Doppler line shapes in the photolysis of laser excited, aligned molecules:Application to the vibrationally mediated photodissociation of HN3

A general expression for the Doppler profile for fragments produced in the photodissociation of laser excited, aligned molecules, as in vibrationally mediated photodissociation (VMP), is presented. In contrast to one-photon d issociation, for which the Doppler profile depends only on the second momen t of the fragment molecular-frame angular distribution, the profile in the case of VMP is sensitive to several moments of the angular distribution, up to k=4. In addition, the profile for a near-prolate molecule depends on th e angle theta(a) between the electronic transition moment mu and the a iner tial axis. This theory is applied to the analysis and interpretation of Dop pler profiles in the laser fluorescence detection of NH(a (1)Delta) fragmen ts, of rotational angular momenta N=7 and 10, from the 532 nm VMP of HN3 ex cited to the second N-H stretch overtone level (3 nu(1)). For both Lambda-d oublets of these rotational levels, the second moment of the molecular-fram e angular distribution beta(2,0)=[P-2(cos theta(m))] was found to be positi ve, in agreement with previous results for high-J fragments from one-photon photolysis of HN3. The profiles are consistent with a value of similar to 0 degrees for the angle theta(a). These values for theta(a) and beta(2,0) a re inconsistent with simple expectations based on planar, prompt dissociati on upon excitation to the lowest singlet excited state ((1)A ") and suggest the importance of nonplanar geometries in the dissociation dynamics. (C) 1 999 American Institute of Physics. [S0021-9606(99)00325-6].