PHOTOFRAGMENT VECTOR CORRELATIONS MEASURED BY TRANSIENT ABSORPTION-SPECTROSCOPY - CN FRAGMENTS FROM ETHYL THIOCYANATE PHOTODISSOCIATION

The correlated angular distribution of translational and rotational mo tion of photofragments is probed by Doppler-broadened transient absorp tion line shapes. The one-photon nature of the probe process reduces t he complexity of the analysis, while maintaining sensitivity to five b ipolar moments of the translational and rotational angular distributio ns. The 193-nm photodissociation of C2H5SCN illustrates the method, wi th CN products detected with a titanium:sapphire ring laser probing th e A2PI-X2SIGMA+ transition. The vector correlations in the high rotati onal states of CN indicate a direct dissociation following a transitio n of mixed symmetry at 193 nm: the recoil is preferentially but not ex clusively parallel to a transition moment, and the perpendicular compo nent is dominated by a rotation axis parallel to a C2H5SCN transition moment. The strong rotational excitation is generated by planar bendin g forces in a bent excited state. Lower rotational states are formed w ith a composite kinetic energy distribution, indicating an additional, slower, less highly polarized channel that partitions more energy int o the unobserved C2H5S radical than does the direct channel. At 248 nm , a weaker absorption also generates CN photofragments with a preferen tially parallel recoil, requiring a reassessment of the excited-state assignments in the alkyl thiocyanates.