Abstract
The line-broadening phenomena for a diatomic polar molecule perturbed by a tetrahedral molecule are analysed. Two points are particularly studied and exemplified in the case of the vibration bands of HCl perturbed by CF4 and by SiF4. (i) The influence of the electrostatic potential energy (i.e. dipole-octopole and quadrupole-octopole couplings) on reorientation during a collision is introduced, and it appears, to a large extent, to be competitive with the effect of the shorter range angle independent contributions to the potential. Consequently only a weak vibrational effect from the 0–1 to the 0–2 bands is observed when the octopole moment of the tetrahedral molecule is taken into account, in agreement with the experiments. (ii) For the systems mentioned above the moments of inertia allow us to apply a convenient simplified formalism for the line-broadening calculations. In this formalism the rotational degrees of freedom of the perturber are considered as classical. The results of these theoretical considerations are in good agreement with the presently available experimental data.
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