A theoretical study of the absorption spectrum of singlet CH2

This paper is a continuation of our earlier work aimed at characterizing th e 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 l inear and therefore participate in a Renner interaction. Our previous refin ement of the (a) over tilde state potential energy surface ignored the Renn er 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 sur faces, allowing for the Renner effect, and making an ab initio calculation of the dipole moment surfaces and transition moment surface, we make a simu lation 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 le vels belonging to the vibrational ground state of the (b) over tilde stare is necessary before we can consider better refinements of the potential sur faces. 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 th at the adiabatic Renner effect correction A(L-z(2)) to the (a) over tilde-s tate potential energy surface causes a significant increase in the energy o f the (a) over tilde state; this means that the experimentally derived vibr ationless singlet-triplet splitting in methylene, T-e((a) over tilde), is r educed by 64 cm(-1) to 3159 cm(-1). This is essentially in perfect agreemen t with the best theoretical value for this quantity. (C) 2000 Elsevier Scie nce B.V. All rights reserved.