Bound and quasibound states of the Na center dot center dot center dot FH van der Waals molecule

A recently developed diabatic representation for the two lowest (2)A(') ele ctronic states of the NaFH system [M.S. Topaler, D.G. Truhlar, X.Y. Chang, P. Piecuch, and J.C. Polanyi, J. Chem. Phys. 108, 5349 (1998)], augmented w ith highly accurate multireference configuration interaction data, is used to construct a new potential energy function for the ground electronic stat e of NaFH. The refined potential is used to calculate the bound and quasibo und rovibrational states of the Na ... FH molecule. The focus is on the qua sibound states corresponding to excitation of the H-F mode in the complex, although states in which the H-F mode is not excited are also studied. All rovibrational calculations are performed within the framework of the Sutcli ffe-Tennyson Hamiltonian for triatomic molecules. The energy positions and lifetimes of quasibound states are obtained using the stabilization method. Three methods are employed to solve the rovibrational problem: (i) the var iational approach, in which the Hamiltonian matrix, as defined by a discret e basis set, is diagonalized, (ii) the coupled-channel method, in which the van der Waals stretching coordinate is handled by direct numerical propaga tion on a grid, and (iii) two perturbative approaches based on the adiabati c separation of vibrational motions. The effect of rotational excitation on the lifetimes of calculated resonances is studied. The main results of thi s study are the strong evidence for the existence of many long-lived rovibr ational resonances corresponding to excitation of the H-F mode in the compl ex and the rationalization of this finding in terms of effective potentials defining adiabatic separations. Possible impact of the results obtained in this study on new experimental ways of probing potential energy surfaces o f the NaFH system, with emphasis on the dynamics of photo-induced charge tr ansfer in Na ... FH, is discussed. (C) 2000 American Institute of Physics. [S0021-9606(99)01048-X].