COHERENT ANTI-STOKES-RAMAN SPECTROSCOPY STUDY OF COLLISIONAL BROADENING IN THE O-2-H2O Q-BRANCH

The fundamental isotropic Raman e branch of oxygen perturbed by collis ions with water vapor has been studied at pressures up to 1.5 atm and for temperatures between 446 and 990 K. The spectra have been recorded by using coherent anti-Stokes Raman spectroscopy (CARS) which has bee n preferred to stimulated Raman spectroscopy (SRS) in order to obtain more signal and higher sensitivity as the mixture has a small percenta ge of oxygen. The high resolution CARS spectrometer uses a seeded Nd:Y AG laser actively stabilized on an external Fabry-Perot interferometer to prevent any frequency drift during the course of the experiment. T he line broadening coefficients have been determined for several rotat ional quantum numbers (up to N=31 at 990 K). The effect of the splitti ng into triplets at lower pressure and the effect of interferences bet ween neighboring lines at higher pressure have been taken into account . The influence of Dicke narrowing has also been considered and specia l care has been taken to avoid Stark broadening. The line broadening c oefficients have been calculated according to a semiclassical model. T he rotational quantum number and temperature dependencies of the exper imental line broadening coefficients have also been studied with anoth er approach based on fitting and scaling laws. Among several laws, the modified exponential energy gap law (MEG), the statistical power-expo nential gap law (SPEG), and the energy corrected sudden law with basis rate constants taken as a hybrid exponential-power law (ECS-EP) have given good results; We have used the fitting and scaling laws to extra polate in temperature the linewidths at 2000 K.