A THEORETICAL CALCULATION OF THE ABSORPTION-SPECTRUM OF CH2+

The ground (X) over tilde(2)A(1) electronic state of CH2+ is quasiline ar with a small barrier to linearity, and at linearity the state becom es degenerate with the (A) over tilde(2)B(1) electronic state forming a (2) Pi(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 couplin g. Tn a previous paper (P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker, J. Mel. Spectrosc. 172 (1995) 194) we calculated the rovibroni c energies of the states using ab initio potential energy surfaces tha t we generated. In the present paper we use the electronic wavefunctio ns of the previous nb initio calculation to determine the dipole momen t and transition moment surfaces, and we develop the theory that allow s us to use these to calculate intensities. As a result we now calcula te both the positions and intensities of the lines in the absorption s pectrum of CH2+, 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 w avenumber range from 0 cm(-1) to beyond 15 000 cm(-1); this involves o nly the (X) over tilde and (A) over tilde electronic states. We hope t hat these results allow experimentalists to search successfully for th e features that we predict, and thereby to achieve a better spectrosco pic characterization of this important molecular ion. (C) 1997 Elsevie r Science B.V.