No abstract

Total differential scattering angular distribution measurements for He scattering from Cl 2 are reported. The scattering data are combined with previous excitation spectroscopy and vibrational predissociation product state distributions for He-CI 2 to determine the potential-energy surfaces for the ground X and excited B electronic states. The potentials are somewhat deeper, De= -38.2 cm-I for the X state, than previously estimated. The X state potential minimum is for the "T" shaped configuration with a Rmin value of 3.55 A. The potentials are quite successful at describing the scattering data, the rotationally resolved spectroscopy of the complex, and the product state distributions for vibrational predissociation. The B state potential underestimates the dependence of the rate of vibrational predissociationon the initial vibrational level.

High-resolution crossed molecular beam measurements of the total differential cross sections (DCS) for the scattering of He by O2, N2, and NO in the thermal energy range are reported. The data show well resolved diffraction oscillations which appear damped with respect to the corresponding isotropic He–Ar case. Information on the anisotropy of the interaction is indirectly obtained from the damping of the diffraction oscillations within the framework of the infinite-order-sudden (IOS) approximation. While large discrepancies, within the same scheme of analysis, are found with respect to anisotropy estimates from similar experiments, but performed at a lower resolution, good agreement is observed with respect to those obtained for He–O2 and He–N2 from state-to-state rotationally inelastic DCS measurements. Potential energy surfaces (PES) are derived for all systems by also simultaneously fitting absolute total integral cross sections and second virial coefficient data. Comparisons are made with previous both empirical and theoretical potentials, which also have been obtained from or tested against experimental bulk and scattering properties within the IOS approximation. The derived He–O2 surface is found to be in good agreement with recent results. For He–N2 we propose an empirical PES which predicts several independent scattering and bulk data. For He–NO the surface obtained is an improvement with respect to that previously proposed.

The anisotropic potential energy surface of He–CO2 is determined by the simultaneous analysis of newly measured high resolution total differential cross sections, differential energy loss spectra, new low temperature second virial coefficients, new diffusion, and viscosity data. The calculations are carried out in the infinite-order-sudden approximation. The repulsive anisotropy of the potential is determined from the rotationally inelastic cross sections and the quenching of the diffraction oscillations, while the absolute scale is fixed by the position of these oscillations. The second virial coefficient data are essentially sensitive to the general features of the spherical effective potential well. The transport data are then correctly predicted by this potential surface which differs both in the anisotropy and the spherical part from the recently derived multiproperty fit potential for this system.

The reaction o('D) + HBr has been investigated by the crossed molecular beams and infrared chemiluminescence methods in an effort to characterize the dynamics of both possible reactive channels. The angular and velocity distribution of the BrO product from theo('D) + HBr + BrO + H pathway have been obtained in crossed beam experiments at collision energies, E,, of 5.0 and 14.0 kcal/mol. The product center-of-mass angular distribution is found to be almost backward-forward symmetric at both E,, with backward scattering being slightly favored, from which it is deduced that two processes contribute to this channel: a dominant one occurring via formation of a long-lived complex, following o('D) insertion, and another one occurring via direct abstraction of the halogen atom and giving rise to a rebound dynamics. The large fraction (~5 0 % ) of available energy released into translation indicates the existence of a potential barrier for H-displacement in the exit channel. From energy and angular momentum conservation arguments, it is inferred that BrO is formed rotationally very hot in the lowest vibrational levels of both 2r1312 and 211112 electronic states. The initial vibrational distribution of the OH product from the o('D) + HBr + OH + Br channel has been measured using fast time-resolved Fourier transform spectroscopy. The vibrational distribution is strongly inverted, from which it is deduced that the HOBr intermediate dissociates very rapidly, before energy randomization occurs. A lower limit to the branching ratio of the relative cross sections for the BrO + H and OH + Br channels is derived (u(Br0 + H)/u(OH + Br) 2 0.16 ? 0.07) and compared to recent bulk work. The dynamical results for the overall reaction are discussed with reference to the relevant singlet and triplet potential energy surfaces and possible molecular configurations involved. Comparison with the dynamics of the ground state reaction o (~P ) + HBr + OH + Br is carried out also, to examine the effect of electronic excitation on the dynamics of the reactions of atomic oxygen with hydrogen halides.