Ab initio MRD-CI Calculations for the decay of molecular excited states via radiative and dissociative mechanisms

Abstract

The ab initio multi-reference single- and double-excitation configuration interaction (MRD-CI) method for the calculation of the energies and properties of electronically excited states is employed to investigate a variety of molecular decay processes. The theoretical basis for the prediction of oscillator strengths and radiative lifetimes is reviewed and the computational problems peculiar to the calculation of matrix elements between different electronic states are discussed. A number of applications is considered for the prediction of radiative lifetimes of molecular excited states and comparison with experimental data is made for C 2, HSO, SiH, C 3 and other systems in which dipole-allowed transitions are involved. Changes in the computational procedure which are required for the treatment of spin-forbidden and otherwise dipole-forbidden transitions are described and some examples for O 2 and related systems are considered. Decay processes are then considered which involve predominantly nuclear motion and the manner in which such phenomena are described quantum mechanically is compared with the methodology employed for radiative decay. Various examples of photodissociation reactions are discussed with emphasis on the different types of mechanisms which commonly occur in such processes: explicit calculations for the systems methanol, HO 2 and O 2 + are considered in this case. The role of non-adiabatic effects in such dissociative processes is investigated in detail and additional cases in which ab initio treatments have been carried out to demonstrate the importance of this type of mechanism are reviewed. Finally a procedure is described for revising the existing MRD-CI program system to allow for the direct computation of natural lifetimes (line widths) of vibronic excited states in terms of complex eigenvalues of non-Hermitian matrices, as prescribed by the Complex Coordinate Method and/or the Direct Siegert Method.