Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H ₂ → HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v ′ = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF( v ′ = 3)-H′ vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.

Dissociative chemisorption of CH₄ on transition-metal surfaces, representing the rate-limiting step in methane steam reforming, has been shown experimentally to be strongly mode selective. To understand the mode selectivity, a twelve-dimensional global potential energy surface is developed for CH₄ interacting with a rigid Ni(111) surface based on a large number of density functional theory points. The reaction dynamics is investigated using an eight-dimensional quantum model, which includes representatives of all four vibrational modes of methane. After correcting for surface effects, key experimental observations, including the mode selectivity, are well reproduced. These theoretical results, along with mechanistic analysis, provide insights into this industrially important heterogeneous reaction.

A new global potential-energy surface for the ground electronic state of $\mathrm{H}{\mathrm{O}}_2\left(X^2{A}^{″}\right)$ has been developed by three-dimensional cubic spline interpolation of more than 15 000 ab initio points, which were calculated at the multireference configuration-interaction level with Davidson correction using the augmented correlation-consistent polarized valence quadruple zeta basis set. Low-lying vibrational states were obtained in this new potential using the Lanczos method and assigned. The calculated vibrational frequencies are in much better agreement with the available experimental band origins than those obtained from a previous potential. In addition, rate constants for the $\mathrm{H}+{\mathrm{O}}_2\leftrightarrow \mathrm{O}+\mathrm{O}\mathrm{H}$ reactions were obtained using a wave-packet-based statistical model. Reasonably good agreement with experimental data was obtained. These results demonstrate the accuracy of the potential.

The dissociative chemisorption of water is an important step in many heterogeneous catalytic processes. Here, the mode selectivity of this process was examined quantum mechanically on a realistic potential energy surface determined by fitting planewave density functional calculations spanning a large configuration space. The quantum dynamics of the surface reaction were characterized by a six-dimensional model including all important internal coordinates of H₂O and its distance to the surface. It was found that excitations in all three vibrational modes are capable of enhancing reactivity more effectively than increasing translational energy, consistent with the “late” transition state in the reaction path.

Signatures of mass-independent isotope fractionation (MIF) are found in the oxygen (¹⁶O,¹⁷O,¹⁸O) and sulfur (³²S,³³S,³⁴S,³⁶S) isotope systems and serve as important tracers of past and present atmospheric processes. These unique isotope signatures signify the breakdown of the traditional theory of isotope fractionation, but the physical chemistry of these isotope effects remains poorly understood. We report the production of large sulfur isotope MIF, with Δ³³S up to 78‰ and Δ³⁶S up to 110‰, from the broadband excitation of SO₂in the 250–350-nm absorption region. Acetylene is used to selectively trap the triplet-state SO₂( ³B₁), which results from intersystem crossing from the excited singlet ( ¹A₂/ ¹B₁) states. The observed MIF signature differs considerably from that predicted by isotopologue-specific absorption cross-sections of SO₂and is insensitive to the wavelength region of excitation (above or below 300 nm), suggesting that the MIF originates not from the initial excitation of SO₂to the singlet states but from an isotope selective spin–orbit interaction between the singlet ( ¹A₂/ ¹B₁) and triplet ( ³B₁) manifolds. Calculations based on high-level potential energy surfaces of the multiple excited states show a considerable lifetime anomaly for³³SO₂and³⁶SO₂for the low vibrational levels of the ¹A₂state. These results demonstrate that the isotope selectivity of accidental near-resonance interactions between states is of critical importance in understanding the origin of MIF in photochemical systems.