Probing Feshbach resonances in F+H2(j=1)-->HF+H: dynamical effect of single quantum H2-rotation

Full quantum state resolved scattering of the F atom reaction with H(2)(j=0) and H(2)(j=1) was investigated at the collision energies of 0.19 and 0.56 kcalmol. Dramatic difference between the dynamics for the F+H(2)(j=0,1) reactions at both collision energies have been observed. Forward scattering HF(v(')=2) products have been observed unambiguously for the F+H(2)(j=1) reaction at low collision energies, which was attributed to the Feshbach resonances. This study provides a unique case of reaction resonances involving a rotationally excited reagent.     

Collision energy dependence of the O(1D) + HCl --> OH + Cl(2P) reaction studied by crossed beam scattering and quasiclassical trajectory calculations on ab initio potential energy surfaces

The dynamics of the O(1D) + HCl --> OH + Cl(2P) reaction are investigated by a crossed molecular beam ion-imaging method and quasiclassical trajectory calculations on the three ab initio potential energy surfaces, the ground 1(1)A' and two excited (1(1)A' and 2(1)A') states. The scattering experiment was carried out at collision energies of 4.2, 4.5, and 6.4 kcal/mol. The observed doubly differential cross sections (DCSs) for the Cl(2P) product exhibit almost no collision energy dependence over this inspected energy range. The nearly forward-backward symmetric DCS indicates that the reaction proceeds predominantly on the ground-state potential energy surface at these energies. Variation of the forward-backward asymmetry with collision energy is interpreted using an osculating complex model. Although the potential energy surfaces obtained by CASSCF-MRCI ab initio calculations exhibit relatively low potential barriers of 1.6 and 6.5 kcal/mol for 1(1)A' and 2(1)A', respectively, the dynamics calculations indicate that contributions of these excited states are small at the collision energies lower than 15.0 kcal/mol. Theoretical DCSs calculated for the ground-state reaction pathway agree well with the observed ones. These experimental and theoretical results suggest that the titled reaction at collision energies less than 6.5 kcal/mol is predominantly via the ground electronic state.     

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.     

Differential Cross Sections of F+HD→DF+H Reaction at Collision Energies from 3.03 meV to 17.97 meV

The prototypical reaction of F+HD→DF+H was investigated at collision energies from 3.03 meV to 17.97 meV using a crossed molecular beam apparatus with multichannel Rydberg tagging time-of-flight detection. Significant contributions from both the Born-Oppenheimer (BO) forbidden reaction F^*(²P_1/2)+HD→DF+H and the BO-allowed reaction F(²P_3/2)+HD→DF+H were observed. In the backward scattering direction, the contribution from the BO-forbidden reaction F^*(²P_1/2)+HD was found to be considerably greater than the BO-allowed reaction F(²P_3/2)+HD, indicating the non-adiabatic effects play an important role in the dynamics of the title reaction at low collision energies. Collision-energy dependence of differential cross sections (DCSs) in the backward scattering direction was found to be monotonously decreased as the collision energy decreases, which does not support the existence of resonance states in this energy range. DCSs of both BO-allowed and BO-forbidden reactions were measured at seven collision energies from 3.03 meV to 17.97 meV. It is quite unexpected that the angular distribution gradually shifts from backward to sideway as the collision energy decreases from 17.97 meV to 3.03 meV, suggesting some unknown mechanisms may exist at low collision energies.     

Differential cross section for the H+D(2)-->HD(v(')=1,j(')=2,6,10)+D reaction as a function of collision energy

We have measured differential cross sections (DCSs) for the HD (v(')=1,j(')=2,6,10) products of the H+D(2) exchange reaction at five different collision energies in the range 1.48< or =E(coll)< or =1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, E(coll)=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v(')=1,j(')=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection.     

