Sheng Der Chao

Forward scattering due to slow-down of the intermediate in the H + HD --> D + H(2) reaction.

Quantum dynamical processes near the energy barrier that separates reactants from products influence the detailed mechanism by which elementary chemical reactions occur. In fact, these processes can change the product scattering behaviour from that expected from simple collision considerations, as seen in the two classical reactions F + H2 → HF + H and H + H2 → H2 + H and their isotopic variants. In the case of the F + HD reaction, the role of a quantized trapped Feshbach resonance state had been directly determined, confirming previous conclusions that Feshbach resonances cause state-specific forward scattering of product molecules. Forward scattering has also been observed in the H + D2 → HD + D reaction and attributed to a time-delayed mechanism. But despite extensive experimental and theoretical investigations, the details of the mechanism remain unclear. Here we present crossed-beam scattering experiments and quantum calculations on the H + HD → H2 + D reaction. We find that the motion of the system along the reaction coordinate slows down as it approaches the top of the reaction barrier, thereby allowing vibrations perpendicular to the reaction coordinate and forward scattering. The reaction thus proceeds, as previously suggested, through a well-defined ‘quantized bottleneck state’ different from the trapped Feshbach resonance states observed before.

A globally smooth ab initio potential surface of the 1 A′ state for the reaction S(1D)+H2

A procedure based on the reproducing kernel Hilbert space (RKHS) interpolation method has been implemented to produce a globally smooth potential energy surface (PES) for the 1 A′ state of the S(1D)+H2 reaction from a set of accurate ab initio data, calculated at the multireference configuration interaction level with augmented polarized quadruple-zeta basis sets and arranged on a three-dimensional regular full grid in the Jacobi coordinates. The procedure includes removing a small number of questionable ab initio data points, implementing a recently developed technique for efficiently handling a partially filled grid, and adopting a sequence of regularizations for attaining additional smoothness. The resulting RKHS PES is analytic, first-order differentiable, and fast to evaluate. Quasiclassical trajectory calculations have been performed and compared with the results based on a recent hybrid PES obtained from a combination of the RKHS interpolation in the entrance channel and Murrell–Carter (MC)-type fi...

The case for a reactive resonance in F+H2

We have investigated the F+H2 reaction on the Stark–Werner potential energy surface using quantum scattering theory, adiabatic theory, and spectral quantization. Clear evidence is found for a transition state resonance on this potential surface. Comparison to experimental results suggests that a resonance is also present in the true reaction.

A fully state-and angle-resolved study of the H+HD→D+ H2 reaction: Comparison of a molecular beam experiment to ab initio quantum reaction dynamics

We present the results of a joint experimental and theoretical investigation of the reaction dynamics of the H+HD→D+H2 chemical reaction. The experiment was performed using a crossed molecular beam apparatus that employed the Rydberg-atom time-of-flight detection scheme for the product D atom. The photolysis of a HI precursor molecule produced a beam source of hot H atoms, which, when crossed with a cold HD beam, yielded two well-defined center-of-mass collision energies, EC=0.498 and 1.200 eV. The resolution of the experiment was sufficient to allow the measurement of the rovibrationally state-resolved differential cross section from the ground state of the HD reagent. The reaction was modeled theoretically using a converged coupled channel scattering calculation employing the BKMP2 potential energy surface: The S matrix was computed on a grid of 56 energies in the range EC=0.245–1.551 eV. It is found that the experimental and theoretical state-to-state differential cross sections are in quantitative agr...

Time delay as a tool to identify the signatures of reactive resonance: F+HD and F+H2 reactions

The formalism of scattering time delay is investigated as a tool to identify the signatures of resonance in reactive molecular collisions. The concept of state and angle resolved time delay is reviewed and applied to model problems. Several numerical pathologies of the time delay formalism are discussed that potentially may obscure a resonance signature, or may lead to a false positive result. The time delay is computed explicitly for the F+HD→HF+D and F+H2→HF+H reactions using the results of full scattering calculations on the Stark–Werner potential energy surface. The reactive resonance known to exist for the F+HD reaction is clearly apparent both in the state resolved and angle resolved time delay functions. On the other hand, the analogous resonance in the F+H2 reaction is masked in the state resolved time delay function through the influence of a nearby energetic threshold. However, the angle resolved time delay does provide a clear signature of the resonance, thus settling a controversy on the exist...

State-to-state dynamics of H+HD→H2+D at 0.5 eV: A combined theoretical and experimental study

The state-to-state dynamics of the H+HD→H2+D reaction at a collisional energy of 0.5 eV was studied experimentally using a crossed molecular beam apparatus using the H atom Rydberg “tagging” time-of-flight detection scheme, and modeled theoretically using a converged quantum scattering calculation on the ground potential energy surface. The experimental results agreed very well with the theoretical calculations without considering the geometrical phase at this collisional energy. The results indicate that the H+HD reaction at this energy is dominated by a simple rebound mechanism along a collinear reaction path. Both experimental and theoretical results show that the rotational state distribution of the H2 product is strongly influenced by nuclear spin statistics.

The search for resonance signatures in H+D2 reaction dynamics

Abstract To investigate the recent report of the observation of a reactive resonance in the title reaction, we have calculated the ro-vibrationally state-resolved integral cross-sections (ICS) and differential cross-sections (DCS), as well as the vibrational branching ratios for the H+D 2 ( v = j =0)→HD( v ′ , j ′ )+D reaction. Oscillatory structure occurring in the state-to-state reaction probabilities does survive partial wave averaging to appear in the integral cross-sections for low j ′ -values, although not significantly for the j ′ =7 state recently reported. The resonance picture interpretation of the oscillations is shown to be problematic.