Ultralow Temperature Chemical Kinetics of OH Radical Reactions of

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Ultralow Temperature
Chemical Kinetics of OH
Radical Reactions of Interest
in Elena
theJiménez1,
Interstellar
Medium
Bernabé Ballesteros1, André
Canosa2, Thomas
M. Townsend1, Beatriz
(ISM)
Cabañas1, and José Albaladejo1
1 Department of Physical Chemistry. University of
Castilla-La Mancha (UCLM), Ciudad Real, SPAIN
2 Institut de Physique de Rennes. Laboratory of
Experimental Astrophysics. CNRS-University of
Rennes 1, FRANCE
Outli
ne Introduction
Ultralow Temperature Chemical Kinetics of OH Radical Reactions of Interest in the Interstellar Medium (ISM)
1.
Introduction
1.
Introductio
Molecules in the Interstellar Medium
n(ISM)
 Densest star-forming regions of the ISM (∼
Molecular Cloud
104 particle cm-3) are very cold (10-100 K).
C8
H
 Besides the most abundant gas in the ISM
(H2), many complex molecules, such as
polyynes, dimethyl ether, PAHs and
fullerenes have been identified during
the past decades.
Accretio
n
Disk Come
t
MeteoritePlane
s
t
(Earth
)
Life-cycle of a star
 Since 1937, when methylidyne radical (CH) was first detected in the ISM
(Swing and Rosenfeld, Astropysical Journal, 86 (1937) 483), more than 170
molecules have been identified.
 Hydroxyl radicals (OH) was first detected in the ISM by N. H. Dieter, H. I.
Ewen. (Nature. 201 (1964) 279).
 Hydrogen peroxide (H2O2) have recently been detected by P. Bergman et al.
(Astronomy & Astrophysics, 531, L8 (2011).
 Methoxyl radicals (CH3O) identified by J. Cernicharo et al. (Astropysical
Journal Letters, 759, L43 (2012)).
http://www.astrochymist.org/astrochymist_ism.ht
ml
http://www.astro.uni-koeln.de/cdms/molecules
4
1.
Processes in Interstellar Clouds: Synergy between
Introductio
Laboratory and Models
nUnderstanding how Interstellar Molecules are created in the
observed
abundances involves:
1) The construction of MODELS, which include large networks of
elementary chemical and some physical processes in the grain surface
and in the gas-phase.
2) The experimental and theoretical efforts to obtain the information
about these processes which is required as input to the models.
GAS-PHASE REACTIONS
AB + CD 
Products
A. Canosa. Russian Chemical Reviews 76
(2007) 1093
5
1.
Introductio
Bimolecular Rate Coefficients: Low-T Extrapolation
n For many years, the field of gas-phase reaction kinetics involving
neutral
partners has been restricted to experimental studies which were carried out
at temperatures, basically 298 K and above 200 K.
k(T) = A(T) exp(-Ea/RT)
Arrhenius expression


k(T) = α
(T/300 K)-β
Three-parameter expression
Extrapolation of k from the
T-expression
at
ultralow
temperatures can have a great
impact on the rate coefficients.
The reaction of OH radicals with
methanol (CH3OH) is an example:
From experiments at T = 210-1000 K
OH + CH3OH  H2O + CH2OH
 H2O + CH3O

