Fragment recoil anisotropies in the photoinitiated decomposition of HNCO

Abstract

The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) N3H+CO, (2) H+NCO, and (3) N1H+CO [where NH3 and NH1 denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the N1H+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of <1 ps (β<−0.6), and >5–10 ps (β≅0.0). With 230.1 nm photolysis, CO(J=0–14) originating in channel (3) is produced with β=−0.8±0.05 via direct dissociation on S1 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 correlated with NH1 in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J=14,15) gives rise to an isotropic recoil distribution, and a hotter correlated NH1 rotational distribution. At the same wavelength, CO correlated with NH3 is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The >5 ps time scale for the appearance of channel (2) implies dissociation on S0 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.