and
the i n c o m p l e t e
4 c.
same peak
Where
(N2+,
peak
several
resolution
RE
width
+
exchange
gave
a
analysis which presupposes discrete
the
(see a b o v e ) . T h e result f o r km(E),
to
dot.
compared
measur-
from
Fig. 6
with
Figure 5. The
good
respond
13.4
method
RE =
de-
pre-
taken
at k = 3 • 1 0 4 s e c - 1 ,
val-
duced f r o m experiments with K r + and A r + only,
cor-
a point
benzonitrile
ues o f km
a b l e s a t e l l i t e p e a k . It w o u l d , b y e x t r a p o l a t i o n ,
to
the
method of
contribute
also
for
Fig.
+
with X e +
k (E)
in
d o m i n a n t c o m p o n e n t is m a r k e d w i t h a h e a v i e r
Charge
well-established
d e c o m p o s i t i o n w a s u s e d a s a test f o r t h e s i m p l i f i e d
A r , C / 0 ) , the p r e s u m a b l y
+
The
FWHM
peaks. T h i s was p r e f e r r e d to
(k)
endother-
mic process.
T h e error bars in F i g . 5 indicate the 9 0 %
of the n
p o i n t s to a d i f f e r e n t m e c h a n i s m , i. e. an
e V . H o w e v e r , this p o i n t w a s n o t i n c l u d e d in F i g . 5
in
as
justification
for
the m o r e
accurate
agreement
to
use
of
the
is
(E)
obtained
only
the determination
k
was
simplified
of
k(E)
benzene
and thiophene.
b e c a u s e the p r o c e s s
2)
Xe+ + C6H5CN - > C6H4+ + HCN + Xe
B e n z e n e
Of
is m o s t likely e n d o t h e r m i c .
appearance potential of
convincing,
pearance
in view of
potential
This
C 6 H 4 + of
follows from
13.9 e V 2 1 .
the uncertainties
measurements,
is
of
the
d e p e n d e n c e o f the satellite p e a k intensity
the
More
all
ap-
unusual
four
important
first
and
third
primary
could
be
used
on
mass
76+
(benzonitrile
p r i m a r y i o n k i n e t i c e n e r g y e U[0Q . C o m p a r e d t o t h e
have
similarly
" o r d i n a r y " increase of intensity t o w a r d s l o w e r
s p e c t i v e l y . T h e f u n c t i o n km(E)
VI,
C2),
the rise w a s
here
4
times
(see Sect.
steeper.
This
qualitatively
to
as d e t e r m i n e d
the
from
first
they
and
a
two
widely
and
39+
be-
process,
re-
exchange
with
km.
Ar+
ed
peak,
so
that
here
only
a
lower
its
special
with
3
the
delayed
The
loss
Consequently
negligible.
S IO7
4>
5
c
3
Q
The
satellite p e a k ,
limit
for
energy
of
one
peak
only
which
deficit
km
mass
distortion
measurable
is h e r e
presented
associated
unit
is
and
tail
are
is
the
quantity
located
very
in
Fig. 8 a
and
b.
From
its
t h e m a i n p e a k ) t h e km(E)
in F i g . 7, w a s derived
o
o
mon
k
15.76
5.0
energy of
6.0
C6H5CN*E(eV)
Fig. 6. Comparison of k(E)
for C„H 5 CN + - > C 6 H 4 + + H C N
as obtained by the complete (solid curve) and a simplified
(dashed curve) mathematical procedure.
lated
value
and
was
15.94
immediately
combine
to
This
from
give
(relative
as f o l l o w s . F o r A r +
attributed
eV.
the
can
km
the
the
to
two
a
determined
if
to
then b e
experimental
k
one makes
SP/MP
SP/MP
an assumption
of
calcu-
(at 1 4 . 0 a n d
observed
com-
RE's
F r o m t h i s e x p e r i m e n t a l o n e &14.0 a n d k ^ j c a n
be
to
shown
f o r this p r o c e s s , as g i v e n
ratio. F o r K r + two values of
eV)
intensity
very
close
t h e m a i n p e a k . It i s e x t r e m e l y i n t e n s e , a s i s
\(f
Excitation
difficulties.
small.
v>
|6
rela-
c o u l d b e stated.
Jc
D
is
states
+
undistort-
T h e i m p o r t a n t p r o c e s s C 6 H 6 + —> C 6 H 5 +
c
Kr+,
with
tively h i g h R E gave already a practically
3
(K+
to
for C6H6+ - > C4H4+,
charge
different
(E).
background
appeared
third
the
km
g i v e n i n F i g u r e 7 . It i s s e e n t h a t t h e t w o K r
yield
IO9
derive
residue)
surfaces),
which was observed in several other cases
to
The second and fourth were masked by
from
U-10n
decompositions
C6H4+, C 4 H 4 + and C 3 H 3 + only
the
22
on
the
C6H6+->C6H5+,
14.7
ratio.
only
about
with the k value f o r A r + , they lie o n a straight line
in
a
l o g k(E)
are
k(E)
plot24.
shown
in
Qu.olQu." = 3.0;
tively)
25.
The
Fig. 7
1.0;
resulting
for
0.33
the
functions
assumed
(curves
a, b ,
c
ratios
respec-
T h e value of k f o r the limiting, but
unlikely
cases
$14.0 = 0
or
$14.7 = 0 ,
quite
k — 3.6 •105
s e c - 1 , is also s h o w n .
An
was
alternative
also
used.
Qu.olQu.i
a n
perimental
adduced.
points
The
d
The
from
rather
shown
first
3)
energy
of C 6 Hg ,
U,on = 8V
M
ul0n250 V
C6H*
J
kinetic
energy
associated
tions,
relative
(?i4.o a n d
were required
charge
good
yielded
agreement
is a l s o i n
ratio measured with N 2 +
accord
(and
N+)
primary
decomposition
occurred
charge
C 4 H 4 S + —> C 2 H 2 S +
with a measurable
exchange
with
Xe+.
delay
only
has
RE's
Xe+
lysis,
values
as
of
Between
described
k
differing
these,
k(E)
above,
by
was
2.4
yielded
orders
afof
ana-
corresponding
of
interpolated
magnitude.
as s h o w n
in
Figure 9.
109
3
I0 8
•2?