Gas-phase SN2 and bromine abstraction reactions of chloride ion with bromomethane: reaction cross sections and energy disposal into products

Reaction cross sections and product velocity distributions are presented for the bimolecular gas-phase nucleophilic substitution (S(N)2) reaction Cl(-) + CH(3)Br --> CH(3)Cl + Br(-) as a function of collision energy, 0.06-24 eV. The exothermic S(N)2 reaction is inefficient compared with phase space theory (PST) and ion-dipole capture models. At the lowest energies, the S(N)2 reaction exhibits the largest cross sections and symmetrical forward/backward scattering of the CH(3)Cl + Br(-) products. The velocity distributions of the CH(3)Cl + Br(-) products are in agreement with an isotropic PST distribution, consistent with a complex-mediated reaction and a statistical internal energy distribution of the products. Above 0.2 eV, the velocity distributions become nonisotropic and nonstatistical, exhibiting CH(3)Cl forward scattering between 0.2 and 0.6 eV. A rebound mechanism with backward scattering above 0.6 eV is accompanied by a new rising feature in the CH(3)Cl + Br(-) cross sections. The competitive endothermic reaction Cl(-) + CH(3)Br --> CH(3) + ClBr(-) rises from its thermochemical threshold at 1.9 +/- 0.4 eV, showing nearly symmetrically scattered products just above threshold and strong backward scattering above 3 eV associated with a second feature in the cross section.     

Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

N(4S)+ NO(X2Π)→N2(X3Σg-)+O(3P) 反应的立体动力学研究

采用准经典轨线方法研究了在不同碰撞能下，碰撞反应N(4S)+NO(X2Π)→ N2(X3Σg- )+O(3P)在两个最低势能面3A 和 3A'上产物与反应物之间的矢量相关. 结果表明，对于不同的碰撞能，在两个势能面上反应产物的转动取向展示了不同的特征和趋势. 随着碰撞能的增加，发生在3A 势能面上的反应主要受外平面机理支配，而发生在 3A' 势能面上的反应倾向于受内平面机理支配. 这些差异来自于两个势能面的不同构型.     

I+HI'(v)→IH(v')+I'反应几率振荡行为的量子散射理论研究

本文基于振动绝热性分析, 用量子散射理论研究I+HI'(v)→IH(v')+I'反应几率的振荡行为, 其中双原子间相互作用势的振动本征态是Morse振子的严格解。在计算反应几率时采用分布高斯基(Distributed Gaussian Basis Sets; DGBS)展于方法, 既保持了反应几率的解析表达式, 又有效地简化了计算。计算结果明显地揭示了反应几率在低碰能区域的振荡行为。对反应机理也作了探讨。     

A pulsed-field ionization photoelectron secondary ion coincidence study of the H2+ (X,upsilon+=0-15,N+=1)+He proton transfer reaction

The endothermic proton transfer reaction, H2+(upsilon+)+He-->HeH+ + H(DeltaE=0.806 eV), is investigated over a broad range of reactant vibrational levels using high-resolution vacuum ultraviolet to prepare reactant ions either through excitation of autoionization resonances, or using the pulsed-field ionization-photoelectron-secondary ion coincidence (PFI-PESICO) approach. In the former case, the translational energy dependence of the integral reaction cross sections are measured for upsilon+=0-3 with high signal-to-noise using the guided-ion beam technique. PFI-PESICO cross sections are reported for upsilon+=1-15 and upsilon+=0-12 at center-of-mass collision energies of 0.6 and 3.1 eV, respectively. All ion reactant states selected by the PFI-PESICO scheme are in the N+=1 rotational level. The experimental cross sections are complemented with quasiclassical trajectory (QCT) calculations performed on the ab initio potential energy surface provided by Palmieri et al. [Mol. Phys. 98, 1839 (2000)]. The QCT cross sections are significantly lower than the experimental results near threshold, consistent with important contributions due to resonances observed in quantum scattering studies. At total energies above 2 eV, the QCT calculations are in excellent agreement with the present results. PFI-PESICO time-of-flight (TOF) measurements are also reported for upsilon+=3 and 4 at a collision energy of 0.6 eV. The velocity inverted TOF spectra are consistent with the prevalence of a spectator-stripping mechanism.     

The excitation function for Li + HF → LiF + H at collision energies below 80 meV

We have measured the dependence of the relative integral cross section of the reaction Li + HF → LiF + H on the collision energy (excitation function) using crossed molecular beams. By varying the intersection angle of the beams from 37° to 90° we covered the energy range 25 meV ≤ E(tr) ≤ 131 meV. We observe a monotonous rise of the excitation function with decreasing energy over the entire energy range indicating that a possible translational energy threshold to the reaction is significantly smaller than 25 meV. The steep rise is quantitatively recovered by a Langevin-type excitation function based on a vanishing threshold and a mean interaction potential energy ∝R(-2.5) where R is the distance between the reactants. To date all threshold energies deduced from ab initio potentials and zero-point vibrational energies are at variance with our results, however, our findings support recent quantum scattering calculations that predict significant product formation at collision energies far below these theoretical thresholds.     

