Phenomena of<i>g-u</i>symmetry-breakdown in HD

Abstract

Phenomena associated with the breakdown of inversion symmetry in the HD molecule are reviewed and discussed. A distinction is made between three kinds of physical effects observed in HD spectra. The existence of a small electric dipole moment in the ground state gives rise to vibrational and pure rotational transitions following selection rules of electric dipole transitions. Coupling between electronic states of g and u symmetry occurs, which is associated with the appearance of forbidden lines in the electronic spectrum. This effect occurs predominantly at near coincidences between levels of opposite inversion symmetry and a recently observed example of strongly interacting states (H̄ ′∑