^
C:
O
deficit
transfer
to fulfill the c o n d i t i o n
analysis26
in
3
c
t>
vo
§ 107
o
\
$14.7 . F u r t h e r m o r e ,
7,
exwere
T h i o p h e n e
o
0)
F i g . 8. M a i n peak (on the left in each scan) and satellite peak
of the process C 6 H 6 + — C 6 H 5 + + H, for two different kinetic
energies e t/j 0 n of the primary ions (in this case C O + ) . T h e
comparatively small increase of the main p e a k : satellite peak
intensity ratio with increasing Uj 0 n demonstrates the inefficient conversion of kinetic into internal energy.
the
Fig.
the
exchange
1 2 . 1 3 a n d 1 3 . 4 4 e V . T h e simplified m e t h o d of
E(eV)
Fig. 7. D e p e n d e n c e of unimoiecuiar d e c o m p o s i t i o n rate c o n stant k on internal energy E of C 6 H 6 + , f o r two different d e c o m position paths. T h e curves a, b , c are f o r three assumed ratios
of the charge exchange cross sections f o r K r + ( 2 P 3 / 2 ) + C 6 H 6 and
K r + ( 2 P j / 2 ) + C 6 H 6 , respectively (see t e x t ) . Boundary values
f o r this k(E) curve are given by the horizontal bar. T w o further points as well as the area " C u o t f f " (see Sect. V I I , E ) all
support the e x c e e d i n g l y slow rise of this k{E). T h e contrasting steep rise of k(E) f o r C 4 H 4 + formation proves that the two
processes are not in competition with each other.
c
<b
t a tr
o c
charge
&14.7
about
ions.
+ C2H2
Excitation
line conditions,
CO+
involved
in
Ä44.0 a n d
assumptions
o b t a i n e d in this w a y
k(E)
The
ter
finding
the
method.
with the S P / M P
primary
of
of
the straight
data
also
with the
method
Instead
cross
sec-
&14.0 a n d
&14.7
that,
together
O
u
<D
3
o
Q 3
Excitation
energy of C4H4S*
E(eV)
Fig. 9. D e p e n d e n c e of unimoiecuiar decomposition rate constant k on internal energy E of C 4 H 4 S + , obtained by the "simp l i f i e d " p r o c e d u r e f r o m charge exchange with X e + ions.
C) Charge Exchange at varying Ion Impact Energy
Measurements were made for ion energies
between
1)
T r a n s 1a t i o n a 1 E n e r g y
T r a n s f e r
All experiments discussed so far w e r e done
SP/MP
with
9 0 e V p r i m a r y ions. B y v a r y i n g the i o n i m p a c t
e r g y the a m o u n t of e n e r g y transferred f r o m
en-
kinetic
to excitation energy c a n b e measured.
T h e ratio S P / M P
is a sensitive i n d i c a t o r f o r
amount of energy transferred. A
spectacular
the
quali-
tative e x a m p l e is g i v e n i n F i g u r e 8 . H e r e t h e k i n e tic e n e r g y
ergy. The
a
in the c . m .
system
changes
by
a
factor
3 0 , f r o m 0 . 6 to 1 8 times the r e c o m b i n a t i o n
of
factor
ratio S P / M P ,
of
1.4.
however,
Therefore
the
only
changes
amount
of
e n e r g y transferred in this charge e x c h a n g e
ure
k(E)
plays
only
obtained
serve
as
such
above
gauges
to
a
(with
minor
role,
measure
Fig-
quantitatively
the
eV
(LAB
field
eU-l0n
"simplified
system).
peak
intensity
were
analyzed
and
ratio
was
strengths F. T h e
method")
eUion
The
de-
results
as
above
yielded
val-
u e s o f k w h i c h , at e a c h R E , i n c r e a s e d
monotonically
with
converted
Uion . This
the
known
variation
to
k{E)
of
yield
was
k
the
change
of
via
internal
e n e r g y E . T h e r e s u l t is s h o w n in F i g u r e 1 1 . It
can
b e seen that less than 0 . 1 % of the c . m . kinetic e n e r g y
is t r a n s f e r r e d
in these charge e x c h a n g e
2)
E x c h a n g e
C h a r g e
C r o s s
processes.
S e c t i o n
T h e relative c r o s s s e c t i o n , as a f u n c t i o n o f
for
formation
a
of
cannot,
a
secondary
in
general,
a measurement
function
of
VI,
CI,
the
total
causes
ion
yield
with
U\0n,
charge
obtained
reason
ex-
directly
k
and
is the
as
transfer
as d i s c u s s e d
in
of
Sect.
h e n c e the f r a c t i o n
collected
Among
U-Wn.
by
of the m a i n peak intensity
U-10n . T h e
which
ion
be
kinetic into excitation energy
change
0.6
energy
the
from
now
250
satellite to m a i n
each
curves
can
of
(using
change
a m o u n t of translational e n e r g y transfer.
o
for
kinetic
the
Z7; on = 9 0 e V )
by
and
termined f o r different
collision
1 0 gives two other examples. S i n c e the
energy
en-
kinetic
i s v e r y s m a l l . T h i s i s t r u e i n all c a s e s s t u d i e d .
8
within
the
the
cases
of
peak
to
studied,
this
effect was negligible only f o r
<t>
C6H5CN + Ar+ - > C6H4+ + HCN +
2 04
Ar,
Q.
c
£
b e c a u s e h e r e k is s o l a r g e
a0
"to
2
10
o
a
small
variation
intensity
0
5
10 20
50
100
U,on(eV,
250
log
for
is
this
(see Figs. 4 and 5 )
insignificant.
process,
For
the
Kr+
primary
sults
of
Sect.
both
k(RE
CI.
mass
c:
<u
«D
and
and
(c).
also
with
into
analogous
energy
of primary
ion X* (eV, log
scale)
Fig. 11. Kinetic to internal energy transfer f o r three charge
exchange processes, obtained from data such as in Fig. 10 via
the k(E) curves of Figs. 5 and 7.
each
given.
benzene,
curves
forming
uncorrected
(c)
give
the
total
contributions
separately,
(a)
the
U- l o n
A
Fig.
12,
13
52
ions;
peak
intensity,
true
Q(Ujon)
for
K r + (2P3/2) species, respectively.
of
curve
curve
) ,
(b)
[curve
curves
the effect o f
Figure
i o n
curves
of
for charge exchange
mass
Kr+(2P!/9)
intensity r a w data
main
yield,
h ^ ( U
comparison
illustrates
peak
ion
from
and
see
rection f o r the k v a r i a t i o n 2 7 .
o
using the refor
= 14.67)
k(RE
I76+(1/2) (^ion)
The main peak
are
(c)
76
two
obtained
C5>
c:
T>
to
obtain
w a s used here to convert the m a i n
at
were
(a) ]
and
analysis,
to
as
C
o
-c
normalized
corrected
in-
Kr+(2P3/2),
o
that
Ar+
(d)].
intensity,
was
the
their relative contributions were obtained. This
and
Q.
