Influence of vibrational excitation on the reaction Li(2 2PJ)+H2(v=1)→LiH(X 1Σ+)+H

New Diabatic Potential Energy Surfaces for the Li + H2 Reaction and Time-Dependent Quantum Wave Packet Studies

New global diabatic potential energy surfaces (DPESs) for the ground (12A') and first excited (22A') states for the Li + H2 system were developed, with more than 30,000 energy points at the IC-MRCI+Q level of theory, utilizing the aug-cc-pV5Z basis set for the H atoms and the cc-pCV5Z basis set for the Li atom, fitted by a single neural network (NN) with symmetry. Product state-resolved quantum dynamics calculations of the nonadiabatic reaction Li (2P) + H2 (X 1 ∑g+, v0 = 0, j0 = 0) → LiH (X 1∑+) + H(2S) were carried out using these new DPESs and also the previous HYLC-DPESs. The numerical results suggested that our newly constructed DPESs provided an accurate description of the LiH2 system.

Non-adiabatic dynamics studies of the H(2S) + LiH(X1Σ+) reaction by time-dependent wave packet method

Non-adiabatic dynamics studies of the H + LiH (v₀ = 0, j₀ = 0) reaction are carried out based on the diabatic potential energy surfaces (PESs) reported by He and co-workers (Sci. Rep., 2016, 6, 25083) in the collision energy range from 0.001 to 1.0 eV. The H(²S) + LiH(X¹Σ⁺) → Li(2²S)/Li(2²P) + H₂(X¹Σ_g⁺) depletion reactions and the H'(²S) + LiH(X¹Σ⁺) → H(²S) + LiH'(X¹Σ⁺) exchange reaction are studied at the state-to-state level of theory. The dynamics properties, such as reaction probability, integral cross section, differential cross section, the ro-vibrational state distributions of the product and specific-state rate constant are calculated. In addition, the dynamics values of the H(²S) + LiH(X¹Σ⁺) → Li(2²S) + H₂(X¹Σ_g⁺) depletion reaction are compared with available theoretical results, which are based on the adiabatic PESs. Large discrepancies are found between the adiabatic and diabatic values, especially at low J values, which indicate that the non-adiabatic effect is very great and cannot be simply ignored. Furthermore, the deviations between adiabatic and diabatic values gradually decrease as the collision energy increases.

Non-adiabatic dynamics studies of the K(4p2P) + H2(X1Σ) reaction based on new diabatic potential energy surfaces

Global diabatic potential energy surfaces (PESs) of the KH2 system corresponding to the ground (12A') and first excited (22A') states were constructed for the first time. In ab initio calculations, the MRCI-F12 method with AVTZ and def2-QZVP basis sets was adopted and 17 865 ab initio energy points were calculated. The mixing angle, which is used to obtain the diabatic energies, was calculated by the molecular properties of the transition dipole moment. The diabatic PESs were fitted individually by the permutation invariant polynomial neural network method and the topographical features of the diabatic PESs are discussed in detail. The non-adiabatic dynamics studies of the K(4p2P) + H2(v0 = 0, 1, j0 = 0) reaction were carried out using the APH method based on the new diabatic PESs. The collision reaction processes K(4p2P) + H2(v0 = 0, 1, j0 = 0) → H + KH and the quenching processes K(4p2P) + H2(v0 = 0, 1, j0 = 0) → K(4s2S) + H2 were studied at the state-to-state level of theory. For the reaction process, the dynamics results indicated that the vibrational excitation of H2 was significantly more effective at promoting the reaction than the translational energy. In addition, the differential cross-sections were forward-biased scattering, which indicated that the direct abstraction mechanism plays a dominant role in the reaction. For the quenching process, the vibrational excitation of H2 molecules could improve the quenching efficiency obviously.

Significant effects of vibrational excitation of reactant in K + H2 → H + KH reaction based on a new neural network potential energy surface

The study of K + H2 collision has a long experimental history, but there have been few theoretical studies due to lack of a global potential energy surface (PES). In this study, a new global PES for the ground state of KH2 system was constructed based on numerous ab initio points, using the permutation invariant polynomial neural network method. The root mean square error (RMSE) of PES is very small (5.64 meV). On the new PES, time-dependent quantum wave packet (TDWP) and quasiclassical trajectory (QCT) calculations were carried out to study the dynamics of the K(2S) + H2(X1Σ+g) → H(2S) + KH(X1Σ+) reaction. Dynamics results show that (i) the K + H2(v = 0) → H + KH reaction scarcely occurred, (ii) the K + H2(v = 1) → H + KH reaction took place in small quantities, and (iii) the K + H2(v = 2) → H + KH reaction occurred in large quantities. This indicates that vibrational energy of the reactant is significantly more effective at promoting the reaction than the translational energy. This characteristic stems from a major physical model in reactive collisions: the vibrationally excited H2 molecule and K atom collide first in a T-geometric configuration and the vibrational motion of the H2 molecule helps separate the two H atoms a large distance after the collision. At a large H-H distance, a broad well exists on the PES, so the heavy K atom could pull back the light H atom to initiate the reaction. Similar to the reactive channel, vibrational excitation of the reactant also has a significant effect on the collision-induced dissociation channel.

