afrl, propulsion directorate
DESCRIPTION
Coherent Anti-Stokes Raman Scattering (CARS) Noninvasive Coherent Laser-Like Signal Spatially and Temporally Resolved Excellent Gas Temperature Data (especially at higher temperatures) Non-resonant background (NRB) interference Lack of accurate rotational Raman linewidth data for important combustion species (e.g., CO2, C2H2) w Stokes r probe pump CARS This has the wrong energy level diagram. It needs to be modified for the rotational experiment as does the beam diagram.TRANSCRIPT
AFRL, Propulsion Directorate
Direct Measurements of Collisionally Broadened S-Branch Raman
Coherence Lifetimes of CO2 Joseph R. Gord Purdue University Sukesh
Roy, Paul S. Hsu, Naibo Jiang, Waruna D. Kulatilaka, Hans U.
Stauffer Spectral Energies, LLC James R. Gord AFRL, Propulsion
Directorate This is collaborative work between ISSI and AFRL.
Coherent Anti-Stokes Raman Scattering (CARS)
Noninvasive Coherent Laser-Like Signal Spatially and Temporally
Resolved Excellent Gas Temperature Data (especially at higher
temperatures) Non-resonant background (NRB) interference Lack of
accurate rotational Raman linewidth data for important combustion
species(e.g., CO2, C2H2) w Stokes r probe pump CARS This has the
wrong energy level diagram.It needs to be modified for the
rotational experiment as does the beam diagram. Brief Review of
ps-CARS
Why Picosecond? Suppression of nonresonant background Investigation
of the collisional and Doppler dependence of signal through
time-resolved measurements and enabling direct measurements of
rotational Raman linewidth Probable minor species detection Brief
Review of ps-CARS (Cont.)
(NRB suppression) Improved temperature measurement in rich
hydrocarbon-air flame Time-Resolved ps-CARS Signals (lasers of
order 150 ps) Resonant Signal Nonresonant Signal S. Roy, T.R. Meyer
& J. R. Gord, Appl. Phys. Lett. 87, (2005) Brief Review of
ps-CARS (Cont.) (Time-Resolved ps-CARS)
Direct measurements of collisionally broadened Q-branch Raman
coherence lifetimes of H2 The direct measured linewidths are in
good agreement with those derived from the linewidths determined in
the previous high-resolution frequency domain studies
Frequency-domain constant Linewidth Kulatilaka et al., APL 97,
(2010) Problems Encountered in Previous CO2 Rotational Raman
Linewidth Measurements
Experiment Theory Correction for overlapping of lines was used in
the previous works, but the accuracy of the obtainedlinewidth is
questionable. Jammu et al. Can-Phys (1966) Significant line
overlapping due to closely spaced CO2 lines Large measurement
uncertainty (~ 15%) (Pinter Opt. Spectry 17, 428 (1964) The
theoretical model based on Modified Energy Gap law (MEG) does not
provide accurate linewidth values. System Schematic 135 ps 0.15
cm-1
Schematic Diagram of the broadband picosecond CARS system is shown
here. 135 ps 0.15 cm-1 Laser Specifications Probe Laser Energy:
~400 J Pulse width: ~100 ps
Bandwidth: ~0.12 cm-1 Diameter at waist: ~ 60 m Interaction length:
~ 2mm Pump/Stokes Lasers Energy: ~1700 J Pulse width: ~115 ps
Bandwidth: ~135 cm-1 Time-Resolved ps-CARS
Spectroscopy of CO2 Direct Measurement of CO2 Rotational Raman
Linewidth
Measured collisional self-broadening coefficients (HWHM) at
room-temperature for the CO2 S branch. With better measurement
accuracy. Measured CO2 Rotational Raman Linewidth Broadened by
N2
Measurements were also performed in binary mixtures of CO2N2
(primary collisional partner in combustor). These measurements can
be useful for accurate CO2 ps-CARS thermometry as well as for
studying various CO2 collisional energy-transfer processes. These
figures need some clean up. Summary & Conclusions The decay
time constants measured for pure H2 are in good agreement with
those derived from the linewidths determined in previous
high-resolution frequency domain studies. We have directly measured
the rotationally resolved Raman coherence lifetimes for pure CO2
and CO2-N2 mixtures and derived the corresponding rotational Raman
linewidth. The experimentally measured S-branch signal decay
lifetimes will be used for making appropriate corrections to
increase the accuracy of ps-CARS thermometry in high-collision
environments where the probe pulse is delayed w.r.t the pump and
Stokes pulses to suppress the NRB contribution. . Acknowledgments
Funding for this research was provided by AFOSR (Drs. Riq Parra and
Chiping Li) and AFRL Backup Slides Molecular
SpectroscopyElectronic, Vibrational, Rotational, & Zeeman
Levels
Rotational Levels Zeeman Sublevels Molecular Population
Distribution Temperature Dependence
T = 300 K T = 1000 K Coherent Anti-Stokes Raman Scattering (CARS)
Spectroscopy
Noninvasive Coherent Laser-Like Signal Spatially and
TemporallyResolved Excellent Gas-TemperatureData n, J n, J
Vibrational Excitation Dynamics
n, n, J n, n, J n, n, J3 n, n, J n, n, J n, n, J n, n, J n, n, J5
18 Rotational Excitation Dynamics
J2 J1 J3 Vibrational Excitation Dynamics 19