Depolarization Ratios of Methane Raman Bands as a Function of Pressure

Abstract

In this work, we measured the intensities of Q-branches of the ν 1 , ν 2 and ν 3 bands in the polarized and depolarized methane Raman spectra in the pressure range of 1-60 atm. It was established that the pressure dependence of depolarization ratios of the ν 2 and ν 3 bands are negligible. In turn, the depolarization ratio of the ν 1 band increases with increasing pressure and reaches approximately 0.0045 at 60 atm. These data are more precise than previously published ones because ν 1 band intensities were determined taking into account the contribution of overlapping lines of ν 3 band. The presented data will be useful in calculating the methane polarizabilities at high pressure, as well as in calculating methane Raman spectra for measuring the natural gas composition using Raman spectroscopy.

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

“…The obtained values of the ρ (ν 1 ) of pure CH 4 , where the subtraction procedure of the simulated spectra was not performed, are also shown in Figure 5 for comparison. It can be seen that the pressure dependence of the ρ (ν 1 ) is observed in this case, which corresponds to the previous results [ 42 ]. The reason for this is that the contribution of the ν 3 and ν 2 + ν 4 lines to the intensity of the ν 1 band (in the 2910–2925 cm −1 range) increases due to the collisional broadening effect.…”

“…The double standard deviation of all measurements is less than 0.0001. It should be noted that much larger values of the ρ (ν 1 ) at 5 MPa were obtained by other authors [ 40 , 41 , 42 ]. We suppose that this discrepancy is caused by the neglect or incorrect accounting of the overlap of the ν 1 peak by the ν 3 and ν 2 + ν 4 bands, in addition to the low signal-to-noise ratio.…”

“…Further, it is necessary to take into account the fluctuations of the laser power to improve the accuracy of the intensity measurement. We used the Q branch of the ν 3 band of CH 4 for this purpose since the ρ of this band does not depend on pressure in the range of 0–5 MPa and equals 0.75 [ 42 , 53 ]. Thus, the ρ (ν 1 ) values were obtained using Equations (2) and (3): where and are the integrated intensities of the Q branch of the ν 3 band in the depolarized and polarized spectra, respectively.…”

“…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 ].…”

“…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 ]. Moreover, as shown in [ 40 , 41 , 43 , 44 , 45 ], the molecular environments also affect the ρ (ν 1 ).…”