A neural network potential energy surface for the NaH2 system and dynamics studies on the H(2S) + NaH(X1Σ+) → Na(2S) + H2(X1Σg+) reaction

In order to study the dynamics of the reaction H(²S) + NaH(X¹Σ⁺) → Na(²S) + H₂(X¹Σ_g⁺), a new potential energy surface (PES) for the ground state of the NaH₂ system is constructed based on 35 730 ab initio energy points. Using basis sets of quadruple zeta quality, multireference configuration interaction calculations with Davidson correction were carried out to obtain the ab initio energy points. The neural network method is used to fit the PES, and the root mean square error is very small (0.00639 eV). The bond lengths, dissociation energies, zero-point energies and spectroscopic constants of H₂(X¹Σ_g⁺) and NaH(X¹Σ⁺) obtained on the new NaH₂ PES are in good agreement with the experiment data. On the new PES, the reactant coordinate-based time-dependent wave packet method is applied to study the reaction dynamics of H(²S) + NaH(X¹Σ⁺) → Na(²S) + H₂(X¹Σ_g⁺), and the reaction probabilities, integral cross-sections (ICSs) and differential cross-sections (DCSs) are obtained. There is no threshold in the reaction due to the absence of an energy barrier on the minimum energy path. When the collision energy increases, the ICSs decrease from a high value at low collision energy. The DCS results show that the angular distribution of the product molecules tends to the forward direction. Compared with the LiH₂ system, the NaH₂ system has a larger mass and the PES has a larger well at the H-NaH configuration, which leads to a higher ICS value in the H(²S) + NaH(X¹Σ⁺) → Na(²S) + H₂(X¹Σ_g⁺) reaction. Because the H(²S) + NaH(X¹Σ⁺) → Na(²S) + H₂(X¹Σ_g⁺) reaction releases more energy, the product molecules can be excited to a higher vibrational state.

Influence of rovibrational excitation on the non-diabatic state-to-state dynamics for the Li(2p) + H2 → LiH + H reaction

The non-adiabatic state-to-state dynamics of the Li(2p) + H2 → LiH + H reaction has been studied using the time-dependent wave packet method, based on a set of diabatic potential energy surfaces recently developed by our group. Integral cross sections (ICSs) can be increase more than an order of magnitude by the vibrational excitation of H2, whereas the ICSs are barely affected by the rotational excitation of H2. Moreover, ICSs of the title reaction with vibrationally excited H2 decrease rapidly with increasing collision energy, which is a typical feature of non-threshold reaction. This phenomenon implies that the title reaction can transformed from an endothermic to an exothermic reaction by vibrational excitation of H2. With the increase of the collision energy, the sideways and backward scattered tendencies of LiH for the Li(2p) + H2(v = 0, j = 0, 1) → LiH + H reactions are enhanced slightly, while the backward scattering tendency of LiH for the Li(2p) + H2(v = 1, j = 0) → LiH + H reaction becomes remarkably weakened. For the reaction with vibrationally excited H2 molecule, both direct and indirect reaction mechanism exist simultaneously.

Nascent rotational distribution for LiH(v=0-3,J) states from collisions with H2(E=4300 and 4800cm-1)

Rotationally state selective excitation of H₂(v=1, J=1 or 3) was achieved by stimulated Raman pumping. The full state-resolved distribution of scattered LiH(v=0-3, J=0~13)molecules from collisions with excited H₂(E=4300 and 4800cm^-1) is reported. Nascent rotational and translational energy profiles for scattered LiH(v=0~3) molecules with J=0~13 were measured using high-resolution transient laser induced fluorescence(LIF). The product translational energy for individual J-states increases by 120% for a 13% increase in donor energy. The scattered LiH(v=0, J=0~13) molecules have a biexponential rotational distribution. Fitting the data with a two-component exponential model yields a low-energy distribution and a high-energy distribution. The rotational distribution is sensitive to donor energy. Rotational distributions of scatted LiH(v=1-3) molecules were also measured. The distribution yielded rotational temperatures at 690K for LiH/H₂(E=4300cm^-1) and 730K for LiH/H₂(E=4800cm^-1), respectively. The rate constants for appearance LiH(v=0-3,J) were determined.

