COLLISIONAL NARROWING AND SHIFTING IN THE RAMAN Q-BRANCH OF OXYGEN ATHIGH-DENSITY

The fundamental isotropic Raman Q-branch of pure oxygen has been studi ed at 295 and 446 K in the pressure ranges 4-1255 and 6.4-176 bar, res pectively. The spectra have been recorded with a high resolution stimu lated Raman spectrometer. In these pressure ranges, the Q-branch profi le mainly results from spectral interferences arising between overlapp ing Q lines coupled by rotationally inelastic collisions. Comparison i s made between the experimental spectra and calculations including the off-diagonal elements of the relaxation matrix to account for line mi xing. Various fitting and scaling laws have been used for the purpose of modeling the relaxation matrix. These include modified exponential energy pp (MEG), statistical power-exponential pp (SPEG) and dynamical ly based angular-momentum scaling laws (based on the energy-corrected sudden-approximation ECS). The line-width of the band calculated with these models is compared with the experimental value as a function of the density. The overall shift of the Q-branch obtained by fitting the calculated spectra to experimental data is in good agreement with low pressure measurements and semi-classical calculations. Although all m odels are qualitatively satisfactory, the MEG law appears to give a be tter prediction of the collisional narrowing at 295 K.