In
to
peak
as well
Itg + ( U i 0 n ) • T h e
Q>
current,
= 14.0)
intensity
o
ion
peak
the true relative cross section Q ( U - w n )
formation
<U
c
a>
c:
the m a i n
that
main
normalized
c u r r e n t , is s h o w n in F i g . 1 2 [ c u r v e
scale)
Fig. 10. Variation of the intensity ratio satellite peak : main
peak with U i 0 n (see Fig. 8) ( F = 1 0 0 V / c m ) .
cm. kinetic
The
the
shows
Kr+
(a)
(b)
corthe
with
is
the
(b)
and
the K r + ( 2 P ! / 2 )
and
Benzene
3
Benzonitrile
-
2 -
(normalized)
2.5
1-
25
50
100 150 Ektncm
(normalized)
~25Ö
(eV)
U,0„ (Kr)
(V)
Fig. 13. Dependence of dissociative charge exchange cross section on relative kinetic energy, for K r + + C 6 H 6 + — > C 4 H 4 + -fC 2 H 2 + K r . See Figure 12.
25
10
5
10
5
5
2.5
25
50
50
25
10
25
100 150
100
50
250
Ekincm(eV)
Uion
100
(Kr*)(V)
U.on (Ar*)(V)
Fig. 12. Dependence of dissociative charge exchange cross section on relative kinetic energy, for K r + + C 6 H 5 C N —*• C 6 H 4 +
+ H C N + Kr (curves a - c )
and
Ar++C6H5CNC6H4++
H C N + A r (curve d) ; (a) gives the uncorrected normalized
main peak intensity, (b) and (c) the true separate contributions, as derived from ( a ) , from K r + ( 2 P ! / 2 ) and K r + ( 2 P 3 / 2 ) ,
respectively. Unlike ( a ) , the normalized main peak data of
curve (d) need not be corrected for kinetic effects.
c)
the
spread
responding
of
thermal
d)
for
the spread o f R E ' s
N2+,
corresponding
whose
benzonitrile
decomposition
siderable
detail.
decompositions
or
broad
picted
in
is
behaviour
the
+
first
is k n o w n
studies
some
of
4
show
structure
II(k)lj5.
that in
con-
unimoiecuiar
or changes
The
results
the present
k
in
de-
experi-
m e n t b e n z o n i t r i l e i o n s w e r e p r e p a r e d i n states w i t h
n a r r o w distributions 77
(k).
peak-
the v a r i o u s f a c t o r s c o n t r i b u t i n g
to
their w i d t h :
a)
T h e limited
The
observed
of
the u n f o l d i n g
cess, resulting in an uncertainty of a factor of
o n the &-scale;
b)
the b r o a d e n i n g
for
each
widths
RE;
and
(FWHM)
4 for Ar+
a
KRAUS
discussed
the
moiecuiar
decay.
of
experimental
noise;
a
7 7 (A;)
5
m a y w e l l b e a (5-func-
generously
estimated
pro2.8
as w e l l
28
possibility
Bunker
of
a s BUNKER
upper
have
29
non-exponential
performed
uni-
classical
traafter
c o l l i s i o n a l a c t i v a t i o n in a heat b a t h . T h i s is a p p r o priate
to
position,
unimoiecuiar
decom-
b u t less s o t o the p r e s e n t , rather
the
usual
chemical
specific
m o d e of excitation. Mies and Krauss considered
in-
d i v i d u a l q u a n t u m m e c h a n i c a l r e s o n a n c e states
(cor-
responding
over-
to the
activated
ion
states)
with
It w a s s h o w n t h a t
departures
f r o m e x p o n e n t i a l d e c a y can o c c u r t h r o u g h the intera c t i o n o f t h e r e s o n a n c e states w i t h t h e
dissociation
continuum.
a non-exponential
decay
existed,
it w o u l d
in
o u r e x p e r i m e n t s h o w u p as a m o r e o r l e s s b r o a d e n e d d i s t r i b u t i o n 7 7 (k).
W e c o n c l u d e , therefore, that
in the present cases the conditions f o r
from
the
and a factor of
jectory calculations o n molecules dissociating
If
resolution
differing by
e) T h e spread of energy transferred f r o m kinetic
l a p p i n g level widths.
T h e s e distributions are r e m a r k a b l y sharply
ed, considering
(0.16 eV for Ar+, 0.29 eV
energy.
MIES
system
in
corfactor
l i m i t t o its w i d t h i s a f a c t o r o f 2 .
HCN
g a v e o n l y average rate constants
distributions
Fig.
now
Previous
at b e s t i n d i c a t e d
the
ion
C6H4
a
factor of 1.8 and 2.8, respectively).
tion
-
(0.1 eV,
k within
to k v a l u e s
f o r N 2 + . T h u s t h e t r u e TI (k)
VII. Discussion
The
of
of 1.4, cf. Fig. 5 ) ;
peaks are a factor of
A) Is the decay law for C E H 5 C N +
an exponential?
energy
to an uncertainty
tial d e c a y
(specified
in
28
non-exponen-
in t e r m s o f level
width
and
spacing
distributions,
in
in
29
terms
drance of intramolecular energy f l o w )
not
obtain.
In
particular,
Mies
and
o u t the possibility that metastable
ions
with
excitation
sociation
Fig.
5
energies
threshold
shows
tonically
may
clearly
shortened
that
as
well
the
the
do
the
dis-
benzonitrile,
lifetime
excitation
is
mono-
energy
in-
creases.
T h e earlier electron impact w o r k
dependence
for
k(E)
also gave
states
at
and
state.
that
the
threshold
Justification
time
electron
given
of
by
law
for
these
the
energy
for
density
excitation
of
assumptions
excellent
"normal"
f r a g m e n t i o n s . T h e k(E)
linear
dependence
1.54/eV30.
