COLLISION EFFECTS IN NITROGEN AND METHANE COHERENT ANTI-STOKES-RAMAN ISOTROPIC Q-BRANCH SPECTRA AT HIGH-DENSITIES

Using coherent anti-Stokes Raman spectroscopy (CARS) the spectral shif t and width of the collisionally narrowed Q-branch structures of nitro gen and the nu(1) symmetric stretch vibration in methane were investig ated at high densities. The gas samples either contained the pure subs tance or, for the case of nitrogen and methane, were diluted with argo n, methane and carbon monoxide or argon and nitrogen, respectively, in the pressure range 50-2000 bar and at temperatures between 300 and 70 0 K. The simultaneous recording of spectra at ambient conditions ensur ed a frequency measurement accuracy of 0.07 cm(-1). Contributions to t he line shapes and frequency shifts are determined that originate from narrowing of the rotational structure and from vibrational dephasing in nitrogen, methane, and its mixtures. The results are compared with quasiclassical calculations of the band shape and shift to determine t hermally averaged collision cross sections for energy relaxation and v ibrational dephasing as a function of temperature. In the investigated density regime, for nitrogen the band shape is dominated by collision al narrowing. The peak position of the band does not strongly depend o n composition of the sample and the maximum red shift of the Raman fre quency diminishes with increasing temperature. For methane at densitie s above 50 amagat effects from rotational relaxation are no longer det ectable and dephasing collisions are dominant. In addition to vibratio n-translation relaxation, vibrational energy transfer is an important process for line broadening at high densities. The frequency shift of the e-band strongly depends on mixture composition and temperature. (C ) 1996 American Institute of Physics.