Phenomena of g-u symmetry-breakdown in HD

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 (H¯ ^{1 +} _g and B¯ ^{1 +} _u ) is highlighted. Electronic coupling between states of g and u symmetry always takes place near dissociation threshold; as a result of the mass difference and the electronic isotope shift the behaviour at long range cannot be described in an adiabatic picture. A procedure is developed to construct long-range potentials near the n = 2 dissociation limit in which the breakdown of g-u symmetry is incorporated.