Isothermal Density Tuning Of The Depolarization Ratios From The Upsilon-1 Mode Of Methane

Pure methane and 10 mol % methane in Ar have been studied in the region of methane's nu1 totally symmetric stretching mode near 2910 cm-1 (omega0, the signal maximum or zeroth spectral moment frequency). Densities from 0.0795 (3 bar) to 15.0 (2100 bar) X 10(21) mol/c.c. were examined using Ar+ laser illumination at 514.5 nm. Frequency shifts of nu1's omega0, and the zeroth moment depolarization ratio, rho(D), values as a function of pressure were recorded. Previous omega0 shifts vs density were generally confirmed and were extended to higher densities. This allowed a shallow minimum to be observed in this data. It was correlated with the rho(D) values. These latter values were examined to see whether or not they were as sensitive as the frequency shift results were to the local reorientational friction coefficient tensor f(r) (potential-energy interactions) in methane and the Ar/CH4 solutions. The initial (3.5-1000 bar data) depolarization results were interpreted with two models of methane in mind. The first, a conventional view that is based upon the concept that higher densities cause distortions in the molecular shape (lower symmetry), and the second, which makes use of the known fact that the symmetry of methane at low pressures is less than T(d). Therefore, as the density and f(r) increase quantum rotation slows and the rotation-vibration interaction effectively ceases and causes the molecules to pack tightly and the well-known T(d) symmetry is achieved.