Benchmarking the polyatomic reaction dynamics of X+methane

Imaging the Ion-Molecule Reaction Dynamics of O- + CD4

While neutral reactions involved in methane oxidation have been intensively studied, much less information is known about the reaction dynamics of the oxygen radical anion with methane. Here, we study the scattering dynamics of this anion-molecule reaction using crossed-beam velocity map imaging with deuterated methane. Differential scattering cross sections for the deuterium abstraction channel have been determined at relative collision energies between 0.2 and 1.5 eV and ab initio calculations of the important stationary points along the reaction pathway have been performed. At lower collision energies, direct backscattering and indirect complex-mediated reaction dynamics are observed, whereas at higher energies, sideways deuterium stripping dominates the reaction. Above 0.7 eV collision energy, a suppressed cross section is observed at low product ion velocities, which is likely caused by the endoergic pathway of combined deuteron/deuterium transfer, forming heavy water. The measured product internal energy is attributed mainly to the low-lying deformation and out-of-plane bending vibrations of the methyl radical product. The results are compared with a previous crossed-beam result for the reaction of oxygen anions with nondeuterated ̧methane and with the related neutral-neutral reactions, showing similar dynamics and qualitative agreement.

A Study on the Effect and Suppression of Hydrogen Permeation Behavior on the Friction Characteristics of PEEK/PTFE Composites via Molecular Dynamics Simulation

To research the effect of hydrogen permeation on the friction characteristics of the seal materials on the hydrogen equipment, the molecular models of 10% PEEK/PTFE composites and its frictional models were established, respectively, and molecular dynamics (MDs) and giant canonical Monte Carlo (GCMC) methods were used to simulate the diffusion coefficient, dissolution coefficient and permeability coefficient of the hydrogen in PEEK/PTFE composites. The effect of a different amount of hydrogen on the friction and wear of PEEK/PTFE composites was also studied. The results showed that few permeations of the hydrogen gas mainly demonstrated having a positive effect on the surface of the PEEK/PTFE composites, and the wear rate of the PEEK/PTFE composites showed a slight decreasing trend. The wear rate of the PEEK/PTFE composites gradually decreased when more hydrogen molecules penetrated the matrix. With the further penetration of the hydrogen molecules, the wear rate and friction coefficient of the PEEK/PTFE composites rapidly increased, showing a negative effect. With the further penetration of the hydrogen molecule, the friction coefficient of the composite displayed a small fluctuation and then a rapid decreasing trend. Meanwhile, effective improvement measures were proposed, and the introduction of the graphene was verified to be effective to reduce the negative effect of the hydrogen permeation, thereby improving the friction performance of the PEEK/PTFE composites.

State-to-State Dynamics in Mode-Specific Reactions of Cl + CH3D(v1-I, v1-II, and v4 = 1; |10⟩): Loss of Memory or Not

Several decades of the study of reaction dynamics culminate in the concept of mode specificity and bond selectivity in polyatomic systems. Until very recently, the main concern of those studies has been total reactivity and little attention has been paid to the mode-specific effects on the more detailed product-state and angular distributions. Conventional wisdom would anticipate that the fine detail should reveal a more pronounced mode dependency. However, a few recent studies showed that the product distributions could appear to be surprisingly insensitive to the modes of internal excitation of reagents. This counterintuitive finding led to a concept of loss of memory. Here, we present detailed experimental results in the reactions of the Cl atom with three distinct stretching-excited CH₃D(v_CH3 = 1) reagents. In conjunction with the previous reports on various aspects of this reaction, such a comprehensive set of data enables us to perform an in-depth examination of the validity of this new concept.

Imaging the Mode-Specificity in Cl + CH3D(v1-I, v1-II, v4 = 1; |jK⟩ = |10⟩) → CH2D(41) + HCl(v)

