Renner–Teller induced predissociation of HNO(A˜ [sup 1]A[sup ″]): Rotational-state dependent linewidths of quasibound states.

The predissociation dynamics in the A˜ [sup 1]A[sup ″] electronic state of HNO is investigated. The quantum mechanical dynamics calculations take into account the Renner–Teller (or electronic Coriolis) coupling with the electronic ground state X˜ [sup 1]A[sup ′], which appears to be the dominant decay mechanism for the quasi-bound vibrational states in the A˜ [sup 1]A[sup ″] upper state. All three internal vibrational degrees of freedom are incorporated and two ab initio potential energy surfaces are used. The linewidths, Γ, are directly calculated by the filter diagonalization method and an absorbing optical potential in the exit channel. They are generally small (∼1 cm[sup -1]) and increase with excitation of the bending mode (bent-to-bent transition). On average, Γ increases with K, the a-axis rotational quantum number. However, for some vibrational states the linewidth shows a non-monotonic behavior with K, which is the result of mixing with highly excited vibrational levels in the continuum of the ground electronic state. This effect is even more striking, when the total angular momentum quantum number J is varied: In some cases, the linewidth exhibits a pronounced resonancelike behavior as function of J. The agreement with recent experimental spectroscopic data is satisfactory. The calculated linewidths are of the same order (within a factor of 2 or so) as the experimental ones. However, because the two ab initio potential energy surfaces do not reproduce sufficiently well the X˜–A˜ excitation energies, the resonancelike effects are not quantitatively described. Potential energy surfaces with considerably higher accuracy are required. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]