Infrared absorption by collisional CH4+X pairs, with X=He, H2, or N2

Abstract

Existing measurements of the collision-induced rototranslational absorption spectra of gaseous mixtures of methane with helium, hydrogen, or nitrogen are compared to theoretical calculations, based on refined multipole-induced and dispersion force-induced dipole moments of the interacting molecular pairs CH4-He, CH4-H2, and CH4-N2. In each case the measured absorption exceeds the calculations substantially at most frequencies. We present the excess absorption spectra, that is the difference of the measured and the calculated profiles, of these supramolecular CH4-X systems at various gas temperatures. The excess absorption spectra of CH4-X pairs differ significantly for each choice of the collision partner X, but show common features (spectral intensities and shape) at frequencies from roughly 200 to 500 cm(-1). These excess spectra seem to defy modeling in terms of ad hoc exchange force-induced dipole components attempted earlier. We suggest that besides the dipole components induced by polarization in the electric molecular multipole fields and their gradients, and by exchange and dispersion forces, other dipole induction mechanisms exist in CH4-X complexes that presumably are related to collisional distortion of the CH4 molecular frame.

Static hyperpolarizability of the van der Waals complex CH4N2

“…[53] Dipole moment surface of the complex was investigated in the framework of long‐range analytical model[54, 55] and was calculated with the use of ab initio methods. [55] Using the dipole moment surface from work,[54] the collision‐induced absorption spectra of CH 4 and N 2 was calculated,[56, 57] which for frequencies from 30 to 250 cm −1 agrees well with existing measurements. [58, 59] Moreover, there are the calculations of the polarizability surface of CH 4 N 2 complex using ab initio methods and within the framework of the long‐range approximation.…”

Update of the HITRAN collision-induced absorption section

“…Therefore, they are best considered to yield smooth absorption spectra that closely reproduce the experimental data, with a smooth temperature dependence. It was shown more recently that pure ab initio calculations are not in agreement with these experimental results, presumably due to frame distortions of the tetrahedral CH 4 molecule, which are not included (Buser and Frommhold, 2005). These were also not included in the older calculations included here (Taylor et al, 1988), but these effectively corrected for this effect by fitting the dipole function to the experimental absorption spectrum.…”