The spectrum of singlet SiH2

We report a theoretical study of the two lowest singlet electronic states ((X) over tilde (1)A(1) and (A) over tilde B-1(1)) of silylene SiH2. These states become degenerate as a (1)Delta(g) state at linear configurations and are subject to the Renner effect. In ab initio calculations we have determined the potential energy and dipole moment surfaces for each state, and the transition moment surface between the states. Parameterized analytical functions have been fitted through the various sets of ab initio points, and the parameter values obtained for the potential energy surfaces have been further refined in fittings to experimental spectroscopic data. In these latter fittings, we use as input data experimentally derived energy differences together with ab initio points. In this manner, we achieve refined potential energy surfaces that behave reasonably also in regions of configuration space that are not sampled by the wavefunctions of the states for which experimentally derived energies are available. The calculation of rovibronic energies, the fittings to experimentally derived energies, and simulations of (A) over tilde B-1(1) --> (X) over tilde (1)A(1) emission spectra of SiH2 have been carried out with the RENNER program system. The higher excited vibrational states of (H) over tilde (1)A(1) SiH2 form polyads of heavily interacting states and many polyad states have been observed in dispersed fluorescence studies. The present theoretical work shows that owing to the heavy interaction between the states in the polyads, it is difficult to obtain unambiguous assignments for them.