NONADIABATIC EFFECTS IN THE PHOTODISSOCIATION OF VIBRATIONALLY EXCITED HNCO - THE BRANCHING BETWEEN SINGLET (A(1)DELTA) AND TRIPLET (X(3)SIGMA(-))NH

Initial vibrational excitation of a state containing three quanta of N -H stretch (3 upsilon(1)) decreases the fractional photolysis yield of NH (a (1)Delta) relative to NH (X (3)Sigma(-)) by a factor of approxi mately two compared to the isoenergetic photodissociation of a 300 K t hermal sample of HNCO. At a total energy of 43 480 cm(-1) NH (a (1)Del ta) accounts for 24% of the total NH yield in the direct photolysis bu t only 10% in the photodissociation of 3 nu(1). At 44 440 cm(-1), the NH (a (1)Delta) yields are 65% and 32% in the single photon and two-st ep photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the prefe rential production of NCO in the photolysis of vibrationally excited H NCO. The initial vibrational excitation has no influence on the rotati onal and vibrational distributions of NH (X (3)Sigma(-)), but it signi ficantly increases the amount of energy in rotation of NH (a (1)Delta) . These results, along with several recent experimental and theoretica l studies, suggest the participation of at least three different poten tial energy surfaces in the photodissociation of isocyanic acid. (C) 1 998 American Institute of Physics.