Multiconfigurational second-order perturbation calculation of the electronic absorption spectrum of trisilane, Si3H8

Abstract

The low-lying singlet excited states of trisilane have been computed using the complete active space self-consistent field (CASSCF) method, second-order perturbation theory (CASPT2), and the extended multistate CASPT2 (MS-CASPT2) approach, and a generally contracted basis set of atomic natural orbitals (ANOs) including Rydberg functions. The ground state structure was obtained from an ab initio optimization at the second-order Møller-Plesset perturbation theory (MP2) level using Dunning's correlation-consistent triple-zeta basis set (cc-pVTZ) and agrees well with experiment. The calculation of the electronic transitions included the lower valence excited states and two Rydberg series converging to the first two ionization potentials. Only the MS-CASPT2 method gave an accurate description of the observed electronic excitations since it provided an effective separation of the valence and Rydberg states. In contrast with previous assignments, three states of valence character were found below the first Rydberg state, located at 7.20eV. As a consequence, the first broad band observed in the spectrum is entirely of valence character. Vertical dipole-allowed valence transition energies are calculated at 6.55, 7.09, 7.32, 7.60, 8.07 and 8.18eV (excitations to 11B2, 21A1, 31A1, 11B1, 51B2 and 21B1, states, respectively). Two vertical dipole-forbidden valence transitions assigned to the 11A2 and 41A2 states are found at 6.89 and 8.52eV, respectively. The 4s, 4p and 3d members of the Rydberg series converging to the first and second ionization potentials are found at 7.20–8.62 eV and 7.82–9.61 eV, respectively. A detailed theoretical interpretation of the absorption spectrum of trisilane based on the new assignments has been proposed, and comments on the ability of the CASSCF, CASPT2 and MS-CASPT2 approaches to describe accurately the absorption spectrum of oligosilanes are made.