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