Jahn-Teller effect in the low-lying K-shell excited states of SiH₄ and their behaviour towards dissociation: an ab initio study

Abstract

The stability of various low-lying resonant K-shell excited states of SiH₄ in the energy range below the K-shell ionization limit is examined, in particular with respect to the question whether or not dissociation can occur from the two lowest K-shell excited states. Photodissociation as a fast decay mechanism in concurrence to the Auger decay has been observed previously for HBr, HJ and CH₃Br. Potential energy curves are calculated for these K-shell excited states of SiH₄ by means of the MRD-CI method; the energies of the lowest fragmentation channels are also calculated by this method. It is shown that a repulsive path exists on the energy hypersurface of the lowest K-shell excited ¹A₁ state in C_3v symmetry; a fast dissociation may occur from this state which can be reached by a vibronically allowed transition. In C_2v symmetry a deep minimum of this state far away from the ground state equilibrium geometry is obtained due to the interaction of this state with the A₁ component of the lowest ¹T₂ K-shell excited state which is subject to a strong Jahn-Teller splitting. The spectroscopic consequences of this Jahn-Teller splitting and the interaction of the lowest A₁ states are discussed. An interpretation of the most intense broad feature of the K-shell photoabsorption spectrum of silane is suggested on the basis of these results.