Calculated vibrational intensities in the (A)over-tilde-(X)over-tilde electronic transition of acetylene

Relative intensities in the vibrational structure of the (A) over tilde (1) A(u) (C-2h)-(X) over tilde (1) Sigma (+)(g)(D-infinityh) electronic transit ion of acetylene are calculated. The calculation takes account of the large change of geometry and the change in the normal coordinates (the Dushinski i effect) between the two states. Because conventional vibrational wavefunc tions for a nonlinear state do not behave correctly at linear geometries th e vibrational integrals are only evaluated approximately. The transition (1 Sigma (-)(u)-(1)Sigma (+)(g)) is forbidden at linear geometries, and so ca lculations are performed without and with a factor proportional to the angl e of bend from linearity. Good agreement with experiment is obtained for th e first few quanta of the absorption spectrum, lending support to the (A) o ver tilde -state harmonic potential of Tobiason et al., J. Chem. Phys. 99, 5762 (1993). Qualitative agreement is obtained for the emission spectrum (J acobson and Field, J. Phys. Chem. 104, 3073 (2000)) when allowance is made for anharmonicity in the (X) over tilde state, but more quantitative agreem ent will require improvements in the empirical potential of the (X) over ti lde -state, or an ab initio potential to higher energies than available at present. (C) 2001 Academic Press.