Determination of temperature from N2 and O2 cars spectra at very high pressure

Abstract

Using least-squares routines, temperatures have been obtained from fitting computer-generated spectral shapes of collapsed N2 and O2 Q-branch coherent anti-Stokes Raman scattering (CARS) spectra to experimental data at high densities between 120 and 725 amagat (1 amagat=2.67×1019 cm−3) in the temperature range from 300–850 K. Experimental data were obtained from high-pressure gases enclosed in a temperature-controlled vessel that is equipped with sapphire windows for optical access. In the fitting codes, for each temperature interation step, densities have been readjusted by calculating pVT data from equations of state. Inelastic rotational energy transfer rate constants, necessary for spectral modeling, were calculated from known energy gap scaling laws. Alternatively, temperature-fitting results from the modeling of unresolved Q-branch band shapes using a quasi-classical description of rotational relaxation are presented. Within the pressure range investigated, for nitrogen at 300 K, best agreement is obtained for the polynominal energy gap (PEG) law, whereas the classical model is reasonably good at 700 K. For oxygen at 300 K, the modified exponential gap (MEG) model and, at higher temperatures, the quasi-classical model are preferred.