State-to-state dynamics analysis of the F + CHD3 reaction: a quasiclassical trajectory study

An exhaustive state-to-state dynamics study was performed to analyze the F + CHD3 --> FD(nu', j') + CHD2(nu) gas-phase abstraction reaction. Quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were performed at different collision energies on an analytical potential energy surface (PES-2006) recently developed by our group. Whereas the CHD2 coproduct appears vibrationally and rotationally cold, most of the available energy appears as FD(nu') product vibrational energy, peaking at nu' = 2 and nu' = 3, with the population in the latter level growing as the energy increases. The excitation function rises from the threshold of the reaction and then levels off at higher energies, with the maximum contribution from the FD(nu' = 3) level. The state-specific FD(nu') scattering distributions correlated with the coproduct CHD2 in the nu4 = 2 and nu3 = 1 states, at different collision energies, show a steady change from backward to forward scattering as the energy increases. This similar behavior for the two coproduct vibrational states, nu4 = 2 and nu3 = 1, agrees qualitatively with the experimental measurements. Comparison with theoretical and experimental results for the isotopic analogues, F + CH4 and F + CD4, shows that the title reaction presents a direct mechanism, similar to the perdeuterated reaction, but contrasts with that of the F + CH4 reaction. These results for the dynamics of different isotopic variants, always in qualitative and sometimes in quantitative agreement with experiment, show the capacity of the PES-2006 surface to correctly describe the title reaction, even though there are differences that could be due to deficiencies of the PES but also to the known limitations of the classical treatment in the QCT method.     

Quasi-classical trajectory study of Si+O2-->SiO+O reaction

Quasi-classical trajectory calculations for the Si(3P)+O2(X 3Sigmag-)-->SiO(X 1Sigma+)+O(1D) reaction have been carried out using the analytical ground 1A' potential energy surface (PES) recently reported by Dayou and Spielfiedel [J. Chem. Phys. 119, 4237 (2003)]. The reaction has been studied for a wide range of collision energies (0.005-0.6 eV) with O2 in its ground rovibrational state. The barrierless PES leads to a decrease of the total reaction cross section with increasing collision energy. It has been brought to evidence that the reaction proceeds through different reaction mechanisms whose contributions to reactivity are highly dependent on the collision energy range. At low collision energy an abstraction mechanism occurs involving the collinear SiOO potential well. The associated short-lived intermediate complex leads to an inverted vibrational distribution peaked at v'=3 and low rotational excitation of SiO(v',j') with a preferentially backward scattering. At higher energies the reaction proceeds mainly through an insertion mechanism involving the bent and linear OSiO deep potential wells and associated long-lived intermediate complexes, giving rise to nearly statistical energy disposals into the product modes and a forward-backward symmetry of the differential cross section.     

Evidence for excited spin-orbit state reaction dynamics in F+H2: theory and experiment

We describe fully quantum, time-independent scattering calculations of the F+H2-->HF+H reaction, concentrating on the HF product rotational distributions in v'=3. The calculations involved two new sets of ab initio potential energy surfaces, based on large basis set, multireference configuration-interaction calculations, which are further scaled to reproduce the experimental exoergicity of the reaction. In addition, the spin-orbit, Coriolis, and electrostatic couplings between the three quasidiabatic F+H2 electronic states are included. The calculated integral cross sections are compared with the results of molecular beam experiments. At low collision energies, a significant fraction of the reaction is due to Born-Oppenheimer forbidden, but energetically allowed reaction of F in its excited (2P 1/2) spin-orbit state. As the collision energy increases, the Born-Oppenheimer allowed reaction of F in its ground (2P 3/2) spin-orbit state rapidly dominates. Overall, the calculations agree reasonably well with the experiment, although there remains some disagreement with respect to the degree of rotational excitation of the HF(v'=3) products as well as with the energy dependence of the reactive cross sections at the lowest collision energies.     

