Collision-induced rototranslational absorption in compressed methane gas

The collision-induced, rototranslational absorption spectrum of compressed methane gas is computed, based purely on the reliably known, leading multipole-induced dipole components of ${\mathrm{CH}}_4$ molecular pairs. In contrast to previous work of the kind no ad hoc empirical corrections of unknown exchange force-induced dipole components are attempted. Not surprisingly, the calculated spectra show a sizeable absorption defect at virtually all frequencies, when compared to existing laboratory measurements. The defect suggests the presence of dipole-induction mechanisms in addition to those due to the leading multipole-induced dipole terms. The excess absorption, the differences between measured and calculated spectra, resembles in certain ways the excess absorption spectra seen at the same frequencies in methane-$X$ gas mixtures, where $X$ stands for helium, hydrogen, or nitrogen, respectively [Buser and Frommhold, J. Chem. Phys. 122, 024301 (2005)]. To a large extent, the excess absorption seems to be related to collisional distortions of the tetrahedral frame of the unperturbed ${\mathrm{CH}}_4$ molecule.

Figure 1

Existing measurements [17] at temperatures of 163, 195, 243, and 297 K (dots) are compared to calculated absorption spectra (solid line) in methane.

Figure 2

Collision-induced rototranslational absorption of methane at 195 K (solid line) as sum of their individual dipole components (dotted lines), labeled with ${\lambda }_1{\lambda }_2\Lambda L$. The dots represent the measured enhancement spectrum in the far infrared.

Figure 3

Excess absorption—the difference of measured and calculated absorption, from Fig. 1.