From Reactive Rainbow to Dynamic Resonance Well

Odyssey in the Wonderland of Chemical Dynamics

This is a recollection of my scientific trajectory. When I look back, I consider myself to be very fortunate for being able to do something I love and on topics of my own will. I am not a competitive person and tend to shy away from the limelight. Nonetheless, I survived in my profession and eventually made some modest contributions, which are beyond what I would have expected. We often forget about the human aspect of scientific endeavor. After all, science is done by individuals; humans have emotions and make mistakes. The frustrations of failures, the joys of finding problems and solutions to them, and the passion for fulfilling curiosity are all parts of this endeavor. Throughout the years, many people-mentors, students, postdocs, collaborators, and colleagues-have accompanied me in this exciting and fruitful journey, for which I am deeply grateful and feel very lucky to have them.

Vibrational State-Resolved Differential Cross Sections of the F + CH4 → HF + CH3 Reaction at the Collision Energies of 3.1-13.8 kcal/mol

We report a high-resolution crossed molecular beam experiment investigating the reaction dynamics of the F + CH4 → HF + CH3 reaction across a broad range of collision energies (3.1-13.8 kcal/mol). By using time-sliced velocity map imaging within the crossed molecular beam apparatus, we obtain correlated angular distributions and branching ratios for various product pairs (CH3(ν), HF(ν')). The resolved reactive rainbow-like features that display a distinct bulge in the angular distribution in different channels reveal the formation of vibrationally ground and excited states of the methyl radical accompanied by different rovibrationally excited HF products. These findings suggest distinct reaction dynamics for channels leading to vibrationally excited methyl radicals compared to those forming ground-state methyl radicals.

Near-ambient pressure velocity map imaging

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the “pressure gap” between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 × 10−3 mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

Glories, hidden rainbows and nearside-farside interference effects in the angular scattering of the state-to-state H + HD → H2 + D reaction

Yuan et al. [Nat. Chem., 2018, 10, 653] have reported state-of-the-art measurements of differential cross sections (DCSs) for the H + HD → H2 + D reaction, measuring for the first time fast oscillations in the small-angle forward region of the DCSs. We theoretically analyse the angular scattering dynamics in order to quantitatively understand the physical content of structure in the DCSs. We study the H + HD(vi = 0, ji = 0, mi = 0) → H2(vf = 0, jf = 0,1,2,3, mf = 0) + D reaction for the whole range of scattering angles from θR = 0° to θR = 180°, where v, j, m are the vibrational, rotational and helicity quantum numbers respectively for the initial and final states. The restriction to mf = 0 arises because states with mf ≠ 0 have DCSs that are identically zero in the forward (θR = 0°) and backward (θR = 180°) directions. We use accurate quantum scattering matrix elements computed by Yuan et al. at a translational energy of 1.35 eV for the BKMP2 potential energy surface. The following theoretical techniques are employed to analyse the DCSs: (a) full and nearside-farside (NF) partial wave series (PWS) and local angular momentum theory, including resummations of the full PWS up to third order. We also use window representations of the scattering matrix, which give rise to truncated PWS, (b) six asymptotic (semiclassical) small-angle glory theories and four N rainbow theories, (c) we introduce "CoroGlo" tests, which let us distinguish between glory and corona scattering at small angles for Legendre PWS, (d) the semiclassical optical model (SOM) of Herschbach is employed to understand structure in the DCSs at intermediate and large angles. Our conclusions are: (a) the small-angle peaks in the DCSs arise mainly from glory scattering. For the 000 → 020 transition, there is also a contribution from a broad, or hidden, N rainbow, (b) at larger angles, the fast oscillations in the DCSs arise from NF interference, (c) the N scattering in the fast oscillation region contains a hidden rainbow for the 000, 020, 030 cases. For the 000 → 020 transition, the rainbow extends up to θR ≈ 60°; for the 000 and 030 cases, the angular ranges containing a N rainbow are smaller, (d) at intermediate and backward angles, the slowly varying DCSs, which merge into slow oscillations, are explained by the SOM. Physically it shows this structure in a DCS arises from direct scattering and is a distorted mirror image of the corresponding probability versus total angular momentum quantum number plot.