Temperature‐Dependent Investigation of the S‐Branch Raman Linewidth in CO ₂ via Time‐Resolved Rotational CARS

ABSTRACT The temperature‐dependent coherence decay behavior of pure CO 2 and the self‐broadened rotational Raman linewidths were investigated between 295 and 1900 K using a picosecond time‐domain pure rotational coherent anti‐Stokes Raman scattering (RCARS) approach. To balance rapid collisional dephasing and temporal resolution, the cell pressure was reduced from 1000 to 750 mbar below 1200 K, which was successfully accounted for during the computation of the Raman linewidth. The decay behavior provides insight into the collision dynamics of the CO 2 molecules in the gas phase, which is important for gas dynamics modeling. Furthermore, the spectral signatures of the collisional dynamics, reflected in the S‐branch Raman linewidth, constitute an important aspect of accurate spectral modeling for gas‐phase diagnostic applications using the RCARS technique. The RCARS spectra for different probe delays exhibit temperature‐dependent changes in the spectral envelope that are distinct from other small molecules. A region of lower Raman linewidths observed around J" = 76 and J" = 42 and higher Raman linewidths at J" = 60 and J" = 90 indicate slower decays and faster decays compared with the expected monotonic trend, respectively. This hints at the existence of multiple decay and energy transfer channels specific to CO 2 . These may be an indication of coherence transfer in equilibrium CO 2 , which was recently found in optically centrifuged CO 2 . These specific collisional dynamics result in a nonmonotonic trend of the S‐branch Raman linewidth with rotational quantum number, which is not accounted for in previously available linewidth data. Therefore, the application of the presented Raman linewidths is expected to greatly enhance spectral modeling and RCARS gas‐phase diagnostics.