Shi Ying Lin

Full-dimensional quantum wave packet study of rotationally inelastic transitions in H2+H2 collision

We report full-dimensional accurate quantum dynamical calculations of the rotationally inelastic collision: para-H2(ν1=0,j1=0)+para-H2(ν2=0,j2=0)→para-H2(ν1=0,j1′)+para-H2(ν2=0,j2′), using a wave packet approach based on the Chebyshev polynomial expansion of Green’s operator. The six-dimensional Hamiltonian within the coupled-states approximation is discretized in a mixed grid/basis representation and its action is computed in appropriate representations facilitated by a series of one-dimensional pseudo-spectral transformations. Both the parity and diatomic exchange symmetry are adapted. The S-matrix elements for the rotational transitions are obtained at all energies by the Fourier transform of Chebyshev correlation functions and used to compute transition probabilities, differential and integral cross sections, and state-resolved thermal rate constants. Results are compared for two recently proposed ab initio based potential energy surfaces and with previous quantum results.

Quantum wave packet study of reactive and inelastic scattering between C(1D) and H2

Using a wave packet method, state-to-state inelastic transition probabilities and initial state specified total reaction probabilities are calculated for the title system (J=0) on a recent ab initio potential energy surface. Both the inelastic and reactive scattering probabilities are found to be strongly oscillatory, indicative of the involvement of long-lived resonances that are supported by a deep CH2 well. The oscillation becomes less pronounced at higher collision energies and with internal excitation of the reactant molecule. The reaction from the (νi=0, ji=0) initial state is clearly dominated by the insertion pathway, and this dominance is largely unaffected by the excitation of the reactant rotation or vibration. In addition, low-lying vibrational states of CH2 have been determined and compared with spectroscopic data.

State-to-state quantum dynamics of O + O2 isotope exchange reactions reveals nonstatistical behavior at atmospheric conditions

The O + O2 exchange reaction is a prerequisite for the formation of ozone in Earth’s atmosphere. We report here state-to-state differential and integral cross sections for several O + O2 isotope-exchange reactions obtained by dynamically exact quantum scattering calculations at collision energies relevant to atmospheric conditions. These reactions are shown to be highly nonstatistical, evidenced by dominant forward scattering and deviation of the integral cross section from the statistical limit. Mechanistic analyses revealed that the nonstatistical channel is facilitated by short-lived osculating resonances. The theoretical results provided an in-depth interpretation of a recent molecular beam experiment of the exchange reaction and shed light on the initial step of ozone recombination.

O+OH→O2+H: A key reaction for interstellar chemistry. New theoretical results and comparison with experiment

We report extensive, fully quantum, time-independent (TID) calculations of cross sections at low collision energies and rate constants at low temperatures for the O+OH reaction, of key importance in the production of molecular oxygen in cold, dark, interstellar clouds and in the chemistry of the Earths atmosphere. Our calculations are compared with TID calculations within the J-shifting approximation, with wave-packet calculations, and with quasiclassical trajectory calculations. The fully quantum TID calculations yield rate constants higher than those from the more approximate methods and are qualitatively consistent with a low-temperature extrapolation of earlier experimental values but not with the most recent experiments at the lowest temperatures.

A wave packet based statistical approach to complex-forming reactions

A wave packet based statistical model is suggested for complex-forming reactions. This model assumes statistical formation and decay of the long-lived reaction complex and computes reaction cross sections and their energy dependence from capture probabilities. This model is very efficient and reasonably accurate for reactions dominated by long-lived resonances, as confirmed by its application to the C((1)D)+H(2) reaction.