This
the
line.
dashed
fixed
was
agreement
of
by
of
The
of
dashed
are those
line
in
is
on
measured
with
Fig.
k = 1 0 7 i s r e a c h e d at E=
E
with
in
Fig.
the
slope
5
by
£-axis
was
-1
.
po-
AP(76+)
fragments
k > 10" s e c
5
was
appearance
fragment C6H4+,
"Normal"
of
"metastable"
on
shown
position
the
= 15.2±0.2 eV4'21.
apparatus
i0log k
function
means
and
o b t a i n e d i n that w o r k
tential o f the " n o r m a l "
the
the
of
the
intensity, f o r K r + through A r + . C o r r e c t i o n
for
13
in
our
Therefore
is p o s i t i o n e d
such
20,
is
spectrum
only
about
(Table
15 — 2 0 %
C 1 2 C 5 H 4 + c o n t r i b u t i o n r e d u c e s it f u r t h e r b y
f a c t o r o f a b o u t 0 . 6 . W i t h this l o w a b u n d a n c e
the
C12C5H4+
13
k(E)
benzonitrile
background
it w a s
directly
the
0.1 eV = average
ion
other or not.
in
are
in
In the
way
of
measured
k
for
with
one
at e a c h R E
the t w o
processes
the
would
b e e n exactly the same, n a m e l y equal to the
decomposition
yields
in
rate
measured
this
case
the
at m a s s
have
h = & 7 7 + k-jQ .
of
been
(Note
parent
76
in
and
the
that in
ion.
ratio
l o w e r p a r t . A t h i g h k,
more
reliable.
electron
The
impact
agreement
the latter c u r v e is
deviation
of
and
in
the
certainly
the f o r m e r
curve
is n o t u n e x p e c t e d in v i e w o f the a s s u m p t i o n s
made
77
reassuring
agreement
between
both
sets
d a t a a l s o l e n d s s t r o n g s u p p o r t t o t h e k(E)
f o r butane and heptane derived f r o m
pact experiments
5.
less
electron
In those cases the s a m e
favourable
n a t e l y , a check w i t h
circumstances.
the present m e t h o d
of
curves
k1&jk-n,
this case
with
and
k76
k-n
cannot b e measured directly, but only inferred f r o m
the
relative
ratio
ion
yields
k-Jk-j-j c o u l d ,
and
of
the
course,
value
of
The
k.)
have been
f o r each R E . F o r the case o f isolated,
different
non-compet-
i n g d e c o m p o s i t i o n s to f o r m 7 6 + and 7 7 + ions,
would
in
general
values
of
k
would
not
at
for
be
the
related
each
two
RE
measure
fragments.
one
different
Also,
to the relative i o n
these
yields.
W e a d o p t the a s s u m p t i o n o f c o m p e t i n g
processes
as the m o r e likely o n e . T h i s m e a n s that the
given
in
Fig. 5
is
to
be
interpreted
as
&77 w o u l d
be
smaller b y
a factor
k{E)
& 7 7 + k-6
of
.
ener-
10.
If,
o n the other h a n d , the m a s s 7 7 f o r m a t i o n is n o t in
competition
t h e t r u e &76
with mass
76
formation,
Fig.
5
gives
( w h i c h is t h u s n o t v e r y s e n s i t i v e a s t o
w h i c h o f t h e t w o a s s u m p t i o n s is m a d e ) , a n d k77
under
Unfortuis
D) Two clearly non-competing decompositions
c6H6+
im-
assump-
t i o n s as f o r b e n z o n i t r i l e w e r e m a d e , t h o u g h
somewhat
ion
would
is
then completely u n k n o w n .
i n its d e r i v a t i o n .
The
have
The
mass
to
overall
gies,
from
are in excellent
an-
value
tions
as o b t a i n e d
for
case, which c o n f o r m s
& 7 6 a l o n e w o u l d b e a b o u t 1 0 % s m a l l e r at all
k(E)
to
as
t h e r m a l e x c i t a t i o n e n e r g y ) . It i s s e e n t h a t t h e f u n c -
charge exchange
a
and
not possible
same
competition
first
the usual ideas of the Q E T ,
that
15.2 - 9 . 7 0 5 + 0.1 = 5.6 e V
( 9 . 7 0 5 e V = I.P. of benzonitrile
the m a s
l e m o f w h e t h e r the losses of C N and H C N f r o m the
the
calculated and m e a s u r e d i o n y i e l d , as a f u n c t i o n
a
76+
the
to
intensity
m e n t w o u l d h a v e resolved u n a m b i g u o u s l y the p r o b -
C6H5CN+ - > C6H4+ + HCN,
t h o u g h only with s o m e assumptions o n the
each
3),
77
m a s s 7 6 . T h i s is u n f o r t u n a t e since such a m e a s u r e -
B) Comparison with k(E) from electron impact
experiments
of
According
mass
measure
4
reaction
C 6 H 5 C N + - ^ C 6 H 5 + + CN
point
long-lived)
above
For
C) The concurrent
hin-
Krauss
(i. e.
exist.
of
probably
impos-
The
C6H6+,
two
concurrent
leading
constitute
a
to
C6H5+
clear-cut
decomposition
and
example
of
paths
of
C4H4+,
respectively,
of
existence
the
of
r e a c t i o n s that are n o t in c o m p e t i t i o n w i t h o n e
an-
other.
de-
Here,
in
contrast
to
the t w o
C6H5CN+
sible b e c a u s e of the lack of suitable c h a r g e e x c h a n g e
c o m p o s i t i o n s discussed in the p r e c e d i n g
partners.
we
did
measure
separately
the
rate
paragraph,
constants
for
the t w o
for
decomposition
them
paths.
the f u n c t i o n s
Figure
8
shows
are completely
k(E)
that
differ-
excitation was used;
tor of
e n t . It is t o b e e m p h a s i z e d that at e a c h R E t h e
two
P{E)
values of
k were obtained
For
pact
example,
with
main
peak
main
peak,
/c 7 8 + _> 5 2 + ,
peak
Ar+
was
primary
identical
in
indicating
while
at
distortion
the
suppressed
ion
an
the
to
shape
mass
52
the m a s s
78
unmeasurably
mass 77
and
leading to k - ^ - *
reactions,
as
quite independently.
a
= 1.56-10° sec-1. For
fast
any
78+ - > 52+
slow
mass
process
77
Thus
"point"
competing
would
have
formation.
Thus,
F i g . 8 d o e s n o t s i m p l y m e a n that a fast a n d a
decomposition
of
C0H6+
course,
be
exist
side
entirely
by
side
compatible
slow
(which
would,
of
QET),
b u t gives p r o o f , b e c a u s e of the principle
with
o u r m e a s u r e m e n t , that these p r o c e s s e s are
17.6 eV
the
of
ed.
an
of
cesses
seems
to
be true f o r
C 0 I I e + —> C 3 H 3 + + C 3 H 3
CGH,J +
CÖH
fortunately
above).
