VECTOR AND SCALAR CORRELATIONS IN STATISTICAL DISSOCIATION - THE PHOTODISSOCIATION OF NCCN AT 193 NM

Nascent Doppler profiles of CN (X(2) Sigma(+)) fragments from the 193 nm photodissociation of NCCN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. This new m ethod is highly suited for Doppler spectroscopy of nascent photoproduc ts. The experimental line shapes suggest an asymptotic available energ y of 5300 +/- 100 cm(-1) and are well described by a model in which th e available energy is partitioned between a statistical reservoir (470 0 cm(-1)) and a modest exit barrier (600 cm(-1)). we have determined s tate dependent v . j correlations. A trend of j becoming increasingly perpendicular to v for the higher rotational states is in accord with phase space theory, although the observed correlations are more than t wice as strong. The v . j correlations can be quantitatively modeled b y further restricting the phase space model with an approximate conser vation of the K-quantum number, the projection of total angular moment um about the linear axis of NCCN. Global rotational and vibrational pr oduct distributions have also been measured. The highest accessible ro tational states are underpopulated, compared to a phase space calculat ion. The global vibrational distribution is substantially colder than the phase space theory predictions. Vibrational branching ratios for c oincident fragments have been measured as a function of the detected C N state from a close analysis of high signal-to-noise Doppler profiles . The correlated vibrational distribution, P(upsilon(1), upsilon(2)), shows an excess of vibrationless coincident fragments, at the expense of dissociation to give one ground state and one vibrationally excited CN fragment. The correlated formation of two vibrationally excited CN fragments is as likely as the phase space prediction, yet the formati on of upsilon = 2 is strongly suppressed. The fragment vector and scal ar correlations provide a highly detailed view of the loose transition state typical for reactions well described by statistical reaction th eories. (C) 1997 American Institute of Physics.