A theoretical study of the absorption spectrum of singlet CH2

This paper is a continuation of our earlier work aimed at characterizing the electronic states of the methylene free radical CH2. In the present paper we focus attention on the lowest pair of singlet states, (a) over tilde(1)A(1) and (b) over tilde(1)B(1). These states are degenerate when they are linear and therefore participate in a Renner interaction. Our previous refinement of the (a) over tilde state potential energy surface ignored the Renner effect. Now we include this effect and refine both the potential energy surfaces by a fitting of the data. Using these two new potential energy surfaces, allowing for the Renner effect, and making an ab initio calculation of the dipole moment surfaces and transition moment surface, we make a simulation of the absorption spectrum associated with this pair of interacting electronic states. These predictions have been of use for the assignment of the spectrum. Further experimental work aimed at identifying rotational levels belonging to the vibrational ground state of the (b) over tilde stare is necessary before we can consider better refinements of the potential surfaces. Also spectra that fill other gaps in the data available for the two states would be highly desirable. One important new feature to emerge is that the adiabatic Renner effect correction A(L-z(2)) to the (a) over tilde-state potential energy surface causes a significant increase in the energy of the (a) over tilde state; this means that the experimentally derived vibrationless singlet-triplet splitting in methylene, T-e((a) over tilde), is reduced by 64 cm(-1) to 3159 cm(-1). This is essentially in perfect agreement with the best theoretical value for this quantity. (C) 2000 Elsevier Science B.V. All rights reserved.