Determination of the absolute scattering cross section for the reaction D + H2(v=1) → HD+H at 0.33 eV

The reaction D+H₂(v=1) has been investigated in a crossed molecular beam experiment at the most probable collision energy of E₀=0.33 eV. Angular and time-of-flight distributions have been measured and the total absolute cross section has been determined to be σ_r(j_i=0, v = 1, E_c.m. = 0.33 eV) = 1.14 ± 0.50 Ų. This value, as well as the distributions, are in good agreement with the results of quasiclassical trajectory calculations (QCT) and the reactive infinite-order sudden approximation (RIOSA).     

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.     

Capture and dissociation in the complex-forming CH(v = 0,1) + D2 → CHD + D, CD2 + H, CD + HD reactions and comparison with CH(v = 0,1) + H2

Rate coefficients for the CH(v = 0,1) + D(2) reaction have been determined for all possible channels (T: 200-1200 K), using the quasiclassical trajectory method and a suitable treatment of the zero point energy. Calculations have also been performed on the CH(v = 1) + H(2) reaction and the CH(v = 1) + D(2) → CH(v = 0) + D(2) process. Most of the results can be understood considering the key role played by the deep minimum of the potential energy surface (PES), the barrierless character of the PES, the energy of the reaction channels, and the kinematics. The good agreement found between theory and experiment for the rate coefficients of the capture process of CH(v = 0) + D(2), the total reactivity of CH(v = 1) + D(2), H(2), as well as the good agreement observed for the related CH(v = 0) + H(2) system (capture and abstraction), gives confidence on the theoretical rate coefficients obtained for the capture processes of CH(v = 1) + D(2), H(2), the individual reactive processes of CH(v = 1) + D(2), H(2), the abstraction and abstraction-exchange reactions for CH(v = 0) + D(2), and the inelastic process mentioned above, for which there are no experimental data available, and that can be useful in combustion chemistry and astrochemistry.     

The H + D2 → HD + D reaction at 0.55, 0.98, 1.10 and 1.30 eV: A distorted-wave study using a new distortion potential

The static-static distorted-wave method has been applied to the H+D₂(v = 0, j = 0) → HD(v', j')+D reaction on the LSTH potential energy surface at four energies. A new static distortion potential has been used that is obtained from the preferred collinear configuration of the atoms together with an average over the unperturbed molecular wavefunction. The rotational distributions agree well with recent experimental data and with other theoretical information.     

Computation of cross sections for the F+H2(v=0,j=0) → FH(v′j)+H reaction by the hyperspherical method

Summary A quantum mechanical calculation of cross sections for the reaction F+H₂(v=0,j=0) FH(vj)+H has been performed on the T5A semiempirical potential surface using hyperspherical coordinates. State-to-state integral and differential cross sections converge rapidly with the number of components of the total angular momentum projection onto the axis of least inertia. Thev=3 differential cross section has a forward peak whose magnitude increases with energy whereas thev=2 differential cross section has a backward maximum, in qualitative agreement with cross-beam experiments. Thev=2 andv=3 rotational distributions are in rather good agreement with experiment, but not the vibrational branching ratios.     

The O((1)D) + H2 (X (1)Σ+, v, j) → OH(X (2)Π, v', j') + H((2)S) reaction at low collision energy: when a simple statistical description of the dynamics works

In this communication, we highlight that statistical approaches for chemical reactions describe reasonably well the low energy dynamics of the title process. Consequently, such methods prove to be valuable to compute rate constants from low to room temperatures. Results are compared with experiment and recent precise quantum wave packet calculations [J. Phys. Chem. A, 2009, 113, 5285].     

Nonadiabatic collision model calculations of ion-pair formation cross sections of Cs+O2→Cs++O2-

This paper has improved Hickman's nonadiabatic collision model by substituting Hickman's constant velocity classical straight line trajectory approximation with the solution of motion equation mR=-dV(R)/dR, and has calculated the cross sections of ion-pair formation Cs+O2 -Cs++O2- with the improved nonadiabatic collision model (INCM). A comparison of our results with other theoretical and experimental results has been made.     

Quasi-classical trajectory-gaussian binning study of the OH + D2 → HOD(v1',v2',v3') + D angle-velocity and vibrational distributions at a collision energy of 0.28 eV

The angle-velocity and product vibrational state distributions for the OH + D(2) reaction at a collision energy of 0.28 eV have been calculated using the quasi-classical trajectory-gaussian binning (QCT-GB) method and the Wu-Schatz-Lendvay-Fang-Harding (WSLFH) analytical potential energy surface. Comparison with high resolution molecular beam experiments shows that, differing from what happens when using the standard QCT method (i.e., histogram binning), very good results are obtained for both distributions. Hence, the strong differences previously observed between QCT and experimental results mainly come from an inadequate pseudoquantization of HOD rather than from other quantum effects. This is probably the first time that such a high level of agreement between theory and high resolution experimental data has been found in polyatomic reaction dynamics.     

