Dynamical calculation of satellite intensities

Abstract

A formalism to calculate molecular electronic spectra is developed which takes account of electronic configuration interaction as well as of vibronic coupling between the electronic states. The formalism allows the dynamical calculation of electronic spectra, i.e., the inclusion of the effects of the nuclear kinetic energy operator on the electronic motion. When neglecting the dynamical effects, the usual statical adiabatic and Franck-Condon approximations are obtained as special cases. The formalism is applied to cyanogen the photoelectron spectrum of which exhibits an unexplained peculiar satellite structure. Ab initio calculations are performed which show that vibronic interaction between close lying 2Σg and 2Πu states takes place through the bending vibration. The vibronic coupling effects are enhanced by the simultaneous excitation of the totally symmetric C–N stretching vibration. Guided by these results a dynamical calculation is performed which reproduces nicely the experimental spectrum. The adiabatic and Franck-Condon approximations turn out to be inapplicable. As a consequence of the strong vibronic interaction with the 2Σg state, the 2πu state has a strongly bent equilibrium geometry.