Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H+H2 --> H2+H to occur predominantly through a 'direct recoil' mechanism: the H--H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H2 product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process. Indirect exchange mechanisms involving H3 intermediates have been suggested to occur as well, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H+D2 --> HD+D, forward scattering features observed in the product angular distribution have been attributed to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.

A 212.8 nm laser initiates the reaction H+D2→HD+D in a mixture of HBr and D2. A second laser state-selectively ionizes the HD(v′=3,j′) reaction product, allowing a determination of the speed distribution and the relative cross section in a velocity-sensitive time-of-flight mass spectrometer. From these measurements we construct differential and integral cross sections for H+D2→HD(v′=3,j′=0–7)+D at 1.64±0.05 eV collision energy. Although the integral cross sections do not show any unusual features, the differential cross sections reveal forward-scattered features that have not been observed in crossed-beam experiments. An analysis of the scattering features in HD(v′=3,j′=1–4) suggests that these states are dominated by classical hard-sphere scattering. This hard-sphere (direct recoil) mechanism, however, cannot account for the dominant forward scattering observed in HD(v′=3,j′=0).

Scattering of the HD(v′=3,j′=0) product from the H+D2 reaction is measured as a function of angle and collision energy from 1.39 to 1.85 eV. The plot of the cross section vs angle and energy is believed to be the first fully experimental plot of its kind reported for this benchmark reaction. Changes in the differential cross section (DCS) are observed in this collision energy range, including a forward-scattering component that peaks at about 1.64 eV and is a strong function of collision energy. This feature has been assigned to result from a barrier resonance, but its full interpretation is presently unsettled. These changes in the DCS do not manifest themselves as variations in the integral cross section (ICS), which varies less than 25% over the energy range measured. Comparisons of the DCSs and the ICS with quantum mechanical calculations show quantitative agreement, although some aspects of the DCS near 1.54 eV are not fully satisfactory.

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H 2 Molecule / Vibrationally Excited / 2+1 REMPI / Rotational AnisotropyWe report the spectroscopic detection of vibrationally excited molecular hydrogen using 2ϩ1 resonantly enhanced multiphoton ionization (REMPI) via the I 1 Π g (v′ ϭ 0) Ϫ X 1 Σ ϩ g (v″ ϭ 3) band ca. 198 nm. Vibrationally excited H 2 was produced by passing roomtemperature hydrogen over a hot ion gauge filament in a high-vacuum chamber. The internal energy distributions were characterized spectroscopically by use of the EF 1 Σ ϩ g ϪX 1 Σ ϩ g 2ϩ1 REMPI detection scheme. We have identified band origins for the S, Q, R, and P rotational branches of the I-X (0,3) band, as well as isolated lines corresponding to two-photon transitions into other nearby H 2 gerade states, including EF 1 Σ ϩ g (v′ ϭ 2, 3, 4), GK 1 Σ ϩ g (v′ ϭ 1), and J 1 ∆g (v′ ϭ 0). We propose the I-X transition as a suitable candidate for the determination of the rotational anisotropy of vibrationally excited ground-state H2 molecules. We support this contention with a calculation of the line strength moments and sensitivities to the second-(quadrupolar) and fourth-rank (hexadecapolar) moments of the rotational angular momentum distributions, which is compared against the well-established Q-branch members of the EF 1 Σ ϩ g ϪX 1 Σ ϩ g two-photon transition.