Collisional effects in Q branch coherent anti-Stokes Raman spectra of N2 and O2 at high pressure and high temperature

Abstract

A temperature and pressure dependent study of coherent anti-Stokes Raman scattering (CARS) Q branch spectra of molecular nitrogen and oxygen has been conducted. Spectra at pressures up to 250 MPa and in the temperature 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 position of collapsed Q branch profiles were measured. Measurements also have been made in synthetic air and in mixtures with argon. A detailed comparison of Q branch CARS band shapes with theoretical models of quantum mechanical and quasiclassical origin has been performed. On the one hand existing scaling laws like the modified energy gap (MEG), energy corrected sudden (exponential) polynomial energy gap [ECS-(E)P], polynomial energy gap (PEG), and statistical polynomial energy gap (SPEG) laws that give analytical expressions for rotational relaxation rates 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 deviations from experimental results in the high pressure regime investigated here. For nitrogen the PEG-law, although not very suitable at lower densities, at room temperature reasonably reproduces the half-widths in the high pressure regime. The same is true for the ECS-EP law at low and high temperatures, whereas the SPEG-law only gives reasonable results at high temperature. For oxygen only the MEG and ECS-EP laws (at room temperature) give half-widths that are within the error limits of the measurement. On the other hand, within experimental error frequency shifts and half-widths of N2 and O2 CARS-spectra are well described by the classical approach throughout the density range. It is found that dephasing contributions to the density induced spectral shift cannot be neglected at room temperature but are less important at higher temperatures. In comparison to experimental data the quasiclassical model provides physical interpretation of temperature dependent cross sections for rotational energy relaxation processes in nitrogen and oxygen at high densities.