DIRECT MEASUREMENTS OF COLLISIONALLY BROADENED (CO$_{2}$-CO$_{2}$) S-BRANCH RAMAN COHERENCE LIFETIMES OF CO$_{2}$

Abstract

Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN 47907; Spectral Energies, LLC, 5100 Springfield Street, Suite 301, Dayton, OH 45431; Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, OH 45433We report direct measurement of S-branch Raman coherence lifetimes of CO$_{2}$ due to CO$_{2}$-CO$_{2}$ collisions by employing picosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy. A custom-built, high-peak-power, nearly transform-limited ps laser system offers an ideal combination of frequency and temporal resolution for such measurements. The rotational S-branch transitions of CO$_{2}$ ground state [0,0$^{0}$,0] with rotational quantum number J$=0-52$ were simultaneously excited by using a broadband ($\sim$3-nm) laser pulse with a full-width-half-maximum (FWHM) of $\sim$100 ps. The coherence lifetimes of self-broadened CO$_{2}$ for a pressure range of 0.05-1.5 bar were directly measured by probing the rotational coherence with a nearly transform-limited, 80-ps-long laser pulse. The measured linewidth of J=6 and J=50 transitions are found to be $\sim$0.106$\pm$0.0002 and $\sim$0.070$\pm$0.0002, respectively. As expected, the energy-transfer from high J levels has a significantly longer coherence lifetime because of the inertia associated with higher angular momentum. These measurements are very significant for performing accurate thermometry or CO$_{2}$ concentration measurements in gas-phase reacting flows