Time-dependent wave packet theory for state-to-state differential cross sections of four-atom reactions in full dimensions: application to the HD + OH → H2O + D reaction

Time-dependent wave packet method has been developed to calculate differential cross section for four-atom reactions in full dimension, utilizing an improved version of reactant-product-decoupling scheme. Differential cross sections for the title reaction were calculated for collision energy up to 0.4 eV. It is found that the differential cross sections for the reaction are all peaked in the backward direction. The majority of H(2)O is produced in the first stretch excited state, with a large fraction of available energy for the reaction going into H(2)O internal motion. As compared in a previous report by Xiao et al. [Science 333, 440 (2011)], the differential cross section at E(c) = 0.3 eV and the differential cross section at the backward direction as a function of collision energy agree with experiment very well, indicating it is possible now to calculate complete dynamical information for some simple four-atom reactions, as have been done for three-atom reactions in the past decades.     

Mean potential statistical theory of the H+ + D2 → HD + D+ reaction

The reactive collision process H(+) + D(2)(ν = 0, j = 0) → HD + D(+) is theoretically analyzed for collision energies ranging from threshold up to 1.3 eV. It is assumed that the reaction takes place via formation of a collision complex. In calculations, a statistical theory is used, based on a mean isotropic potential deduced from a full potential energy surface. Calculated integral cross sections, opacity functions, and rotational distributions of the HD products are compared with recent statistical and quantum mechanical calculations performed using a full potential energy surface. Satisfactory agreement between the results obtained using the two statistical methods is found, both of which however overestimate the existing quantum mechanical predictions. The effects due to the presence of identical particles are also discussed.     

H(D) → D(H) + Cu(111) collision system: molecular dynamics study of surface temperature effects

All the channels of the reaction dynamics of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Cu(111) surface have been studied by quasiclassical constant energy molecular dynamics simulations. The surface is flexible and is prepared at different temperature values, such as 30 K, 94 K, and 160 K. The adsorbates were distributed randomly on the surface to create 0.18 ML, 0.28 ML, and 0.50 ML of coverages. The multi-layer slab is mimicked by a many-body embedded-atom potential energy function. The slab atoms can move according to the exerted external forces. Treating the slab atoms non-rigid has an important effect on the dynamics of the projectile atom and adsorbates. Significant energy transfer from the projectile atom to the surface lattice atoms takes place especially during the first impact that modifies significantly the details of the dynamics of the collisions. Effects of the different temperatures of the slab are investigated in this study. Interaction between the surface atoms and the adsorbates is modeled by a modified London-Eyring-Polanyi-Sato (LEPS) function. The LEPS parameters are determined by using the total energy values which were calculated by a density functional theory and a generalized gradient approximation for an exchange-correlation energy for many different orientations, and locations of one- and two-hydrogen atoms on the Cu(111) surface. The rms value of the fitting procedure is about 0.16 eV. Many different channels of the processes on the surface have been examined, such as inelastic reflection of the incident hydrogen, subsurface penetration of the incident projectile and adsorbates, sticking of the incident atom on the surface. In addition, hot-atom and Eley-Rideal direct processes are investigated. The hot-atom process is found to be more significant than the Eley-Rideal process. Furthermore, the rate of subsurface penetration is larger than the sticking rate on the surface. In addition, these results are compared and analyzed as a function of the surface temperatures.     

Infinite order sudden approximation treatment of the H + D2 → HD + D reaction

Results of a study of the H + D₂ → HD + D reaction within the quantum infinite order sudden approximation (RIOS) are reported here for the total energy range 1.0317 ⩽ E_t ⩽ 2.1417 eV. We present the vibrationally selected integral and total integral cross sections, and the latter are compared with both classical and experimental results. The RIOS results are in excellent agreement with the experimental results at 1.0317 eV and in reasonable agreement (≈ 20% high) at 2.1417 eV.     

