The electronic structure of HgCl2 and HgBr2 and its relationship to photodissociation

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Abstract
The low‐lying excited states of HgCl2 and HgBr2 are investigated using effective core potentials in conjunction with a double‐zeta plus polarization quality basis set and a moderate‐size POL(1) CI wave function. A relativistic effective core potential is used for Hg, while nonrelativistic potentials are used for Cl and Br. The results are interpreted in terms of simple valence bond and molecular orbital pictures to give qualitative insight into the electronic structure of these species. The three lowest absorption features in HgCl2 and HgBr2 are assigned as 1 1Πu, 1 1Σ+u, and 2 1Σ+u, respectively. The initial two are assigned for the first time. Correlation and energy‐level diagrams are used to explain the previous photodissociation experiments. The results are discussed briefly in relation to the development of mercury halide lasers. Finally, the recent experimental work of Husain, Wiesenfeld, and Zare on HgBr2, reported in the following paper, confirms in detail our calculations on the 1 1Σ+u state which gives rise to the second absorption band.