State resolved reaction rates of the spin-forbidden predissociation of N2O:: A quantum dynamics study of the rotational effect

The predissociation of N2O into the 1(3)A' and 1(3)A' dissociative states for the total angular momentum J > 0 is studied by quantum dynamics calculations. The effective Hamiltonian for describing the predissociation is derived from time-dependent wave packet propagation calculations on the triplet potential energy surfaces. The decay rates of individual rovibrational states in the singlet manifold are obtained by diagonalizing the effective Hamiltonian represented in terms of the singlet rovibrational wave functions. The Fermi golden rule is also applied to test its validity in estimating the decay rate distribution. For J = 1 and 2, the rovibrational Hamiltonian is constructed by recoupling the coupled state wave functions obtained by a filter diagonalization. For higher values of J, a random coupling model deduced from the calculations for J = 1 and 2 is introduced to estimate the decay rate distributions. In order to compare the calculated decay rate distributions with those by a random matrix/transition state theory (RM/TST), the transition states are defined as the eigenvectors of decay rate matrix whose eigenvalues are used for calculating the RM/TST distributions. It is found that the fluctuation of decay rate distribution decreases with increasing J though the calculated distribution shows significant deviation from the RM/TST prediction even for J = 20. A simple model is employed to interpret the origin of the decrease of fluctuation in decay rates with J and the deviation from RM/TST. It is concluded that a sharp decay rate distribution comes from an increase of the absolute number of singlet rovibrational states accessible to the transition state due to the K-mixing, though such a K-mixing is limited. The deviation of calculated distribution from the RM/TST one is thus attributed to incomplete energy randomization in the singlet state N2O. (C) 2000 American Institute of Physics. [S0021-9606(00)00504-3].