4+
+ Ho
could
Thus,
breakage
drogen
of
[slow
not
it
be
seems
originate
[steep
that
7 is
extrapolated
for decay
from
and
accurately
un(see
involving
those involving
different
types
of
hyex-
(«^10
_ 6
sec)
is
available
31.
for C
supports
this f u n c t i o n :
usual
behavior
ciations
can
modified
of k (E)
CGH6+
for
—>-
C6H5+
though
H
of
uncertainties of
t h e k(E)
from
the c o m p l i c a t i o n s
Kr+,
cannot
sion.
Furthermore,
the
simplest
forbidden
Its
possibly
from
k(E)
1 in
not
is
of
obtain-
slow
rise
3 eV
k(E)
all C 6 H 6 +
explained
by
probably
between
un-
dissothe
in
un-
terms
electronic
states,
problem.
LONSSON a n d L I N D H O L M h a v e r e c e n t l y
the
breakdown
exchange
35.
with
a
the t w o
invalidate
there
is
for
the
77+,
RE
+ H
comparison
with
32)
resulting
values
above
additional
diagram
From
of
it t h e y
the
of
conclu-
support
for
electron
(see, e. g .
large satellite:main peak intensity
ra-
of
in
will p r o d u c e
k(E)
upper
77+
part
onset
of
the
first
C6H5+ — C
potential
of
and
H
C4H3+
t h e d i s t r i b u t i o n P(E)
electrons
4
7.
The
lower
the satellite p e a k ,
contributes
predominantly
peak. The extrapolation
presumably
Figure
has to b e
secondary
3 +
is
it i s
a g r e e s w e l l w i t h o u r k(E)
in
tion,
agreement
with
1 3 . 8 ± 0.2 eV36.
pearance
potential
14.5 + 0.2 e V 3 3 .
by
a
For
better
KRAUSS
to
The
17.6 ± 0.1 eV
C6H5+
f o r P(E)
for
appear33.
formation
the present
charge
and
somewhat
cited
appearance
from
C6H6+
without
In
the
as
this
dated
by
k(E)
curves.
lowest
Lindholm
the
1963,
of
find
similar
unaltered
by
for
and
For
has
of
of
the
now
the
the
formed
dissociation
ironically,
virtue
might
between
various fragments
onsets
However,
value,
ROSENSTOCK
competition.
is
about
extrapolation.
differences
argument
ap-
impact
(13.8 + 0.2 e V ) ,
evidence
effective
C6H-+/C4H4+
the
electron
uncertain
large
potentials
the
Also,
has
eV
to
b e k n o w n . F o r lack of electron i m p a c t data, a r o u g h
estimate
for
photoioniza-
it a p p e a r s t h a t a n e v e n l o w e r
still37.
38
in F i g . 7 ;
from
C4H4+
reported
Jonsson
a
value
paths
case
been
of
invali-
corresponding
the
conclusion
the different
slopes
G) Kinetic energy transfer
terminated
o f the states e x c i t e d b y 7 0
leading
po-
while
to
dicomposition,
+ C2H2 .
charge
appearance
13.8 eV,
also
of these v e r y same curves.
for
k{E)
obtained
measured
with
one,
polation
of
benzene
tentials f o r C G H 3 + a n d C 4 H 4 + ions. T h e f o r m e r
remains
ance
be
transitions
tios w e r e f o u n d . T h e y can be predicted b y an extra-
the
of
uncertain
+ H
5 +
of
elucidation,
a b o u t t h e s a m e as f o r C 6 H 5 +
impact experiments. In those experiments
at
rather
H
6
an ^-interval
remains an interesting
be
main
C
6 +
the extraordinarily
certainly
F r o m o u r k(E)
with electron impact metastables
part
at the " c u t o f f " e n e r g y
- 1
6
Over
of
QET.
14.0 eV,
E) Comparison
the
im-
increases b y only one order of magnitude. This
pro-
which
processes
and
of
k(E)]
rise],
measured
a C — C bond
loss
the p a i r
k(E)
n o t p o s s i b l e w i t h i n the t i m e
Fig.
in Fig.
non-com-
c i t e d states, b e t w e e n w h i c h e n e r g y r a n d o m i z a t i o n
this
electron
F) Appearance potentials
same
The
this
with
additional,
k(E)
peting.
The
17 e V 2 6 . W i t h
34.
It a g a i n
of
and
measured
i f k(E)
such that k = 3 • 1 0 6 s e c
main
observed,
14
ratio
is o b t a i n e d ,
seems to d r o p b y a fac-
P(E)
2 between
the S P / M P
large
considerable
a satellite p e a k w e r e
about
exchange
A
fundamental
processes
X
+
problem
+ A —> A
+
+ X
with
charge
exchange
is the q u e s t i o n
of
how
m u c h k i n e t i c e n e r g y is t r a n s f e r r e d f r o m the X
+
into
energy
excitation
transfer
is
energy
negligible
of
A+.
has
the
Only
if
this
excitation
of A + the well-defined value
£I = R E ( X
+
)
—LP.(A)
+£
T H
(A)
ions
energy
E\
(cf.
abscissa
of
Fig.
5).
Lindholm's
experiments
w e r e d e s i g n e d s o as t o c i r c u m v e n t t h i s p r o b l e m
By
extracting
the
ions
pendicular geometry")
A+
at r i g h t
to the X +
i o n s A + s h o u l d b e observed
angles
6
:
("per-
beam, only
those
which are f o r m e d
with
tailed k n o w l e d g e of the d e c o m p o s i t i o n
struments
token,
kinetic
cannot
make
any
state-
m e n t a b o u t the extent to w h i c h k i n e t i c e n e r g y
can
be transformed into excitation energy.
In
fact
it is k n o w n
from
have parallel
paths
experiments
(like
that t h e k i n e t i c e n e r g y o f X
electronic
thermic
energy
process
the r e a c t i o n
by
of
A+.
with
the
An
GUSTAFSSON
and
the p e r p e n d i c u l a r
discriminated
Lindholm's
+ Kr
LINDHOLM
patterns,
of
to
in
For
with
these
fulfilled
against
the
which
are
all
based
in
spite
processes
ionic
on
by
the
charge
method.
for
like
E\ m a y
of
incomplete
with
momentum
s o that g l a n c i n g
Accurate
measurements
of
be
mo-
kinetic
which
energy.
of
kinetic
desir-
geometry.
the p e r p e n d i c u l a r
of
s o m e kinetic
reactions
the
energy
with
in
based
a
on
straightfordward
in
Sect.
dis-
transfer
complex
mole-
The
t o m a i n p e a k r a t i o , as a f u n c t i o n o f
transfer,
it w o u l d
affect both
The
the
results
first
presented
in
content
of
an i o n
is c h a n g e d b y s o m e a m o u n t <5E, t h e b r a n c h i n g
2 +
extremely
difficult.
as a f u n c t i o n
It w o u l d
require
A+
ratio
, . . . , i. e . t h e
mass spectrum, will also change. H o w e v e r , a
dE
data
are
of
not
quan-
U-wn
only
is
de-
of
ki-
Sect. V I
CI
U-Wn .
