Isothermal density tuning of the depolarization ratios from the ν1 mode of methane

Abstract

Pure methane and 10 mol % methane in Ar have been studied in the region of methane’s ν1 totally symmetric stretching mode near 2910 cm−1 (ω0, the signal maximum or zeroth spectral moment frequency). Densities from 0.0795 (3 bar) to 15.0 (2100 bar)×1021 mol/c.c. were examined using Ar+ laser illumination at 514.5 nm. Frequency shifts of ν1’s ω0, and the zeroth moment depolarization ratio, ρD, values as a function of pressure were recorded. Previous ω0 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 ρD 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 fr (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 Td. Therefore, as the density and fr increase quantum rotation slows and the rotation-vibration interaction effectively ceases and causes the molecules to pack tightly and the well-known Td symmetry is achieved.

Depolarization Ratios of Methane Raman Bands as a Function of Pressure

“…The data given in Figure 4b are the sum of intensities of ν1 and ν3 bands. Thus, they can be represented using Equation 3: As follows from Figure 6, the depolarization ratio increases significantly less compared to the data of Rose et al [21]. In the studied pressure range, an increase in ρ ν 1 is close to a linear dependence.…”

“…According to the data shown in Figure 4, both a decrease in intensity in the polarized spectrum and an increase in intensity in the depolarized spectrum are observed. This results from lower methane molecule symmetry at high pressure [21,22]. To verify the fact that the increase in the intensity in Figure 4b is not due to the features of the experiment, Figure 5 shows the dependence of the integrated intensity of the depolarized spectrum of the ν 3 band in the range of 2820-2870 cm −1 where there are no ν 1 lines.…”

“…In the studied pressure range, an increase in ρ ν 1 is close to a linear dependence. According to [21], ρ ν 1 increases with pressure up to approximately 1000 atm. In practice, to analyze the composition of natural gas, it is necessary to take into account changes in spectroscopic characteristics Molecules 2020, 25, 1951 6 of 7 in the pressure range of 1-100 atm.…”

“…These data are correct for low pressure, when the molecules do not significantly affect each other. Rose et al [21] and Wright et al [22] showed that the depolarization ratio of ν 1 in pure methane increases with pressure in the range of 1-1000 bar. The main disadvantage of these works is the low signal-to-noise ratio of the experimental setups used, and as a consequence, the high uncertainty of the measured values.…”

Depolarization Ratio of the ν1 Raman Band of Pure CH4 and Perturbed by N2 and CO2

“…More precise measurements were performed using the photoacoustic Raman spectroscopy method by Yu et al [ 39 ], where the ρ (ν 1 ) = 0.002 ± 0.002 was obtained at a pressure of less than 15 kPa. According to the data reported in [ 40 , 41 , 42 ], the ρ (ν 1 ) is a function of pressure. However, there is a discrepancy between the results obtained, for example, ρ (ν 1 ) = 0.11 at 4 MPa [ 40 ], ρ (ν 1 ) = 0.067 at 5 MPa [ 41 ], and ρ (ν 1 ) = 0.0045 at 6 MPa [ 42 ].…”