COLLISIONAL EFFECTS IN Q-BRANCH COHERENT ANTI-STOKES-RAMAN SPECTRA OFN-2 AND O-2 AT HIGH-PRESSURE AND HIGH-TEMPERATURE

A temperature and pressure dependent study of coherent anti-Stokes Ram an scattering (CARS) Q branch spectra of molecular nitrogen and oxygen has been conducted. Spectra at pressures up to 250. MPa and in the te mperature range 298 K<T<850 K have been obtained using a scanning CARS apparatus. The full-width at half-maximum (FWHM) as Well as peak posi tion of collapsed e branch profiles were measured. Measurements also h ave been made in synthetic air and in mixtures with argon. A detailed comparison of Q branch CARS band shapes with theoretical models of qua ntum mechanical and quasiclassical origin has been performed. On the o ne hand existing scaling laws like the modified energy gap (MEG), ener gy corrected sudden (exponential) polynomial energy gap [ECS-(E)P], po lynomial energy gap (PEG), and statistical polynomial energy gap (SPEG ) laws that give analytical expressions for rotational relaxation rate s are used in a CARS code to calculate half-widths of the collapsed Q branch of nitrogen and oxygen. Many of these models show significant d eviations from experimental results in the high pressure regime invest igated here. For nitrogen the PEG-law, although not very suitable at l ower densities, at room temperature reasonably reproduces the half-wid ths in the high pressure regime. The same is true for the ECS-EP law a t low and high temperatures, whereas the SPEG-law only gives reasonabl e results at high temperature. For oxygen only the MEG and ECS-EP laws (at room temperature) give half-widths that are within the error limi ts of the measurement. On the other hand, within experimental error fr equency shifts and half-widths of N-2, and O-2 CARS-spectra are well d escribed by the Classical approach throughout the density range. It is found that dephasing contributions to the density induced spectral sh ift cannot be neglected at room temperature but are less important at higher temperatures. In comparison to experimental data the quasiclass ical model provides physical interpretation of temperature dependent c ross sections for rotational energy relaxation processes in nitrogen a nd oxygen at high densities.