The spectroscopy and photodissociation dynamics of the vinoxy ͑CH 2 CHO͒ radical B(2 AЉ) ←X(2 AЉ) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: ͑1͒ CH 3 ϩCO, and ͑2͒ HϩCH 2 CO, with a branching ratio of Ϸ1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH 3 ϩCO, and a considerably smaller barrier for HϩCH 2 CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the Ã(2 AЈ) or C(2 AЈ) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions.

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B̃(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three-body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B̃ state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B̃ state surface; their rotational excitation results from the marked angular anisotropy of the B̃ state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower Ã(1B1) or X̃(1A1) surfaces. The wavepacket calculations show that B̃■X̃ transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B̃■Ã radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a-axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.

A general formulation is presented to model photodissociation processes in which internal conversion is followed by unimolecular dissociation over an exit barrier; this classification of dissociation mechanism results in a nonstatistical product state distribution. The energy available to products is divided into independent statistical and impulsive energy reservoirs. The statistical reservoir considers direct projections of a vibrational microcanonical ensemble at the transition state ͑TS͒ onto product quantum states, conserving vibrational adiabaticity and angular momentum. The impulsive reservoir represents the energy released in passing from the TS to products; this reservoir is treated assuming sudden dissociation of the zero-point TS wave function using a combination of Franck-Condon and impulsive models. We derive the statistical adiabatic impulsive model, which convolutes these two energy reservoirs, to predict the product translational energy distribution for nonstatistical dissociation over a barrier. Two test cases are modeled and compared with experimental data: unimolecular dissociation of acetyl radicals and photodissociation of vinoxy radicals via the B 2 AЉ-X 2 AЉ band.