ROTATIONAL-LEVEL-DEPENDENT RADIATIVE LIFETIMES AND BRANCHING RATIOS IN NO(B2-PI) (V = 7, OMEGA + 1/2,3/2)

The previous study [G. E. Gadd and T. G. Slanger, J. Chem. Phys. 92, 2194 (1990)] of the upsilon = 0-6 levels of the NO(B 2-PI) valence state has been extended to the upsilon = 7 level, which is of particular interest because it lies at the first dissociation limit of the molecule, and also in a region where strong homogeneous mixing with the C 2-PI Rydberg state manifests itself. Thus, the upsilon = 7 radiative lifetime can be affected by both these interactions, which are dependent on rotational level. It has often been argued that the B 2-PI-3/2 spin-orbit component interacts strongly with C 2-PI-1/2,3/2, whereas the B 2-PI-1/2 component is unperturbed. We show here that "unperturbed" is a relative term and that a sensitive way to demonstrate that even the B 2-PI-1/2 levels are mixed with the C 2-PI state is to measure the intensity branching in the B-X 7-upsilon" vibrational progression from fluorescent spectra, for different rotational levels. In the present study, excitation of these levels is carried out on the B-X 7-0 band at 191-192 nm. We have investigated the lifetimes of both spin-orbit components of B 2-PI (upsilon = 7) and find that the decreasing interaction with the C 2-PI state with increasing rotational level, in spite of the increasing energy above the dissociation limit, leads to a steady increase in radiative lifetime. The value reached at high J is 350-400 ns, whereas the lifetimes of the OMEGA = 1/2 levels that lie below the dissociation limit approach a value of 600-700 ns, consistent with the extrapolation of lifetime data for upsilon = 0-6.