Global diabatic potential energy surfaces and quantum dynamical studies for the Li(2p) + H2(X(1)Σ(+)g) → LiH(X(1)Σ(+)) + H reaction

The global diabatic potential energy surfaces which are correlated with the ground state 1A' and the excited state 2A' of the Li(2p) + H2 reaction are presented in this study. The multi-reference configuration interaction method and large basis sets (aug-cc-pVQZ for H atom and cc-pwCVQZ for Li atom) were employed in the ab initio single-point energy calculations. The diabatic potential energies were generated by the diabatization scheme based on transition dipole moment operators. The neural network method was utilized to fit the matrix elements of the diabatic energy surfaces, and the root mean square errors were extremely small (3.69 meV for , 5.34 meV for and 5.06 meV for ). The topographical features of the diabatic potential energy surfaces were characterized and the surfaces were found to be sufficiently smooth for the dynamical calculation. The crossing seam of the conical intersections between the and surfaces were pinpointed. Based on this new analytical diabatic potential energy surfaces, time-dependent wave packet calculation were conducted to investigate the mechanism of the title reaction. At low collision energies, the product LiH molecule tends to forward scattering, while at high collision energies, the forward and backward scatterings exist simultaneously.

Computing a three-dimensional electronic energy manifold for the LiH + H <==> Li + H2 chemical reaction

We present a new three-dimensional potential energy surface (PES) for the electronic ground state of the LiH + H <==> Li + H2 reaction and further analyze specific aspects of the lower four excited electronic states. Our reactive PESs are calculated using a CASSCF method followed by an MRCI treatment of the correlation energy. The ground-state three-dimensional surface is then fitted by using our own version of the Aguado-Paniagua interpolation form [Aguado, A.; Paniagua, M. J. Chem. Phys. 1992, 96, 1265]. A review of the previous computational work on this system, to which we compare our present findings, is given in the introduction of the paper: with respect to such earlier calculations of the ground-state PES [Dunne, L. J.; Murrell, J. N.; Jemmer, P. Chem. Phys. Lett. 2001, 336, 1], our data confirm the absence of a barrier along the path to the LiH depletion reaction and further reveal possible spurious features of the earlier computed surface which may in turn affect the resulting rates from low-energy dynamic studies of the title system.

Coriolis-Coupled Wave Packet Dynamics of H + HLi Reaction

We investigated the effect of Coriolis coupling (CC) on the initial state-selected dynamics of H+HLi reaction by a time-dependent wave packet (WP) approach. Exact quantum scattering calculations were obtained by a WP propagation method based on the Chebyshev polynomial scheme and ab initio potential energy surface of the reacting system. Partial wave contributions up to the total angular momentum J=30 were found to be necessary for the scattering of HLi in its vibrational and rotational ground state up to a collision energy approximately 0.75 eV. For each J value, the projection quantum number K was varied from 0 to min (J, K(max)), with K(max)=8 until J=20 and K(max)=4 for further higher J values. This is because further higher values of K do not have much effect on the dynamics and also because one wishes to maintain the large computational overhead for each calculation within the affordable limit. The initial state-selected integral reaction cross sections and thermal rate constants were calculated by summing up the contributions from all partial waves. These were compared with our previous results on the title system, obtained within the centrifugal sudden and J-shifting approximations, to demonstrate the impact of CC on the dynamics of this system.

Influence of vibrational excitation on the nonadiabatic reactions of metal atoms with H2

The reactions of alkaline earth metal atoms, Mg(3s3p 1P1) and Ca(4s4p 1P1), with H2(v = 1, j) are studied using a pump-probe technique combined with stimulated Raman pumping and coherent anti-Stokes Raman spectroscopy. For the Ca(4 1P1) case, the energy deposited in the v = 1 level enlarges the H2 bond distance to help facilitate the reaction without opening an additional pathway. For the Mg(3 1P1) case, the vibrational excitation of H2 leads to enhancement of the low rotational component of the rotational distribution and the MgH(v = 0)/MgH(v = 1) ratio. These results can be predicted with quasi-classical trajectory calculations and interpreted with a kinematic collision model.

Reaction pathway and potential barrier for the CaH product in the reaction of Ca(4s4p 1P1)+H2→CaH(X 2Σ+)+H

The nascent CaH product in the reaction Ca(4s4p1P1) + H2 --> CaH(X2Sigma+) + H is obtained using a pump-probe technique. The CaH(v = 0,1) distributions, with a population ratio of CaH(v = 0)/CaH(v = 1) = 2.7+/-0.2, may be characterized by low Boltzmann rotational temperature. According to Arrhenius theory, the temperature dependence measurement yields a potential barrier of 3820+/-480 cm(-1) for the current reaction. As a result of the potential energy surfaces (PES) calculations, the reaction pathway favors a Ca insertion into the H2 bond along a (near) C2v geometric approach. As the H2 bond is elongated, the configurational mixing between the orbital components of the 4p and nearby low-lying 3d state with the same symmetry makes significant the nonadiabatic transition between the 5A' and 2A' surface in the repulsive limbs. Therefore, the collision species are anticipated to track along the 5A' surface, then undergo nonadiabatic transition to the inner limb of the 2A' surface, and finally cross to the reactive 1A' surface. The observed energy barrier probably accounts for the energy requirement to surmount the repulsive hill in the entrance. The findings of the nascent CaH product distributions may be reasonably interpreted from the nature of the intermediate structure and lifetime after the 2A'-1A' surface transition. The distinct product distributions between the Ca(4 1P1) and Mg(3 1P1) reactions with H2 may also be realized with the aid of the PES calculations.