A theoretical calculation of the absorption spectrum of CH2+

The ground X 2 A 1 electronic state of CH 2 + is quasilinear with a small barrier to linearity, and at linearity the state becomes degenerate with the A 2 B 1 electronic state forming a 2 Π u state. Because of the nonzero electronic angular momentum the rovibrational basis states belonging to the two electronic states strongly interact due to both the Renner effect and spin-orbit coupling. In a previous paper (P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker, J. Mol. Spectrosc. 172 (1995) 194) we calculated the rovibronic energies of the states using ab initio potential energy surfaces that we generated. In the present paper we use the electronic wavefunctions of the previous ab initio calculation to determine the dipole moment and transition moment surfaces, and we develop the theory that allows us to use these to calculate intensities. As a result we now calculate both the positions and intensities of the lines in the absorption spectrum of CH 2 + , and its deuterated isotopomers, making full allowance for the effects of the Renner interaction and of spin-orbit coupling. We predict the appearance of the absorption spectrum over the whole wavenumber range from 0 cm −1 to beyond 15 000 cm −1 ; this involves only the X and A electronic states. We hope that these results allow experimentalists to search successfully for the features that we predict, and thereby to achieve a better spectroscopic characterization of this important molecular ion.