Far-infrared absorption in gaseous CH₄and CF₄

Abstract

We present a simple theory of the gas-phase far-infrared collisioninduced absorption spectra of two molecules with tetrahedral symmetry, methane (CH4) and tetrafluoromethane (CF4). We assume that in these molecules CIA arises through octopolar and hexadecapolar induction. By neglecting the small anisotropies in the intermolecular CH4-CH 4 and CF4-CF 4 potentials, we can express the spectral profiles as convolutions of single-molecule free rotation spectra with intermolecular translational components. Information theory is used to estimate the 'least biased' form for the latter, based on our knowledge of their spectral moments (the calculation of the zeroth through sixth moments is described in an appendix). In this way we avoid the need to make questionable assumptions about the system dynamics. The computed lineshapes are in excellent agreement with the available experimental data.

Far infrared absorption in liquid methane: experiment and theory

“…If we substitute equations (23) and (24) in (22) and (21), and substitute equation (21) in (5), and neglect the small anisotropy in the CH 4 potential, we can perform the averages of the T ( n) and of X and U separately [7,8],…”

“…In this calculation of l i j we extend the work of Joslin et al [7,8] by including overlap induced dipoles. We have…”

“…The quantities X and U are the octopole and hexadecapole tensors, respectively. Their magnitude are related to the spherical moments Q l by [8] Q 3 = (12 /5 ) 1 /2 X , Q 4 = (12 /7 ) 1 /2 U .…”

“…A basic problem in such calculations is ® nding a suitable potential function and constructing a reasonable induced dipole model. Our approach is based on earlier studies by Joslin et al [7,8], but extended to include quantum corrections and overlap-induced dipoles. An important assumption is the use of the Kirkwood superposition approximation to calculate the three-body correlation function.…”

“…where q is the density, and the CH 4 polarizability was taken to be [8] a = 2.59 A Ê 3 . As seen in table 1, the absorption in the liquid increases 14± 16% due to the enhancement of the applied ® eld in the liquid.…”