Should
momentum
similarly
11
appear
of internal excitation energy with a large
and
to
be
increase
variation
i n r e l a t i v e t r a n s l a t i o n a l e n e r g y is c o n s i s t e n t w i t h t h e
picture
of
near-resonant,
large
impact
parameter
collisions.
This
finding
at t h e s a m e t i m e s u p p o r t s t h e v a l u e
of the r e c o m b i n a t i o n
would
energies R E
assumed
1 ) , which were derived b y
close
data. A s
collisions
break
down,
and
in
out6,
selection
different R E ' s
this
Lindholm
he has pointed
the optical
rules
would
ob-
tain.
GURNEE a n d
MAGEE h a v e treated
near-resonant
charge exchange processes semi-quantitatively
calculation,
they
compute
nant charge
resonance,
Their
£= 0
using
the i m p a c t
the
probability
43.
parameter
for
as
result
a function
of
the
energy
(i. e. e x a c t r e s o n a n c e ) , w h o s e w i d t h
43,
e.
around
increases
Fig. 9 ) .
p a r a m e t e r e n t e r i n g i n t o t h e w i d t h is t h e
exact
deficit
is a G a u s s f u n c t i o n , c e n t e r e d
with the relative velocity v (see
In
me-
near-reso-
transfer, relative to the c a s e o f
Another
screening
f a c t o r o f the Slater t y p e w a v e f u n c t i o n s u s e d , which
was chosen rather arbitrarily.
In o u r
experiment, f o r K r + + C 6 H 5 C N ,
2 . 5 - I O 6 c m / s e c . A c c o r d i n g to
kinetic
+
peaks
Fig.
energy
of
our
data o f this k i n d . T h e v e r y small
e x c h a n g e m a s s spectra with the X
titative evaluation
of
w o u l d c a n c e l out in their intensity ratio.
tive
o f its d e c o m p o s i t i o n p r o d u c t s B j + , B
I V c,
results of
charge
If the e n e r g y
(see
manner.
true f o r the present p r o b l e m
transfer:
cules m a y b e inferred f r o m the variation of
eUion35'41'42-
in-
spectra,
the e v a l u a t i o n
there b e s o m e i o n los due to transverse
thod,
of
occurrence
mass
are d e r i v e d f r o m m e a s u r e m e n t s of the satellite p e a k
exchange
section40.
the extent
is p a r t i c u l a r l y
their
able in o r d e r to assess the i m p o r t a n c e
The
dissimilar
O u r m e a s u r e m e n t s o f dE
shown
ours.
crimination
even f o r exothermic
was
netic energy
some
to excitation energy transfer are thus highly
effect of
as
spectroscopic
Exo-
collisions will p r e d o m i n a t e
and
effects, but
in v e r y
transfer.
a large cross
t r a n s f e r v e r y little m o m e n t u m
6.
from
discrimination
e x c i t e d level of A + . N e a r - r e s o n a n t c h a r g e
with
quite
is a l s o
were
well
lecule A will usually b e n e a r l y r e s o n a n t with
to o c c u r
Also,
CI)
(Table
thermic charge exchange of X + with a c o m p l e x
is e x p e c t e d
VI,
work
investigations
reactions,
relation
of
success
of
excitation
exothermic
case
cannot
the general
Lindholm's
above
with
detection
exchange, lends strong s u p p o r t to his
concerned
found
39
measurements
monoenergetic
of
Because
occur
against
Nevertheless,
assumption
endo-
been
employed
effectively
breakdown
10.
has
39
into
is t h e
a r g u e s that in this
numerous
The majority
instrument)
example
2 +
geometry
the H o + p r o d u c t .
of
our
K r + + D o —> D . , + + K r
Kr+ + H2 — H
large cross section, Maier
have
in
A+
can be converted
+
give
insensitive to transfer o f transverse m o m e n t u m . T h i s
"longitudinal" type of apparatus, where X + and
ions
can
d e p e n d i n g o n U;on
Sect.
Lindholm
but
paratus. In fact, longitudinal and perpendicular
little o r n o t r a n s f e r o f k i n e t i c e n e r g y . B y t h e s a m e
though,
kinetics
also o f the d i s c r i m i n a t i o n characteristics o f the ap-
city
velocities
only
range
C6H5CN+
from
states
43,
relato
at o u r l o w e s t v e l o -
in very
close
with R E ( K r + )
could be excited
eV),
f o r the highest velocity
whereas
the
v = 6.5 • 105 cm/sec
resonance
(within about
states
u p to a b o u t 0 . 1 5 e V off resonance w o u l d b e
0.02
lying
acces-
sible. T h e sign of e d o e s not enter, so that the excited energy band widens up symmetrically
around
e x a c t r e s o n a n c e a s t h e i m p a c t v e l o c i t y is i n c r e a s e d .
2.2 ± 2 0 % .
However,
("P1/2)
f o r l a r g e m o l e c u l e s the density of
increases very rapidly with energy
net
result
will
excitation
be
energy
an
of
upward
44-46
shift
CcH5CN+.
We
states
, s o that t h e
of
the
mean
believe,
there-
The
intensity
ratio
is p r o b a b l y 2 : 1
similar cross
(see
of
6
: Kr+
Kr+(2P3/2)
) , which indicates a
section f o r the t w o species in
agree-
ment with Fig. 12.
1) Double ionization by charge exchange
f o r e , that o u r r e s u l t s s h o w n i n F i g . 1 1 c a n , as f a r
as t h e o r d e r o f m a g n i t u d e is c o n c e r n e d , b e e x p l a i n e d b y the t h e o r y of
tative c o m p a r i s o n
Gurnee and Magee. A
would
require
an adaptation
of
t h e i r c a l c u l a t i o n t o o u r s p e c i f i c c a s e s as w e l l as
calculation
of
the
density
distribution
A s a p r o d u c t of charge exchange between H +
quanti-
of
a
excited
states o f t h e o r g a n i c i o n s a r o u n d t h e R E v a l u e s
in
benzonitrile
the
ion
C6H5CN++
C6H5CN+.
