VECTOR AND SCALAR CORRELATIONS IN THE PHOTODISSOCIATION OF NCCN

The CN photofragments from the photodissociation of NCCN at 193 nm hav e been measured by high-resolution transient absorption spectroscopy. Doppler-broadened profiles of isolated rotational lines in the 2-0 and 3-1 vibrational bands of the CN A-X transition were observed under co llisionless conditions with a tunable, single-frequency Ti:sapphire ri ng laser. Analysis of the Doppler profiles reveals a vector correlatio n between the translation and rotation of CN photoproducts, with the a ngular momentum of the high rotational states increasingly perpendicul ar to the recoil velocity. After correction for vector correlations, t he laboratory-frame scalar speed distribution of state-selected photop roducts can be determined. The mean squared laboratory velocity is dir ectly related to the average internal energy of coincident CN fragment s. The wings of the Doppler profiles indicate that the available energ y for a pair of ground state CN photoproducts following 193 nm dissoci ation of NCCN at 295 K is 5300 +/- 150 cm-1, which includes the averag e vibrational energy of the parent molecules selected by the photolysi s laser. Phase space theory with an optimized available energy of 5300 cm-1 produces laboratory speed distributions that are in qualitativel y reasonable agreement with the kinetic energy measurements, but overe stimate the total internal energy of the photofragments. The measureme nts are good enough to warrant comparison with more sophisticated mode ls of unimolecular decomposition.