exp(-γ /T)
Overall k decreases when the
temperature decreases and channel
producing CH2OH accounts for
89–96% of the overall products.
1.
Introductio
n The reaction of OH radicals with methanol (CH3OH) is a good example:
 H2O + CH3O
From extrapolation
Estimation
@
100
K:
KIDA
k(T) = α
(T/300 K)-β
http://kida.obs.u-bordeaux1.fr
CH2OH Formation
k ∼ 8.2 x 10-14 cm3
molecule-1 s-1
37
%
CH3O Formation
k ∼ 1.4 x 10-13 cm3
molecule-1 s-1
63
%
k(100K) ∼ 2.2 x 10-13 cm3
molecule-1 s-1
exp(-γ /T)
1.
Introductio
n The reaction of OH radicals with methanol (CH3OH) is a good example:
 H2O + CH3O
From experiments @ 63 and 80 K (Dwayne Heard’s group. University of
Leeds (UK)
The measured overall rate
coefficient was:
k(63
11 cm3
cm3
k(80 K)
K) ∼∼ 4
3x
x 1010-11
molecule-1
s-1
molecule-1
s-1
KIDA estimates k(100K) ∼ 2.2 x 10-13 cm3
molecule-1 s-1
CH3O was detected at 80 K and a
Master equation calculation predicts
a branching ratio of 0.99 at 70 K for
this channel.
ULTRALOW TEMPERATURE
KINETIC (AND PRODUCT)
STUDIES ARE EXTREMELY
IMPORTANT
R. J. Shannon, PhD thesis, University of Leeds (2012).
ALMA
Hersch
el
2. Objectives
within
ASTROMOL
Project
2.
Molecular Astrophysics: The Herschel and ALMA
Objectives
Era (2009-2014)
Coordination: Prof. José Cernicharo
Centro de Astrobiología. INTA-CSIC
IP UCLM: Prof. Beatriz Cabañas
Departamento de Química Física
Project CSD2009-00038
Chemical Physical Laboratory Measurements
•
Study of gas-phase reactions of OH radicals, O and N
atoms with small hydrocarbons and PAHs of interest for
the interstellar medium at T of high mass star-forming
regions.
•
Photochemistry of small hydrocarbons
Implementation of New Techniques
•
Pulsed Supersonic Uniform Gas Expansion to get Ultralow Temperatures
CRESU (Reaction kinetics in Supersonic Uniform Flow) Technique
10
2.
1.
Introductio
Objectives
n
B. Ballesteros
Bertrand Rowe
(AEROCHOP)
September 2010
(Photo taken by André Canosa)
11
2.
1.
Introductio
Objectives
n
Pipes/Pumps
Electronic Control
(5000 m3/h)
He
Cylinders
Vacuum
Chamber/ Laval Nozzle
Excitation
Laser
Photolysis
Laser
September 2012
12
Reservoir
Laval nozzle
rotary disk
3. Implementation
of the Pulsed
Supersonic Gas
Expansion System
3. Laval Nozzle
System
Fundamentals: Pulsed Gas
Expansion
P0 , T0
High
Pressure
(337 mbar)
Rotary Disk
P1 ,
Tamb
P1 , T1
Detection zone
Laval Nozzle
Low
Pressure
(0.62 mbar)
The gas expansion from a
high-P region through a Laval
nozzle to a low-P zone generates
a
cold
supersonic
jet
(T1<<T0).
The aerodynamic chopper
or rotary disk has two
symmetrical apertures which
allows
the
pulsed
gas
expansion through the nozzle.
Rotary
disk
Two apertures
Reservoir +
600 rpm ≡ 10 Hz
Rotary Disk +
Ultralow Temperature
Kinetics of OH Radical Reactions of Interest in the Interstellar Medium (ISM)
Laval Chemical
Nozzle
3. Laval Nozzle
System
Fundamentals: Pulsed Gas
Expansion
P0 , T0
High
Pressure
(337 mbar)
Rotary Disk
P1 ,
Tamb
P1 , T1
Detection zone
Pi
Laval Nozzle
Low
Pressure
(0.62 mbar)
The impact pressure (Pi) was measured
by a Pitot tube at a certain distance
from the nozzle.
Disk
Close
d
Disk
Open
(0.1 s =
10 Hz)
Disk
Close
d
Shock wave
Pitot Tube
15
3. Laval Nozzle
System
Characterization of the flow: Determination of the
Mach number
P1 ,
Tamb
P0 , T0
High
Pressure
(337 mbar)
Detection zone
Low
Pressure
(0.62 mbar)
A supersonic flow is characterized by a Mach number, M,
(defined as the ratio of flow velocity, u, to the speed of sound,
c) greater than 1.
Knowing P0 and measuring the impact pressure (Pi) at
several distances from the Laval nozzle, M
can be
calculated from:
γ
1
 γ −1
Pi  ( γ + 1) M 2  γ + 1 
γ +1

=
2
2


P0  ( γ − 1) M + 2 
2
γ
M
+
γ
+
1




γ, adiabatic expansion coefficient (= 1.67
for Helium)
P1 , T1
16
u
M =
c
M = 6.04
3. Laval Nozzle
System
Characterization of the Temperature of the Jet
Once the Mach number is calculated, the temperature of the jet (T1) can be
determined as follows:

T0
γ-1 2
=1+
M
T1
2
Pulsed Flow
T1 = (22.4 ± 1.4) K


M = 6.04
T0 = 298 K
γ (He) = 1.67
Continous Flow
T1 = 23.5 K
Advantages of the Pulsed CRESU
 Reduction of Gas Flow ⇒ Lower pumping speed
⇒ Lower Gas Consumption
 Increase of the hydrodynamic time ⇒ Measurement of lower rate coefficients
17
4. Application to
Kinetics of OH
Radicals
Pulsed Laser Photolysis/ Laser
Induced Fluorescence Technique
4. Gas-phase Kinetics at Very Low
Temperatures
Pulsed Laser Photolysis: Source of OH radicals
H2O2(gas) is introduced into the chamber by bubbling He through an
aqueous solution at a constant flow rate.
OH
radicals
are
produced
by
UV
photolysis at 248 nm.
λp
h
24 oto
8 n lys
is
m
Temperatures
Laser Induced Fluorescence: Detection of OH
radicals
OH radicals produced in the photolysis of H2O2 are excited at 282 nm
(frecuency doubled from a dye laser) and the subsequent fluorescence
emission is detected by a photomultiplier tube (PMT).
 Laser Excitation:
OH(X2Π) + hνexcit  OH(A2Σ+)
 Laser Induced
Fluorescence:
OH(A2Σ+)  OH(X2Π) + hνLIF
PMT
Filter
PCX
Lenses
HR Mirror
λ
LIF=308-310
20nm
Temperatures
First test: Detection of OH radical at 298 K
In the absence of reactant, the decay of the LIF signal from OH
radicals is due to the reaction with the photochemical precursor
H2O2:
OH + H2O2  H2O +
OH  other
losses
HO2
As the rate coefficient for this reaction is well-known at room
temperature (1.7×10-12 cm3 molecule-1 s-1), we performed a test
experiment using a continuous flow without gas expansion.
The experimental conditions were:
 Preservoir = Pchamber = 74
Torr
 T = 298 K
 Flow He = 11 sLpm
 Flow He/H2O2 = 500 sccm
Química
21
de Radicales OH
Current and Future
Work
Validation
22
ACKNOWLEDMENT
ACKOWLEDGMENTS
S
THANK YOU FOR YOUR ATTENTION

Ultralow Temperature Chemical Kinetics of OH Radical Reactions of

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