We report a crossed-beam imaging experiment on the title reactions at two collisional energies (E_c) of 5.3 and 10 kcal mol^-1. Both the integral cross sections relative to the ground-state reactivity and the differential cross sections were measured and compared. We found that one-quantum excitations of the CH₃-stretching vibrations of the CH₃D reagent exerted profound mode-specificity in forming the umbrella-mode-excited CH₂D(4₁) products with the vibrational efficacy of v₄ > v₁-I > v₁-II at both E_c values. The concomitantly formed HCl(v) coproducts were vibrationally cold. Interestingly, the branching ratios of (v = 1)/(v = 0) appeared invariant to the initial stretch-modes of excitation at E_c = 5.3 kcal mol^-1, yet exhibited a pronounced mode-specific dependency in the order of v₁-II > v₁-I > v₄ at E_c = 10.3 kcal mol^-1. This large and E_c-dependent disparity between the two Fermi-coupled reagents, v₁-I and v₁-II, is particularly significant and could be another facet─in addition to that in the recently reported vibrational enhancement factors─of the Fermi-phase-induced interference effect manifested in the product vibrational branching ratio. The pair-correlated angular distributions (v_CH2D, v_HCl)_s = (4₁, 0)_s in the three stretch-excited reactions were globally alike and resembled that of the ground-state reaction pair (0₀, 0)_g, suggestive of a direct abstraction mechanism of the peripheral type. This is in sharp contrast to all other vibrationally excited pairs of (1₁, 0)_s, (3₁, 0)_s, and (6₁, 0)_s previously reported in the CH₂D + HCl isotopic channel, for which both the direct abstraction and a time-delayed resonance pathway partake.

Pair-Correlated Imaging of Cl + CH3D(v4, v1-I, v1-II = 1, |jK⟩) → CH2D(vi) + HCl(v)

The title reactions were studied at a collisional energy of 10.0 kcal mol^-1 in a crossed-beam, product-imaging experiment. In terms of integral cross sections, all three CH₃-stretching excited CH₃D(v_CH3 = 1) reagents promote the reactivity in forming the predominant product pair of (v_CH2D, v_HCl)_s = (0₀, 0/1)_s with a prominent mode-propensity of v₄ > v₁-I > v₁-II, where v₄ denotes the degenerate mode of CH₃ asymmetric stretch and v₁-I and v₁-II are a pair of Fermi-coupled, symmetric-stretch states. The vibrationally excited CH₂D product pairs of (6₁, 0)_s, (1₁, 0)_s, and (3₁, 0)_s appear to be minor channels and display a reverse propensity of v₄ < v₁-I ≈ v₁-II for (6₁, 0)_s, while v₄ > v₁-I for (1₁, 0)_s. Based on the observed angular distributions, we conjecture that, irrespective of the initial mode of excitation, the (0₀, 0)_s product pair proceeds by a direct abstraction of the peripheral type, whereas the (0₀,1)_s pair is mediated via a resonance pathway. Intriguingly, the angular distributions of the excited product pairs-(6₁, 0)_s, (1₁, 0)_s, and (3₁, 0)_s-are remarkably similar and comprise the traits of both the peripheral mechanism and resonance pathway. Possible interpretation and implication are suggested. In addition, due to the spectral overlap of the REMPI bands and heavily congested image features, a robust data analysis method is developed, which enables us to extract the dynamics attributes of a weak feature buried in the proximate, more intense ones with high fidelity.

A crossed molecular beam apparatus with multi-channel Rydberg tagging time-of-flight detection

We report a new crossed molecular beam apparatus with the H atom Rydberg tagging detection technique. The multi-channel detection scheme with 15 microchannel plate (MCP) detectors enables simultaneously accumulating time-of-flight spectra over a wide range of scattering angles (112°). The efficiency of data acquisition has been enhanced by an order of magnitude. The angular distribution of H atoms from photodissociation of CH₄ at 121.6 nm was used for calibrating the detection efficiency of different MCP detectors. The differential cross section of the reaction F + H₂ → HF + H at the collision of 6.9 meV was measured, demonstrating the feasibility and accuracy of this multi-channel detection method. This apparatus could be a powerful tool for investigating the dynamics of reactions at very low collision energy.

Vibrational overtone excitation of D2 in a molecular beam with a high-energy, narrow-bandwidth, nanosecond optical parametric oscillator/amplifier

We have built a high-energy, narrow-bandwidth, nanosecond light source for efficient preparation of vibrationally excited molecules in a molecular beam. It consists of an injection-seeded optical parametric oscillator and two optical parametric amplifiers. Pumped by the second harmonic of a commercial injection-seeded Nd:YAG laser, it can generate pulse energies up to 377 mJ at 655 nm with a bandwidth smaller than 200 MHz. Its stability is excellent, with a standard deviation of pulse energy of 5.2 mJ and a wavelength stability of 0.001 cm^-1. We demonstrated this light source in a crossed-molecular-beam experiment of the H + D₂ (v = 2, j = 0) → HD + D reaction, in which it was used for overtone excitation of D₂ molecules from (v = 0, j = 0) to (v = 2, j = 0) with an overall excitation efficiency of 2.5%.