Nonadiabatic interactions between the ground and low-lying excited electronic states:: Vibronic states of the Cl-HCl complex

The Cl-HCl radical complex is investigated by a combination of accurate ab initio quantum chemical methods for the evaluation of the three lowest electronic potential energy surfaces and nonadiabatic couplings between them, and quantum evaluation of vibronic states using wave function propagation in imaginary time within a close coupling scheme. The sensitivity of the vibronic energies on the quality of the potential surfaces is clearly demonstrated. Moreover, it is shown that nonadiabatic couplings between the three lowest electronic states play an important role, especially for highly excited vibronic states. Since under experimental conditions the complex is prepared in a superposition of excited vibronic states close to the dissociation limit, the inclusion of nonadiabatic effects is crucial for a quantitative interpretation of future higher resolution spectroscopic experiments. (C) 2001 American Institute of Physics.