Accurate quantum mechanical calculations of differential and integral cross sections and rate constant for the O+OH reaction using an ab initio potential energy surface

The authors report accurate quantum mechanical studies of the O+OH reaction on the improved Xu-Xie-Zhang-Lin-Guo potential energy surface. The differential cross section was obtained at several energies near the reaction threshold using a time-independent method. The dominant forward and backward peaks in the angular distribution are consistent with a complex-forming mechanism, which is also confirmed by the extensive rotational excitation in the O2 product. However, the asymmetry of these peaks suggests a significant nonstatistical component. The initial state (upsilon i=0, j i=0) specified integral cross section, which was calculated up to 1.15 eV of collision energy using the Chebyshev wave packet method, shows no energy threshold and decreases with the increasing collision energy, consistent with the barrierless nature of the reaction. The resulting rate constant exhibits a negative temperature dependence for T>100 K and decays as the temperature is lowered, in qualitative agreement with available experimental data.

Rate constant for OH(Π2)+O(P3)→H(S2)+O2(Σg−3) reaction on an improved ab initio potential energy surface and implications for the interstellar oxygen problem

The authors report a global potential energy surface for the ground electronic state of HO2(XA″2), which improves upon the XXZLG potential [Xu and et al., J. Chem. Phys. 122, 244305 (2005)] with additional high-level ab initio points for the long-range interaction potential in the O+OH channel. Exact J=0 quantum mechanical reaction probabilities were calculated on the new potential and the rate constant for the title reaction was obtained using a J-shifting method. The calculated rate constant is in good agreement with available experimental values and our results predict a significantly lower rate at temperature range below 30K, offering a possible explanation for the “interstellar oxygen problem.”

Quantum statistical and wave packet studies of insertion reactions of S(D1) with H2, HD, and D2

A thorough theoretical investigation of the reactions between S(1D) and various hydrogen isotopomers (H2, D2, and HD) has been carried out using a recent ab initio potential energy surface. State-resolved integral and differential cross sections, thermal rate constants, and their dependence on energy or temperature were obtained from quantum mechanical capture probabilities within a statistical model. For comparison, the J=0 reaction probabilities were also computed using an exact wave packet method. The statistical results are in excellent agreement with available exact differential and integral cross sections. The comparison with experimental results shows that the agreement is reasonably good in general, but some significant differences exist, particularly for the SD/SH branching ratio in the S(1D)+HD reaction.

Fully Coriolis-Coupled Quantum Studies of the H + O2 (υi = 0-2, ji = 0,1) → OH + O Reaction on an Accurate Potential Energy Surface : Integral Cross Sections and Rate Constants

We present accurate quantum calculations of the integral cross section and rate constant for the H + O2 --> OH + O combustion reaction on a recently developed ab initio potential energy surface using parallelized time-dependent and Chebyshev wavepacket methods. Partial wave contributions up to J = 70 were computed with full Coriolis coupling, which enabled us to obtain the initial state-specified integral cross sections up to 2.0 eV of the collision energy and thermal rate constants up to 3000 K. The integral cross sections show a large reaction threshold due to the quantum endothermicity of the reaction, and they monotonically increase with the collision energy. As a result, the temperature dependence of the rate constant is of the Arrhenius type. In addition, it was found that reactivity is enhanced by reactant vibrational excitation. The calculated thermal rate constant shows a significant improvement over that obtained on the DMBE IV potential, but it still underestimates the experimental consensus.

Exact quantum dynamics of N(D2)+H2→NH+H reaction: Cross-sections, rate constants, and dependence on reactant rotation

Using an exact Chebyshev wave packet method, initial state-specified (upsilon(i)=0, j(i)=0,2) integral cross-sections and rate constants are obtained for the title reaction on the latest ab initio potential energy surface. Reaction probabilities up to J=29 are dependent on the reactant rotation and show mild oscillations superimposed on a broad background. Due to a barrier in the entrance channel, the cross sections increase with energy with clear thresholds and the rate constants vary with temperature in the Arrhenius form. The calculated canonical rate constant is in good agreement with the experimental measurements. Our results also indicate that the quasiclassical trajectory method underestimates the rate due to the neglect of tunneling, while the quantum statistical approach overestimates because of the short lifetime of the reaction intermediate.