HF( v′ = 3) forward scattering in the F + H ₂ reaction: Shape resonance and slow-down mechanism

Abstract

Crossed molecular beam experiments and accurate quantum dynamics calculations have been carried out to address the long standing and intriguing issue of the forward scattering observed in the F ؉ H2 3 HF(v ‫؍‬ 3) ؉ H reaction. Our study reveals that forward scattering in the reaction channel is not caused by Feshbach or dynamical resonances as in the F ؉ H2 3 HF(v ‫؍‬ 2) ؉ H reaction. It is caused predominantly by the slow-down mechanism over the centrifugal barrier in the exit channel, with some small contribution from the shape resonance mechanism in a very small collision energy regime slightly above the HF(v ‫؍‬ 3) threshold. Our analysis also shows that forward scattering caused by dynamical resonances can very likely be accompanied by forward scattering in a different product vibrational state caused by a slow-down mechanism.chemical reaction dynamics ͉ crossed molecular beam experiment ͉ potential energy surface C hemical reactions occur when one reactant collides with another and some rearrangements among reactants take place along a path connecting reactants to products. The path is called the reaction coordinate for a chemical reaction, along which the reactants will go through an intimate region to reach the product side. In a typical chemical reaction with an energetic barrier, no discrete quantum structure could exist along the reaction coordinate. However, quantized states do exist along coordinates perpendicular to the reaction coordinate. For each quantized state, there is an effective, vibrationally adiabatic potential. In certain cases, transiently trapped quantum states could exist on these vibrational adiabatic potentials along the reaction coordinate. Such quasi-bound quantized states along the reaction coordinate in the intimate region of a chemical reaction are normally called dynamical resonances, or reaction resonances. Because reaction resonances are very sensitive to the potential energy surface governing a chemical reaction, they provide possibilities for probing the critical region of the potential energy surface more directly. As a result, reaction dynamics has been a central topic in the study of chemical reaction dynamics in the last few decades (1-4).Probing of dynamical resonances experimentally is essential to the study of the resonances in chemical reactions. A key signature of reaction resonance is the product forward scattering caused by the time delay of the reaction system trapped in quasi-bound resonance states. However, forward scattering in a scattering experiment does not necessarily come from reaction resonances. Recently, Zare and coworkers (5) have attributed the forward scattering in the H ϩ D 2 reaction to a time delay mechanism. In the study of the H ϩ HD system by Harich et al., the forward scattering was attributed to the time delay when the reaction system passes over a specific reaction barrier with little translational speed (6, 7). Therefore, distinguishing which mechanism is causing the time delay and the forward scattering product in a specific reaction h...

Dynamics of the C(1D)+H2reaction: A comparison of crossed molecular beam experiments with quantum mechanical and quasiclassical trajectory calculations on the first two singlet (11A′ and 11A″) potential energy surfaces

“…In recent years, quantum mechanical (QM) dynamical calculations on accurate potential energy surfaces (PESs) have been successful in reproducing detailed experimental observables, such as integral (ICSs) and differential cross sections (DCSs) for direct abstraction reactions (see Refs. [1][2][3] and references therein) and, latterly, for the more complex family of insertion reactions [4][5][6][7][8][9][10]. Insertion reactions occur on PESs with deep wells associated with bound intermediates that support a large number of quantum states.…”

A hierarchical construction scheme for accurate potential energy surface generation: An application to the F+H2 reaction

“…Quantum dynamics studies show that the FXZ PES is able to describe very well the resonance states in both the F + H 2 and F + HD reaction 23 and the slow-down mechanism in the F + H 2 → HF͑vЈ =3͒ + H reaction. 22 The FXZ PES was constructed in terms of the internal coordinates ͑r HH , r HF , and ͒ shown in Fig. 1͑a͒, where r HH is the distance between the two H atoms, r HF is the distance between F and one of the H atoms, and is the enclosed bond angle.…”

Shape resonance in the H + D2O → D + HOD reaction: a full-dimensional quantum dynamics study

“…5,24 To the best of our knowledge, shape resonances have been rarely investigated in any chemical reaction dynamics, though small contributions from a shape resonance mechanism in a narrow collision energy range were discovered to produce the forward scattering HF (v 0 ¼ 3) channel from the F + H 2 reaction. 23 In this study, we will provide the first evidence for a shape resonance in the tetra-atomic H + D 2 O / D + HOD reaction from full-dimensional quantum mechanical calculations.…”

“…Since the first theoretical prediction of reaction resonances in the F + H 2 reaction in the early 1970s, 9-11 the search for evidence of such resonances in this reaction has attracted much attention from many top research groups in this field. [12][13][14][15][16][17][18][19][20] Recently, the F + H 2 (HD) reaction was studied extensively in an effort to understand the dynamical resonances in this important system, 8,[21][22][23] using the high-resolution crossed molecular beams technique in combination with full quantum scattering calculation based on accurate potential energy surfaces. The advances made in these studies have provided a clear physical picture of dynamical resonances in this benchmark system that has eluded us for decades.…”