Elevated Pressure Elevated pressure results in an increase in the rate of decay of the

CARS signal due to increased collisions between molecules.

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.

Joseph D. Miller,1 Chloe E. Dedic,1 Sukesh Roy,2

James R. Gord,3 and Terrence R. Meyer1,2,4

1Department of Mechanical Engineering, Iowa State University, Ames, IA

2Spectral Energies LLC, Dayton, OH

3Air Force Research Laboratory, Wright-Patterson Air Force Base, OH

4Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander University, Erlangen-Nürnberg, Germany

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.

Summary The accuracy of CARS temperature measurements are highly dependent

on the ability to :

Measure the spectral and temporal phase and intensity of each laser

pulse

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.

Apparent RCARS Temperature Shift

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 time-

delayed 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 time constant is measured using a single exponential fit to the

experimental decay. The linewidth is determined by:

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.

Investigation of the Collision-Free Assumption for

Hybrid fs/ps CARS

Percent Error 1 atm [N2,O2] 10 atm [N2,O2] 20 atm [N2,O2]

2.5% 30 ps, 50 ps 8 ps, 11 ps NA

5% 40 ps, 70 ps 10 ps, 17 ps 6.5 ps, 8 ps

CARS intensity as a

function of pressure for

N2-N2 (top) and O2-O2

(bottom) environments.

The nonresonant response

(NR) is measured in

Argon.

The open symbols are

experimental data points.

The solid lines are

simulations of the CARS

intensity using the MEG

model for transition

linewidths.

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.

At high pressure the

apparent temperatures

deviate significantly from

the actual temperature,

even within 20 ps.

O2-O2

N2-N2

13.5 ps

300 ps

1

2J

CARSc

500 K

306 K

Nonresonant