Renner–Teller and spin–orbit interactions in SiH2

Experimental data on SiH2 have been reanalyzed to show the existence of a large number of strong, but erratic rotational perturbations which are correlated with anomalous fluorescence lifetimes. These results have been interpreted using a model for the combined effects of Renner–Teller coupling between the two singlet states and spin–orbit coupling with the lowest triplet. In this model, the vibrational and K-type rotational levels of the X̃ 1A1 and the à 1B1 states of SiH2 have been fitted by least squares to give a pair of Born–Oppenheimer curves for the combining electronic states. These have been used to calculate the interaction matrix elements of the spin–orbit coupling with the ã 3B1 state. Large interactions between the 1B1 and the 3B1 states are calculated even though the direct interaction of the states is forbidden. Thus we can rationalize the existence of both the strong rotational perturbations and the unusual radiative lifetimes. Lack of knowledge of the exact location of the triplet state, and of the rovibronic structure associated with this state, precludes a detailed fit of the type recently made for CH2. We have also calculated the rotational constants for a range of vibronic states of SiH2. Analogous rotational constants and band origins of SiD2 have also been calculated using the potential curves derived from the fit to SiH2. The agreement between the calculated origins and those measured experimentally is very satisfactory.