Anomalous behaviour in the vibrational Raman spectra of N₂, O₂, and CO under near critical conditions

Abstract

Experimental studies of the isothermal density dependence of frequency shift and line-width of the Raman Q-branches of N₂, O₂ and CO gases have been done. Light (514.5nm Wavelength) from an Ar-ion laser was focused at the centre of a temperature controlled cell containing the sample, and scattering at 90° was analyzed and recorded by photon counting techniques using a piezoelectrically scanned Fabry-Perot interferometer, photo-multiplier tube and a data acq1uisition and stabilization system. -- The isothermal Raman frequency shift and line-width (FWHM) data were collected over a density range of 10 Amagat to 450 Amagat (70 psi - 1000 psi) for a set of temperatures close to the critical temperature (Tc) for each gas (N₂, O₂ & CO). It was found that the density dependences of both the shift and width exhibited anomalous behavior under near-critical conditions. The anomaly reduced for temperatures away from the critical temperature, and was found to vanish completely in the case of frequency shift at T-Tc ≥ 3K. -- The observed effects are qualitatively interpreted in terms of ‘local density’ fluctuations associated with molecular cluster formation. It is assumed that cluster condensation occurs at a particular value of density ρcc, characterized by a large and sudden change in the mean cluster life-time. It is interpreted that for densities ρ < ρcc, the observed Raman spectrum is mainly due to unclustered molecules and for densities ρ > ρcc, the observed Raman spectrum is dominantly due to molecular clusters. Finally, it is concluded that the Raman effect could be of great importance as a probe of density fluctuations in the critical region.