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

The correlated angular distribution of translational and rotational motion of photofragments is probed by Doppler-broadened transient absorption line shapes. The one-photon nature of the probe process reduces the complexity of the analysis, while maintaining sensitivity to five bipolar moments of the translational and rotational angular distributions. The 193-nm photodissociation of C2H5SCN illustrates the method, with CN products detected with a titanium:sapphire ring laser probing the A2PI-X2SIGMA+ transition. The vector correlations in the high rotational states of CN indicate a direct dissociation following a transition of mixed symmetry at 193 nm: the recoil is preferentially but not exclusively parallel to a transition moment, and the perpendicular component is dominated by a rotation axis parallel to a C2H5SCN transition moment. The strong rotational excitation is generated by planar bending forces in a bent excited state. Lower rotational states are formed with a composite kinetic energy distribution, indicating an additional, slower, less highly polarized channel that partitions more energy into the unobserved C2H5S radical than does the direct channel. At 248 nm, a weaker absorption also generates CN photofragments with a preferentially parallel recoil, requiring a reassessment of the excited-state assignments in the alkyl thiocyanates.