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.

Recent studies of state-resolved angular distributions show the participation of reactive scattering resonances in the simplest chemical reaction. This review is intended for those who wish to learn about the state-of-the-art in the study of the H + H2 reaction family that has made this breakthrough possible. This review is also intended for those who wish to gain insight into the nature of reactive scattering resonances. Following a tour across several fields of physics and chemistry where the concept of resonance has been crucial for the understanding of new phenomena, we offer an operational definition and taxonomy of reactive scattering resonances. We introduce simple intuitive models to illustrate each resonance type. We focus next on the last decade of H + H2 reaction dynamics. Emphasis is placed on the various experimental approaches that have been applied to the search for resonance behavior in the H + H2 reaction family. We conclude by sketching the road ahead in the study of H + H2 reactive scattering resonances.

Differential cross section polarization moments: Location of the D-atom transfer in the transition-state region for the reactions Cl+C 2 D 6 →DCl (v ′ =0,J ′ =1)+ C 2 D 5 and Cl+CD 4 →DCl (v ′ =0,J ′ =1)+ CD 3 A 1:4 mixture of HBr and D 2 is expanded into a vacuum chamber, fast H atoms are generated by photolysis of HBr ca. 210 nm, and the resulting HD (vЈ, JЈ) products are detected by (2ϩ1) resonance-enhanced multiphoton ionization ͑REMPI͒ in a Wiley-McLaren time-of-flight spectrometer. The photoloc technique allows a direct inversion of HD (vЈ, JЈ) core-extracted time-of-flight profiles into differential cross sections for the HϩD 2 →HD(vЈϭ1, JЈϭ1,5,8)ϩD reactions at collision energies ca. 1.7 eV. The data reveal a systematic trend from narrow, completely backward scattering for HD (vЈϭ1, JЈϭ1) toward broader, side scattering for HD (vЈϭ1, JЈϭ8). A calculation based on the line of centers model with nearly elastic specular scattering accounts qualitatively for the observations.

Product state(s)-resolved differential cross section of the reaction O ( 1 D)+ HD→OH (v,j)+ D The photoloc technique with core extraction of the nascent product laboratory speed distribution in a Wiley-McLaren time-of-flight spectrometer has been used to measure differential cross sections for the reaction HϩD 2˜H D (vЈϭ2, JЈϭ0,3,5)ϩD at collision energies ϳ1.55 eV. We find that the peak of each angular distribution shifts from complete backward scattering toward side scattering as the rotational excitation of the product increases. We found the same trend in our previous study of HϩD 2˜H D (vЈϭ1, JЈϭ1,5,8)ϩD at ϳ1.70 eV. We conclude that the same type of correlation exists between impact parameter and rotational quantum number in both product vibrational manifolds. Further analysis of the HD (vЈϭ2, JЈ) differential cross section data reveals, however, a clear tendency of this vibrational manifold to scatter sideways at lower JЈ than HD(vЈϭ1, JЈ). Within the framework of a line-of-centers model with nearly elastic specular scattering, this result implies that smaller impact parameters lead to more vibrationally excited products.