The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the à state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and à states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the à state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.

The photodissociation dynamics of jet-cooled propionaldehyde have been investigated at a wavelength of 309.1 nm by monitoring the resultant nascent HCO fragments by laser induced fluorescence spectroscopy.HCO was formed only in the %(O,O,O) state. The population distribution of different rotational states characterized by Nand K, is reasonably described by a Boltzmann distribution at a temperature of 480 f 50 K, which corresponds to an average energy in rotation of 6.0 f 0.6 kJ mol-'. Careful measurement of the width of individual K, = 0 lines in the LIF spectrum revealed that the average translational energy of the fragments is 23 f 4 kJ mol-' of HCO. These measurements have allowed us to estimate that the ethyl radical sibling fragment is born with almost no internal energy. The observed energy partitioning in the fragments is consistent with a model in which the HCO rotational and translational excitation is determined mostly by the fixed energy in the exit channel. By analogy with acetaldehyde and considering the lack of vibrational excitation, the barrier to dissociation is predicted to lie around 15 kJ mol-' below the photon energy.

The dynamics of the photolysis reaction, CFBr+hν→CF+Br, have been investigated for photolysis energies in the range, ν̄=23 500–26 000 cm−1 (λ=385–435 nm). These energies correspond to excitation into the Ã(1A″) state of CFBr with 2500–5000 cm−1 of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (Ω, Λ, J) of the nascent CF fragments (X 2Π, υ=0) was probed by laser induced fluorescence spectroscopy. Two distinct types of product state distributions were observed. At energies above T00+3360 cm−1 the populations of the Π1/22 and Π3/22 spin–orbit states of CF were equal, while A″ lambda doublet states were preferred over A′. These populations are consistent with a direct dissociation mechanism on the à state, over a barrier with a height of 3360 cm−1. The strong state mixing in the vicinity of the barrier ensures a statistical mixture of final spin–orbit states. The preference for the A″ lambda doublet states is consistent with the two lone electrons in in-plane orbitals pairing up in the final CF product, leaving one unpaired electron in an out-of-plane orbital, lying parallel to the J vector of the recoiling fragment. For excitation at energies below T00+3360 cm−1 the ground spin–orbit state of CF (2Π1/2) is preferred, while the lambda doublet populations are equal. The interpretation of these populations is that at these energies à state CFBr is stable with respect to dissociation over the barrier. The molecule crosses to either the X̃ or ã state where it encounters a deep attractive potential well. The subsequent slower dissociation rate allows the molecule to follow a more adiabatic pathway producing the lowest spin–orbit state of CF, and for any preference for lambda doublet states to be lost.

The dynamics of NO2 dissociation at 309.1 nm have been explored by examining the nascent distribution of NO rotational, vibrational, spin–orbit, and lambda-doublet states. The NO fragment is produced with a monotonically decreasing vibrational distribution over the energetically accessible vibrational states (υ=0–3), and nonstatistical rotational distributions within each vibrational manifold. The distribution within υ=0 and 1 is strongly peaked near J=25.5 with a fairly narrow spread, the distribution within υ=2 is fairly flat, terminating at the limit of available energy, and the υ=3 distribution is oscillatory, also terminating at the limit of available energy. The 2Π1/2 spin–orbit state is more strongly populated than the 2Π3/2 state by a factor of 1.9 for every vibrational state. The differences in lambda-doublet populations are, in general, minor; each Λ state being roughly equally populated, although oscillations are again evident. These results are discussed in relation to results at similar available energy at room temperature and in the free jet at different available energies. It is found that the results are intermediate between the previous data at low excess energy and at high available energy, the distributions showing aspects of both regimes. From the data it is inferred that the dissociation dynamics of NO2 vary continuously from a regime where phase space theory considerations with quantum overtones dominate the product state distributions to the regime where dynamics on the exit channel determine the distributions.

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