Radiative singlet-triplet transition properties from coupled-cluster response theory:: The importance of the S0→T1 transition for the photodissociation of water at 193 nm

Expressions for first-order induced electronic transition matrix elements are derived within the coupled-cluster response theory framework. When combined with electric-dipole and spin-orbit operators, these matrix elements allow the calculation of radiative transition probabilities between singlet ground and triplet excited states. An implementation employing an atomic mean-field representation of the spin-orbit operator is presented at the coupled-cluster singles and doubles level. The suitability of this operator for the calculation of radiative transition probabilities is checked in test calculations for BH and AlH which are compared to full configuration interaction results obtained with the full Breit-Pauli spin-orbit operator. In a first application, we investigate the importance of the S-0-->T-1 transition relative to the S-0-->S-1 transition in the first absorption band of the electronic spectrum of H2O. The potential importance of the S-0-->T-1 transition for understanding the photodissociation in the low energy regime of this band is confirmed and accurate estimates are given for the energy difference between the S-1 and T-1 state as well as the transition dipole moments for excitations to these states. In addition, the geometry dependence of these properties is analyzed. (C) 2002 American Institute of Physics.