COMPETITION BETWEEN SINGLET AND TRIPLET CHANNELS IN THE PHOTOINITIATED DECOMPOSITION OF HNCO

The spin-forbidden channel, NH(X (3) Sigma(-))+CO, has been observed d irectly in the photodissociation of jet-cooled HNCO following S-1<--S- 0 excitation. The NH(X (3) Sigma(-)) yield spectrum is obtained in the energy regimes near the thresholds to H+NCO and NH(a (1) Delta)+CO ch annels. The spectrum is similar to the NCO yield spectrum when direct dissociation on S-1 is not significant. At photolysis energies >43 400 cm(-1), state specific differences between the NH(X (3) Sigma(-)), NH (a (1) Delta) and NCO yield spectra are observed, and at energies >44 000 cm(-1) all structure in the NH(X (3) Sigma(-)) yield spectrum is l ost, while the NH(a (1) Delta) yield spectrum remains structured. The results are interpreted in terms of the different time scales for inte rsystem crossing and dissociation. (C) 1997 American Institute of Phys ics.