With
Ne+,
no
C6H5CN++
ed
In
Figs.
12
and
13
we
find
a decrease
transfer processes be a factor of
benzene,
toluene
and 2 4 . 5 8 e V . F o r
and
rather l o w values 2 6 . 0 ;
of
cross section f o r our particular dissociative
the
tively
charge
3 in g o i n g
from
naphthalene
T h i s a p p e a r s t o b e t h e first k n o w n e x a m p l e o f a
charge
exchange
process
X+ + A
A++ + X + e .
tion
calculated
by
of
over
the
energy
same
crease b y about
for
diatomic
ments
1/3
interval,
for atomic
systems.
The
MAGEE43,
amounts
which,
to
a
de-
and a factor of
corresponding
the
type
X++ + A — A
example, in R e f .
(see
The
direct
present
way
of
experiment
measuring
the
unimolecular
sections f o r charge exchange of A r + , K r +
the
excitation
with
C6H5CN
despite
the
fall
off
differing
in
a
very
masses
of
similar
the
CO+
manner,
reactant
ions.
Thus, charge exchange with aromatic molecules
processes
reported,
for
a novel
and
34.
I V , c ) . T h i s is s u p p o r t e d b y t h e f a c t that t h e c r o s s
and
as
VIII. Conclusion
we
b e l i e v e it u n l i k e l y t h a t o u r s t e e p d e c r e a s e is a n a r t i fact caused b y transverse m o m e n t u m transfer
+ X,
+ +
2
experi-
also s h o w this s l o w e r fall-off. H o w e v e r ,
40
the
respec-
46.
M u c h l e s s s u r p r i s i n g is t h e o c c u r r e n c e o f
and
comioniz-
all h a v e
24.5 and 22.8 eV,
a b o u t 5 t o 1 5 0 V c . m . . T h i s is m o r e t h a n t h e v a r i a GURNEE
is
f o u n d , so that the a p p e a r a n c e potential o f C 6 H 5 C N + +
p a r i s o n , the a p p e a r a n c e potentials o f d o u b l y
H) Charge exchange cross section
was
o b s e r v e d . It is a t t r i b u t e d to a u t o i o n i z a t i o n o f an intermediary
m u s t lie b e t w e e n 2 1 . 6 e V
question.
and
= 51.5)
(mje
re-
primary
tection
constitutes
the
dependence
decomposition
energy.
rate
Conceivably,
of
k(E)
constant
by
employing
mass selection and m o r e sophisticated
techniques
like
signal
on
averaging,
this
deme-
t h o d c o u l d b e a p p l i e d to a m u c h l a r g e r variety
of
ally s e e m s to d e p e n d m o r e s t r o n g l y o n the relative
substances. T h i s w o u l d o p e n u p the m o s t direct ex-
velocity
perimental
than with smaller
systems. A
rough
mea-
s u r e o f t h e r e l a t i v e c r o s s s e c t i o n s , at a f i x e d i m p a c t
energy,
of
t h e IJ. (k)
various
ions X
distributions
can be
+
in F i g . 4
obtained
by
beam.
heights
at R E = 1 4 . 0
1
2
3
For
e x a m p l e the ratio
and
RE = 14.7
of
in
the
Fig.
Acknowledgments
the
in
peak
4
is
W e are i n d e b t e d to the D e u t s c h e F o r s c h u n g s g e m e i n schaft f o r financial s u p p o r t in b u i l d i n g the a p p a r a t u s
and to P r o f . O . OSBERGHAUS f o r his interest and stimulating discussions.
For a recent review, see H. M. ROSENSTOCK, Adv. Mass
Spectrom. 4, 523 [ 1 9 6 8 ] ; also: C. E. KLOTS, J. Phys.
Chem. 75, 1526 [ 1 9 7 1 ] .
7
O . OSBERGHAUS
8
and
CH. OTTINGER,
Phys. Lett. 16,
121
CH. OTTINGER,
Z . N a t u r f o r s c h . 2 2 a,
A
brief account of the present work has been
[1971],
F o r a r e v i e w , s e e : B . S . R A B I N O V I T C H a n d M . C . FLOWERS,
i n : Quarterly Reviews 18 ( 2 ) , 122 [ 1 9 6 4 ] . For N 0 2 , see
also:
H . GAEDTKE
U.
I. HERTEL
40
5
I . HERTEL
[1967],
see U . LÖHLE and
6
A review has been given by E. LINDHOLM, i n : Ion-Molecule
Reactions in the Gas Phase, Adv. in Chemistry Ser. No.
58, (Amer. Chem. Soc., Washington 1966), p. 1.
and Ion Physics 5, 265
[1967],
9
CH. OTTINGER,
J. TROE,
Z. Naturforsch. 2 5 a ,
789
[1970].
4
and
given:
B . ANDLAUER a n d C H . OTTINGER, J. C h e m . P h y s . 5 5 , 1 4 7 1
[1965].
CH. OTTINGER, Z. Naturforsch. 22 a, 20 [1967].
and
ki-
from
dividing
relative peak heights b y the relative a b u n d a n c e s
the i o n
a p p r o a c h to unrevel the dissociation
netics of c o m p l e x systems.
Z . N a t u r f o r s c h . 2 2 a, 1 1 4 1
10
In contrast, some very weak metastable peaks in acetylene
double their intensity at a pressure of only 2 - 1 0 — 7 Torr,
CH. OTTINGER, Int. J. M a s s
[1970].
W. B. MAIER II, J. Chem. Phys. 4 2 , 1 7 9 0 [ 1 9 6 5 ] .
Spectrom.
11
12
13
14
R . F . STEBBINGS, B . R . T U R N E R , a n d J . A .
E . E . MUSCHLITZ, S c i e n c e 1 5 9 , 5 9 9
15
16
17
18
19
20
RUTHERFORD,
J. Geophys. Res. 71, 771 [ 1 9 6 6 ] .
Unfortunately, symmetric charge exchange processes, e. g.
A r + - f A r — A r + A r + , could not be used for this calibration,
since the secondary ions would have been difficult to distinguish from those primary ions which originate in regions
in the ion source with a potential close to the molecular
beam potential (see Fig. 2 ) .
