Elevated Pressure Collision-Free Assumption Summary Apparent

Investigation of the Collision-Free Assumption for
Hybrid fs/ps CARS
Joseph D.
1
Miller,
James R.
1Department
Chloe E.
3
Gord, and
4Graduate
Terrence R.
2
Roy,
1,2,4
Meyer
of Mechanical Engineering, Iowa State University, Ames, IA
2Spectral
3Air
1
Dedic, Sukesh
Energies LLC, Dayton, OH
Force Research Laboratory, Wright-Patterson Air Force Base, OH
School in Advanced Optical Technologies (SAOT), Friedrich-Alexander University, Erlangen-Nürnberg, Germany
Summary
Elevated Pressure
The accuracy of CARS temperature measurements are highly dependent
on the ability to :
Elevated pressure results in an increase in the rate of decay of the
CARS signal due to increased collisions between molecules.
 Measure the spectral and temporal phase and intensity of each laser
pulse
CARS intensity as a
function of pressure for
N2-N2 (top) and O2-O2
(bottom) environments.
The nonresonant response
(NR) is measured in
Argon.
 Determine the nonresonant contribution of the Raman polarization
 Model the effects of collisional energy transfer on the CARS spectral
lineshape and temporal decay
In hybrid fs/ps CARS, 0.1-10 ps transform-limited pulses are used to
suppress nonresonant background by 1000× while maintaining spectral
resolution for frequency domain detection. In this work we investigate the
influence of collisional energy transfer on the transition-dependent decay
rate and accuracy of rotational O2 and N2 CARS thermometry. Up to a
pressure of 20 atm, collisional effects can be neglected with less than
5% temperature error at a probe delay of 6.5 ps for both N2 and O2.
The open symbols are
experimental data points.
The solid lines are
simulations of the CARS
intensity using the MEG
model for transition
linewidths.
Apparent RCARS Temperature Shift
13.5 ps
300 ps
Each transition with
initial state, J, in the
RCARS spectra decays
with a rate constant, τJ,
which is a function of
temperature and pressure
due to collisions.
Since low J levels decay
more quickly, then timedelayed detection of
CARS spectra will exhibit
an apparent shift to higher
J levels and over-predict
temperature. This bias
becomes less important at
high temperature.
The decay of the integrated CARS signal is highly dependent on
pressure. As pressure is increased, the decay approaches the NR signal.
Collision-Free Assumption
The RCARS model accuracy and its sensitivity to time-varying
effects of temperature and pressure on the collisional linewidth can be
greatly reduced if CARS data are collected when collisional effects are
completely negligible immediately following the impulsive excitation.
The open symbols are
best-fit temperatures
neglecting the J-level
dependent decay of each
transition for N2-N2 (top)
and O2-O2 (bottom)
environments. The solid
line is power law fit. To
the data. The dashed
black lines represent a
5% error of the measured
ambient temperature.
N2-N2
The time constant is measured using a single exponential fit to the
experimental decay. The linewidth is determined by:
1
J 
2 c CARS
As temperature is increased the decay rate decreases and the linewidths
become nearly equal across J-levels. As a result, the apparent temperature
shift is reduced at higher temperature. Thus the room temperature
condition exhibits the largest error.
O2-O2
At high pressure the
apparent temperatures
deviate significantly from
the actual temperature,
even within 20 ps.
500 K
306 K
Nonresonant
Percent Error
2.5%
5%
1 atm [N2,O2] 10 atm [N2,O2]
30 ps, 50 ps
8 ps, 11 ps
40 ps, 70 ps
10 ps, 17 ps
20 atm [N2,O2]
NA
6.5 ps, 8 ps
Acknowledgments: Air Force Research Laboratory, Air Force Office of Scientific Research, National Science Foundation, Stephen Danczyk and Douglas Talley of
the Air Force Research Laboratory, Mikhail Slipchenko of Purdue University, Hans Stauffer of Spectral Energies, LLC, and Mark Johnson of Iowa State University.