Photofragment vector correlations measured by transient absorption spectroscopy: cyanogen fragments from ethyl thiocyanate photodissociation

Abstract

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 C[sub 2]H[sub 5]SCN illustrates the method, with CN products detected with a titanium:sapphire ring laser probing the A[sup 2][Phi]-[Chi][sup 2][Sigma][sup +] 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 C[sub 2]H[sub 5]SCN 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 C[sub 2]H[sub 5]S 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 alkylmore » thiocyanates. 42 refs., 7 figs., 7 tabs.« less