Direct observation of forward-scattering oscillations in the H+HD→H2+D reaction

Abstract

Accurate measurements of product state-resolved angular distributions are central to fundamental studies of chemical reaction dynamics. Yet, fine quantum-mechanical structures in product angular distributions of a reactive scattering process, such as the fast oscillations in the forward-scattering direction, have never been observed experimentally and the nature of these oscillations has not been fully explored. Here we report the crossed-molecular-beam experimental observation of these fast forward-scattering oscillations in the product angular distribution of the benchmark chemical reaction, H + HD → H + D. Clear oscillatory structures are observed for the H(v' = 0, j' = 1, 3) product states at a collision energy of 1.35 eV, in excellent agreement with the quantum-mechanical dynamics calculations. Our analysis reveals that the oscillatory forward-scattering components are mainly contributed by the total angular momentum J around 28. The partial waves and impact parameters responsible for the forward scatterings are also determined from these observed oscillations, providing crucial dynamics information on the transient reaction process.

Observation of the geometric phase effect in the H+HD→H2+D reaction below the conical intersection

“…1) exhibits a series of well-resolved rings coming from different ro-vibrational states of the H 2 product, which are clearly assignable. Fine oscillatory features in the forward angular distributions are successfully captured, similar to those observed at 1.35 eV and 2.77 eV 21,36 . There are two very pronounced rings labeled by yellow arrows in Fig.…”

“…Later, it was found that the adiabatic quantum dynamics predictions without the GP (i.e. NGP) agree with experiments at some collision energies quite well, suggesting that the GP may not be important in this reaction and its isotopologues 16 – 21 . Mead and Truhlar found that the GP would only influence observables when the nuclear wavefunction encircled the CI, and the GP effect could be involved by the introduction of a vector potential 22 .…”

“…Very recently, with a high-resolution time-sliced velocity map imaging (VMI) -CMB apparatus, we detected D products of the H+HD reactions by using the near threshold ionization scheme 21,36 . The product ro-vibrational state-resolved DCS with fast oscillatory structures in the forward angular distributions have been observed at the collision energies of 1.35 and 2.77 eV 21,36 . At 1.35 eV, which is far below the CI, the reaction appears to take place via a simple direct abstraction mechanism, where the GP plays a negligible role.…”

Dynamical interference in the vibronic bond breaking reaction of HCO

“…One such example is the inelastic scattering of NO by Ar ( 2 , 3 ), where a Δ j = even propensity can be attributed to the weakly broken symmetry of the diatom ( 4 ). Various forms of interferences have also been found in reactive scattering ( 5 – 7 ). More closely related to the current work is the photodissociation of water in its trueB~ band, where nonadiabatic transitions between the trueB~1italicA′ and trueX~1italicA′ states of H 2 O near two conical intersections (HOH and HHO) provide distinct dissociation pathways leading to the same OH + H products, manifested by oscillations in the OH rotational state distribution ( 8 ).…”

Inelastic Scattering of H Atoms from Surfaces

“…Conventional crossed molecular beam methods were applied 5 − 10 and then improved with the use of photolytic atom sources. 11 − 14 The development of resonance enhanced multiphoton ionization (REMPI) 15 − 19 led to field free ion time-of-flight (TOF), Rydberg atom tagging detection 20 − 36 and ion imaging 37 − 39 eventually enabling measurement of the differential scattering cross sections for selected quantum states of the reaction products with controlled incidence translational energy. As experiments improved, so did theory, and by the end of the last century, the technology was sufficiently developed so that converged quantum dynamics calculations on an accurate Born–Oppenheimer potential energy surface (PES) could be achieved.…”