Pressure Broadening of the Ammonia Inversion Line by Foreign Gases: Quadrupole-Induced Dipole Interactions

Abstract

The broadening of the 3,3 line of the inversion spectrum of ammonia by foreign gases which are not expected to have dipole or quadrupole moments has been measured accurately by Smith and Howard. This broadening is greater than that previously computed by the author using the interaction of the molecule's dipole moment with the induced dipole on the foreign gas atom. In this paper the broadening is explained quantitatively using the interaction of the induced dipole on the foreign gas atom with the quadrupole moment of ammonia. It is concluded that a model of the ammonia molecule using bond dipoles of the appriate size to give the known dipole moment, or a model with point charges at the atoms, again adjusted to give the correct dipole moment, both give quadrupole moments which explain the broadening cross sections with good accuracy.