Table 4 contains a few examples of ions suspected to originate from Penning ionization. As a test, in that case all
primary ions were blocked of! by changing the ion source
potentials appropriately. Although this did suppress the
secondary ions in question, it is not a proof against Penning processes taking place, because changing the ion
source conditions might also have reduced the yield of excited neutrals.
See E. W . MCDANIEL, Collision Phenomena in Ionized
Gases, J. Wiley, New York 1964, Table I V , p. 732 ff.;
[1968];
V.
22
23
24
27
28
29
30
31
B . WALLENSTEIN a n d M . KRAUSS, J . C h e m . P h y s .
32
33
34
J. L . FRANKLIN, J. G . DILLARD, H . M . ROSENSTOCK, J.
C . LIFSHITZ
and
B . G . REUBEN,
J. C h e m . P h y s . 50,
951
[1969].
Similar considerations concerning the influence of the k(E)
cutoff position on the ratio metastable: normal fragment
ions were already presented in Refs. 4 and 5 .
35
B . - Ö . JONSSON a n d E . LINDHOLM, A r k . F y s i k 3 9 , 6 5
B. BREHM, Z. Naturforsch. 2 1 a, 196 [ 1 9 6 6 ] .
T h e possible objection that C 4 H 4 + might be formed by
endothermic charge exchange with K r + ( 2 P 3 / 2 ) , R E = 14.0
eV, was disproved by measuring the relative yields I (14.0)
and 1(14.7) at R E = 14.0 and 14.7 eV, respectively, as a
function of the Kr + translational energy e Uion • Between
E W = 2 0 and 250 V the ratio I ( 1 4 . 0 ) / I (14.7) increased
by only 5%.
37
34,
T.
Potentials,
Appearance Potentials, and Heats of Formation of Gaseous
Positive Ions, NSRDS-NBS 26, U.S. Govt. Printing Office,
Washington 1969.
In R e f . 4 , 15.2 eV is given as A . P . (C 6 H 4 + ) ; this refers to
" n o r m a l " fragments C 6 H 4 + ; according to 4 the A . P . for
C 6 H 4 + as a "metastable" is lower by 1.3 eV.
T h e main peak could not be used for this experiment because it was largely due to charge exchange with X e + + .
Furthermore, the large excess of parent ion in the X e + +
C 6 H 5 C N mass spectrum caused a contribution to the satellite peak on mass 76 from collision-induced dissociation,
which had to be subtracted.
Of course, for an endothermic process the ion yield must
eventually tend to zero with decreasing impact energy. For
the very small endothermicity in the present case this portion is below the accessible energy range.
A measurement of S P / M P at a higher value of the drawoutfield F would obviate these assumptions by providing additional experimental information, but is not feasible because of the broadening of both peaks with increasing F.
In addition to these factors, the relative contributions from
R E = 14.0 and 14.7 eV were multiplied by the abundance
ratio K r + ( 2 P 3 / 2 ) / K r + ( 2 P 1 / 2 ) = 2 : 1 (see Ref. 8 and Table 1).
B. ANDLAUER, Diplomarbeit, Freiburg 1970.
For charge exchange with CO + , the mass 76 main peak intensity varied with C/i0n almost exactly as for the Kr +
charge exchange (the same is true for the satellite peak intensities, respectively). For CO + no reduction to obtain
Q(U'wd) was attempted because of the complications by
the comparatively minor contributions from R E 16.8 eV.
It is very likely, however, that the relative cross section for
mass 76 + formation by CO + impact approximately follows
curve (c) ( R E = 1 4 . 0 e V ) .
F. H. MIES and M . KRAUSS, J. Chem. Phys. 45, 4455 [1966],
D. L. BUNKER, J. Chem. Phys. 4 0 , 1 9 4 6 [ 1 9 6 4 ] .
See Eq. (1) in 4 ; the additive term —21.7 merely results
from the uncorrected Ue\ scale used in that paper.
Similar, but much less direct evidence for incomplete energy randomization can be found i n : U. LÖHLE and CH.
OTTINGER, Int. J. Mass Spectrom. and Ion Physics 5, 265
[1970].
CH. OTTINGER, Z. Naturforsch. 2 0 a, 1229 [ 1 9 6 5 ] .
36
929 [ 1 9 6 1 ] .
W . A . CHUPKA, J. Chem. Phys. 3 0 , 1 9 1 [ 1 9 5 9 ] .
H E R R O N , K . D R A X L , a n d F . H . FIELD, I o n i z a t i o n
21
26
CERMÄK,
J. Chem. Phys. 44, 1318 [ 1 9 6 5 ] . Other long-lived states
with excitation energies close to the I. P. are also unlikely
to contribute (H. HOTOP, private communication).
This is, for example, very important for the reaction
Kr + + C 6 H 5 CN, where a small impurity of air in the krypton would lead to a spuriously small satellite: main peak
intensity ratio (see Figs. 4 and 5 ) .
E. STIEFEL, Einführung in die numerische Mathematik,
B. G. Teubner, Stuttgart 1961, p. 52.
Catalog of Mass Spectral Data, American Petroleum Institute, Research Project No. 44, Carnegie Institute of Technology, Pittsburgh (Pa.) 1953.
M.
25
38
H . M . ROSENSTOCK a n d M . KRAUSS, A d v . M a s s
[1969].
Spectrom.
2 , 2 5 1 [1963].
39
E . GUSTAFSSON
40
[1960].
E. W . MCDANIEL, Collision Phenomena in Ionized Gases,
J. Wiley, New Y o r k 1964.
41
R.
42
43
44
45
and
E . LINDHOLM,
Ark.
Fysik
S . LEHRLE, J. C . ROBB, J. SCARBOROUGH,
18,
and
219
D.
W.
THOMAS, Adv. Mass Spectrom. 4, 687 [ 1 9 6 8 ] .
V . L. TALROSE, Pure and Appl. Chem. 5, 455 [ 1 9 6 2 ] .
E . F . GURNEE a n d J. L . MAGEE, J. C h e m . P h y s . 2 6 ,
1237
[1957].
P. C. HAARHOFF, M o l . Phys. 7 , 1 0 1 [ 1 9 6 3 ] .
G . Z . WHITTEN a n d B . S. RABINOVITCH, J. C h e m . P h y s .
38,
2466 [ 1 9 6 3 ] .
46
M . VESTAL, A . L . WAHRHAFTIG, a n d W . H . JOHNSTON, J.
47
F . H . D O R M A N U. J . D . M O R R I S O N , C a n a d . J . P h y s . 3 5 , 5 7 5
Chem. Phys. 37, 1276 [ 1 9 6 2 ] .
[1961]. *