Experimental and molecular dynamics study of the pressure dependence of Raman spectra of oxygen

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Abstract
The pressure dependence of Raman spectra of gaseous O2 at 300 K has been studied experimentally and by molecular dynamics (MD) simulation. Experimental spectra are reported for the pressure range of 40–3000 bar and MD spectra for four thermodynamic states in the pressure range of 130–3000 bar. The MD trajectories are calculated using the Lennard‐Jones atom–atom intermolecular potential. The interaction‐induced effects on the system polarizability are evaluated using the first order dipole–induced dipole (DID) approximation. In the case of depolarized Raman scattering, the experimental line shapes and time correlation functions agree very well with the MD results. The density dependence of the experimental second spectral moment is also in excellent agreement with the MD predictions. The MD results indicate that the relative contribution of the interaction‐induced polarizability to the depolarized spectrum increases with increasing density, but remains small within the density range considered, and that the spectrum is dominated by orientational relaxation of the molecular polarizability. The experimental depolarized Raman and MD orientational time correlation functions are compared to the results of J‐diffusion and Steele models of relaxation. We find that neither of these models can account for single molecule reorientation in oxygen gas over the entire range of pressures. At high pressures, the experimental results for the frequency‐dependent depolarization ratio η differ significantly from the ‘‘classical’’ value of 3/4 over most of the accessible frequency range. Similar behavior is found for the corresponding Rayleigh depolarization ratios. The MD calculations predict a much smaller deviation of η from the value of 3/4, suggesting that induction mechanisms other than DID are needed to explain the experimental data. The experimentally observed pressure dependence of the Q branch of the Raman spectrum is explained using the motional narrowing model of Brueck.