Photodissociation dynamics of ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy $D_0^0\mathrm{(H–OH)}$ is determined to be $\text{41151±5\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}.$ Two clear alternations in the $\mathrm{OH}\text{(X,v=0)}$ rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited $\mathrm{OH}\mathrm{(X)}$ products (up to $\text{v=9)}$ have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in O–H–H geometries. The effect of parent rotational excitation on the $\mathrm{OH}\mathrm{(A)}$ product state distribution and anisotropy distribution was observed for the first time. Experimental results also show clear evidence for the triple dissociation channel, $\mathrm{O}{(}^3\mathrm{P)+}\mathrm{2H}.$ Accurate branching ratios of different product channels have been determined. Results of detailed QCT calculations agree well with the experimental results in this work.

Photodissociation of water at a wavelength of 121.6 nanometers has been investigated by using the H-atom Rydberg tagging technique. A striking even-odd intensity oscillation was observed in the OH(X) product rotational distribution. Model calculations attribute this oscillation to an unusual dynamical interference brought about by two dissociation pathways that pass through dissimilar conical intersections of potential energy surfaces, but result in the same products. The interference pattern and the OH product rotational distribution are sensitive to the positions and energies of the conical intersections, one with the atoms collinear as H–OH and the other as H–HO. An accurate simulation of the observations would provide a detailed test of global H ₂ O potential energy surfaces for the three (X̃/Ã/B̃) contributing states. The interference observed from the two conical intersection pathways provides a chemical analog of Young's well-known double-slit experiment.

The energy partition in the products of ethylene photodissociation (including ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_4,$ ${\mathrm{C}}_{\mathrm{2}}{\mathrm{D}}_4,$ ${\mathrm{D}}_{\mathrm{2}}{\mathrm{CCH}}_2,$ cis- and trans-HDCCDH) at 193 and 157 nm and the rate constants of H loss channels were computed based on ab initio ethylene ground-state surfaces of which most were reported earlier. In the calculations of the energy partitions, a simple model was used in which the excess energy above the transition state is distributed statistically and the energy released by the exit barrier is described by the modified impulsive model. The rate constants of the ethylene $\mathrm{H}\mathrm{(D)}$ elimination were calculated according to the variational RRKM (Rice–Ramsperger–Kassel–Marcus) theory, and the RRKM rate constants with tunneling corrections were obtained for vinyl decomposition at 193 nm. In contrast with previous conclusions drawn by LIF (laser induced fluorescence) studies, the rate constant calculations suggest that the H loss may be a nonstatistical process. However, the computed variational transition states for H loss appear reasonable as indicated by the translational energy. That with present investigation indicates that the atomic elimination proceeds via the predicted transition states though the process is nonstatistical. Analysis of the ${\mathrm{H}}_2$ translational energy measured at 193 and 157 nm by molecular beam experiments gives evidence that the overall mechanisms of the molecular elimination are different at the two wavelengths, which is also in disagreement with previous belief. At 193 nm, both ${\mathrm{H}}_2$ elimination channels may occur through the predicted transition states. On the other hand, further comparison of the theoretical and experimental translational energy of hydrogen molecule at 157 nm suggests that the observed $\text{(1,1E)}$ reaction path is most likely of much higher “exit barrier” than the one computed. For the $\text{(1,2E)}$ channel, the calculations are still in support of the computed transition state being the one along the experimentally observed pathway at 157 nm.