Quasi-classical trajectory study of the F + CD4 reaction dynamics

To analyze the F + CD4 gas-phase abstraction reaction, an exhaustive state-to-state dynamics study was performed. Quasi-classical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were used on an analytical potential energy surface (PES-2006) recently developed by our group for collision energies in the range 0.3-6.0 kcal mol-1. While the CD3 coproduct appears vibrationally and rotationally cold, in agreement with experiment, most of the available energy appears as FD(nu') product vibrational energy, peaking at nu' = 3, one unit colder than experiment. The excitation function reproduces experiment, with the maximum contribution from the most populated FD(nu' = 3) level. The state-specific scattering distributions at different collision energies also reproduce the experimental behavior, with a clear propensity toward forward scattering, this tendency increasing with the energy. These dynamics results show the capacity of the PES-2006 surface to correctly describe the title reaction.     

Luminescence measurements of Xe(+) + N2 and Xe(2+) + N2 hyperthermal charge transfer collisions

Luminescence spectra are recorded for collisions between Xe(+)/Xe(2+) and molecular nitrogen at energies ranging from 4.5 to 316 eV in the center-of-mass frame. In the Xe(+) + N(2) collision system, evidence for luminescent charge-transfer products is only found through Xe I emission lines. The most intense features of the luminescence spectra are attributed to atomic N emissions observed above ～20 eV. Intense N(2)(+) A (2)Π(u) - X(2)Σ(g)(+) and B(2)Σ(u)(+) - X(2)Σ(g)(+) radiance is observed from Xe(2+) + N(2) collisions. The B state formation cross section decreases with collision energy until 20 eV, after which it becomes independent of impact energy with an approximate value of 3 ?(2). The cross section for N(2) (+) A (ν > 0) formation increases with energy until 20 eV, after which it remains nearly constant at ～1 ?(2). The N(2)(+) product vibrational distributions extracted from the spectra are non-Franck-Condon for both electronic product states at low collision energies. The distributions resemble a Franck-Condon distribution at the highest energies investigated in this work.     

New ab initio potential energy surface and quantum dynamics of the reaction H(2S) + NH(X3Σ-) → N(4S) + H2

A new global potential energy surface is reported for the ground state ((4)A(")) of the reaction H((2)S) + NH(X(3)Σ(-)) → N((4)S) + H(2) from a set of accurate ab initio data, which were computed using the multi-reference configuration interaction with a basis set of aug-cc-pV5Z. The many-body expansion and neural network methods have been used to construct the new potential energy surface. The topographical features of the new global potential energy surface are presented. The predicted barrier height is lower than previous theoretical estimates and the heat of reaction with zero-point energy is closer to experimental results. The quantum reactive scattering dynamics calculation was carried out over a range of collision energies (0-1.0 eV) on the new potential energy surface. The reaction probabilities, integral cross-section, and rate constants for the title reaction were calculated. The calculated rate constants are in excellent agreement with the available experimental results.     

A guided-ion beam study of the O+(4S) + NH3 system at hyperthermal energies

We have measured absolute cross section for the reaction of ground-state O(+) with ammonia at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) method. Measurements were also performed using ammonia-d3 to aid in mass assignments. The reaction is dominated at low collision energies by charge transfer; however, the cross section for this exothermic channel is rather small, decreasing sharply with energy from approximately 40 A(2) for normal ammonia at near-thermal energies and leveling off at 3.7 A(2) above 6 eV; the cross section is slightly smaller for ammonia-d3. Other channels, corresponding to the production of NH2(+) and NO(+), and possibly OH(+), were detected. The NO(+) channel, although nominally exothermic, is very small and exhibits a threshold at approximately 7 eV. Product recoil velocity distributions were also determined at selected collision energies, using GIB time-of-flight methods.     

Crossed Beam Study on the F+D2→DF+D Reaction at Hyperthermal Collision Energy of 23.84 kJ/mol

We presented an experimental apparatus combining the H-atom Rydberg tagging time-of-flight technique and the laser detonation source for studying crossed beam reactions athyperthermal collision energies. The preliminary study of the F+D₂→DF+D reaction at hyperthermal collision energy of 23.84 kJ/mol was performed. Two beam sources were used in this study: one is the hyperthermal F beam source produced by a laser detonation process, and the other is D₂ beam source generated by liquid-N₂ cooled pulsed valve. Vibrational state-resolved di erential cross sections (DCSs) of product for the title reaction were determined. From the product vibrational state-resolved DCS, it can be concluded that products DF(v'=0, 1, 2, 3) are predominantly distributed in the sideway and backward scattering directions at this collision energy. However, the highest vibrational excited product DF(v'=4), is clearly peaked in the forward direction. The probable dynamical origins for these forward scattering products were analyzed and discussed.