Phenomena ofg-usymmetry-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 e ects 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 e ect 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 di erence 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 ˆ 2 dissocia-tion limit in which the breakdown of g±u symmetry is incorporated.