Collision effects in nitrogen and methane coherent anti-Stokes Raman isotropic Q-branch spectra at high densities

Abstract

Using coherent anti-Stokes Raman spectroscopy ͑CARS͒ the spectral shift and width of the collisionally narrowed Q-branch structures of nitrogen and the 1 symmetric stretch vibration in methane were investigated at high densities. The gas samples either contained the pure substance or, for the case of nitrogen and methane, were diluted with argon, methane and carbon monoxide or argon and nitrogen, respectively, in the pressure range 50-2000 bar and at temperatures between 300 and 700 K. The simultaneous recording of spectra at ambient conditions ensured a frequency measurement accuracy of 0.07 cm Ϫ1 . Contributions to the line shapes and frequency shifts are determined that originate from narrowing of the rotational structure and from vibrational dephasing in nitrogen, methane, and its mixtures. The results are compared with quasiclassical calculations of the band shape and shift to determine thermally averaged collision cross sections for energy relaxation and vibrational dephasing as a function of temperature. In the investigated density regime, for nitrogen the band shape is dominated by collisional narrowing. The peak position of the band does not strongly depend on composition of the sample and the maximum red shift of the Raman frequency diminishes with increasing temperature. For methane at densities above 50 amagat effects from rotational relaxation are no longer detectable and dephasing collisions are dominant. In addition to vibration-translation relaxation, vibrational energy transfer is an important process for line broadening at high densities. The frequency shift of the Q-band strongly depends on mixture composition and temperature.

Experimental and theoretical study of line mixing in methane spectra. III. The Q branch of the Raman ν1 band

“…There have been, to our knowledge, no previous measurements for the CH 4 -He system but experiments have been presented for CH 4 -Ar. [17][18][19] The study of Ref. 17 was made for densities significantly higher and does not provide reliable information for the pressures of interest here.…”

“…14 where 13 lines of the 2 2 band have been studied at low densities. More experimental results are available at higher pressures and in the 1 band: spectra of pure methane [15][16][17][18][19] and CH 4 -͑Ar, N 2 ,...) [17][18][19] mixtures have been recorded in the Q branch up to about 600 amagat. These studies show that, above a few atmospheres, the Q branch tends to behave as a single symmetric line whose broadening and shifting coefficients have been determined from recorded spectra.…”

Crossed Two-Beam Coherent Anti-Stokes Raman Spectroscopy in Dispersive Media

“…However, in systems at very high pressures, as in supercritical water oxidation environments, for example, the angle grows large enough to be useable. There have been several CARS m easurements at quite high pressures (up to 2500 bar [22][23][24][25][26][27][28][29][30][31] ); however, m ost of these works are concerned less with spatial resolution than with other fundamental issues. Hence, the X-CARS issue has not been raised.…”

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy

“…1(a)]. They were further employed to investigate the rotational energy transfer in molecular collisions [40][41][42] between the vibrationally coherent CH 4 molecules and other collisional partners, e.g., N 2 and argon (Ar). In the context of combustion diagnostics, the CH 4 CRS signal was recorded at temperatures as high as 1273 K in a furnace, 43 and CH 4 ns CRS was used to measure the temperature in a laminar CH 4 /air diffusion flame 44 and in supercritical liquid oxygen (LOX)/CH 4 combustion.…”