Fragment recoil anisotropies in the photoinitiated decomposition of HNCO

The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, beta, and correlated state distributions in the S-1 ((1)A ")<--S-0((1)A') photoinitiated decomposition of HNCO into three compe ting channels: (1) (NH)-N-3+CO, (2) H+NCO, and (3) (NH)-N-1+CO [where (NH)- N-3 and (NH)-N-1 denote NH(X(3)Sigma(-)) and NH(a(1)Delta), respectively]. In particular, the region in the vicinity of the (NH)-N-1+CO threshold is i nvestigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of <1 ps (beta<-0.6), and >5-10 ps (beta cong ruent to 0.0). With 230.1 nm photolysis, CO(J = 0-14) originating in channe l (3) is produced with beta = -0.8 +/- 0.05 via direct dissociation on S-1 above a barrier of 470 +/- 60 cm(-1). CO at low J-states appears with most of the available energy in the translational degree of freedom and is corre lated with (NH)-N-1 in its lowest rotational states. A small contribution t o channel (3) from S-0 dissociation (observed mainly for J = 14, 15) gives rise to an isotropic recoil distribution, and a hotter correlated (NH)-N-1 rotational distribution. At the same wavelength, CO correlated with (NH)-N- 3 is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S-1 --> S-0 --> T-1. H-atom signals from channel (2) have isotropic angu lar distributions at photolysis wavelengths 243-215 nm; this places a lower limit of 8140 cm(-1) on the barrier to direct dissociation on S-1 to chann el (2). The >5 ps time scale for the appearance of channel (2) implies diss ociation on S-0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings. (C) 1999 American Institute of Physics. [S0021-9606(99)01704-3].