Valence and Rydberg excited states of H2S: An SCF-Xα-SW molecular orbital study

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Abstract
SCF-Xα-SW calculations have been performed for H2S in the ground state, in ionic states resulting from the vertical removal of a valence electron, and in 36 singlet and triplet excited states resulting from the vertical excitation of an electron from the 2b1 or the 5a1 molecular orbitals to various virtual orbitals. Excellent agreement for the transition energies is found both with the experimental ultraviolet and photoelectron spectra and with recent large-scale configuration interaction calculations. The average absolute difference between the Xα-SW and CI results for the 26 cases where both exist is only 0.24 eV. The intrinsic advantages of the Xα-SW method over the traditional LCAO or one-center approaches for the treatment of Rydberg states are delineated. Contour plots of the excited orbitals have been generated and these are used to illustrate some of the conceptual problems related to the Rydberg-valence distinction. Finally, a brief speculative discussion of the possible photochemical implications of the character of the two excited orbitals involved in the first absorption band is given.