Ab initio MRD-CI study of the electronic spectrum of SiH

Abstract

Potential curves and other properties of the ground and excited states of SiH are calculated at close to full CI accuracy in DZP AO basis sets including f functions. The C ²Σ⁺ state of this system is shown to correspond to a second potential minimum of the B ²Σ⁺ species which occurs because of an avoided crossing with a dissociative state of the same symmetry. The D ²Δ and E ²Σ⁺ states are assigned as members of Rydberg series involving upper orbitals of dδ and dσ symmetry respectively. In addition, a number of quartet states have been computed including the ⁴Π and two ⁴Σ^- species which have apparently not yet been characterized experimentally. The ⁴Π state is found to be strongly repulsive, causing predissociation of the 4s and 4p members of the lowest Rydberg series; this observation is consistent with the fact that transitions to such upper states have yet to be observed experimentally. Computed spectroscopic constants, dipole and transition moments and dissociation energies are all found to agree quite well with experiment wherever comparison is possible, including results for spin-orbit constants as a function of vibrational level in the X ²Π_r ground state. Comparison is also made with results for the isovalent systems PH⁺, CH and NH⁺ at various levels and it is noted that NH⁺ exhibits a number of unusual features because of the relatively high value of the nitrogen atom IP.