The ultraviolet spectrum of OCS from first principles: electronic transitions, vibrational structure and temperature dependence.

Global three dimensional potential energy surfaces and transition dipole moment functions are calculated for the lowest singlet and triplet states of carbonyl sulfide at the multireference configuration interaction level of theory. The first ultraviolet absorption band is then studied by means of quantum mechanical wave packet propagation. Excitation of the repulsive 2 (1)A' state gives the main contribution to the cross section. Excitation of the repulsive 1 (1)A" state is about a factor of 20 weaker at the absorption peak (E(ph) ≈ 45,000 cm(-1)) but becomes comparable to the 2 (1)A' state absorption with decreasing energy (35,000 cm(-1)) and eventually exceeds it. Direct excitation of the repulsive triplet states is negligible except at photon energies E(ph) < 38,000 cm(-1). The main structure observed in the cross section is caused by excitation of the bound 2 (3)A" state, which is nearly degenerate with the 2 (1)A' state in the Franck-Condon region. The structure observed in the low energy tail of the spectrum is caused by excitation of quasi-bound bending vibrational states of the 2 (1)A' and 1 (1)A" electronic states. The absorption cross sections agree well with experimental data and the temperature dependence of the cross section is well reproduced.