Semiclassical glory analyses in the time domain for the H + D2(v(i) = 0, j(i) = 0) → HD(v(f) = 3, j(f) = 0) + D reaction

We make the first application of semiclassical (SC) techniques to the plane-wavepacket formulation of time-domain (T-domain) scattering. The angular scattering of the state-to-state reaction, H + D(2)(v(i) = 0, j(i) = 0) → HD(v(f) = 3, j(f) = 0) + D, is analysed, where v and j are vibrational and rotational quantum numbers, respectively. It is proved that the forward-angle scattering in the T-domain, which arises from a delayed mechanism, is an example of a glory. The SC techniques used in the T-domain are: An integral transitional approximation, a semiclassical transitional approximation, a uniform semiclassical approximation (USA), a primitive semiclassical approximation and a classical semiclassical approximation. Nearside-farside (NF) scattering theory is also employed, both partial wave and SC, since a NF analysis provides valuable insights into oscillatory structures present in the full scattering pattern. In addition, we incorporate techniques into the SC theory called "one linear fit" and "two linear fits", which allow the derivative of the quantum deflection function, Θ?(')(J), to be estimated when Θ?J exhibits undulations as a function of J, the total angular momentum variable. The input to our SC analyses is numerical scattering (S) matrix data, calculated from accurate quantum collisional calculations for the Boothroyd-Keogh-Martin-Peterson potential energy surface No. 2, in the energy domain (E-domain), from which accurate S matrix elements in the T-domain are generated. In the E-domain, we introduce a new technique, called "T-to-E domain SC analysis." It half-Fourier transforms the E-domain accurate quantum scattering amplitude to the T-domain, where we carry out a SC analysis; this is followed by an inverse half-Fourier transform of the T-domain SC scattering amplitude back to the E-domain. We demonstrate that T-to-E USA differential cross sections (DCSs) agree well with exact quantum DCSs at forward angles, for energies where a direct USA analysis in the E-domain fails.     

Photoelectron branching ratios and partial photoionization cross sections for production of the (2a1)−1 state of H2O up to 200 eV

The photoelectron branching ratio for the production of the (2a₁)⁻¹ state of H₂O⁺ has been measured in the 50–200 eV photon-energy range using synchrotron radiation and magic-angle photoelectron spectroscopy. Partial photoionization cross sections are derived from the measured branching ratios using previously reported absolute photoabsorption cross sections. The results are consistent with earlier measurements from threshold to 60 eV obtained with dipole (e, 2e) spectroscopy.     

Quantum-mechanical calculation of the reactions D + FH(ν = 0, 1, 2) → DF(ν′) + H and H + FD(ν = 0, 1, 2, 3) → HF(ν′) + D on a realistic potential energy surface

Collinear (two-mathematical-dimensional (2MD)) coupled-channel quantum-mechanical calculations have been performed on the reactions D + FH(ν = 0, 1, 2) → DF(ν′) + H and H + FD(ν = 0, 1, 2, 3) → HF(ν′) + D on a potential energy surface with a 40 kcal/mole barrier to exchange. This barrier height is close to that predicted by ab initio calculations and suggested by experiments. The relative effectiveness of reagent vibrational and translational excitation to promote reaction is considered. A one-mathematical-dimensional (1MD) model for these reactions is constructed and is shown to work very well for the D + FH reaction at high temperatures, and less well for that reaction at lower temperatures as well as for the reverse H + FD reaction. Possible reasons for the breakdowns of this model are discussed.     

The gas-phase and solution mechanism of the isotope exchange reaction OH- + D2 = OD- + HD: Beam study of the solvated-ion reaction OH-. H2O + D2 in the collision energy range 0–2 eV

In a tandem mass spectrometer we have measured the excitation functions (reaction cross section as a function of collision energy) for the following solvated-ion reactant pairs: OH^-.(H₂O) + H₂; OD^-.(D₂O) + D₂; and OH^-.(H₂O) + D₂—in the collision energy range 0–2 eV. Product channels include H₃O^--type production, collision-induced dissociation of reactants and products (OH^- and H^- types) and isotopic mixing. These solvated-ion reactions are used to correlate the reactivity of the isotope exchange reaction OH^- + D₂→OD^- + HD occuring in the gas phase and solution, identifying a proton-transfer mechanism occuring within an H₃O^- intermediate.     

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.     

The absorption cross section of the HCO radical at 614.59 nm and the rate constant for HCO+HCO → H2CO+CO

Using a combination of XeCl exciplex laser flash photolysis of gas-phase glyoxal and formaldehyde and time-resolved cw dye laser absorption at 614.59 nm, we have determined the ratio k₁/σ for the reaction HCO+HCO → H₂CO+CO (1) at 295 ±2 K. Similar studies involving the 308 nm photolysis of a variety of aldehydes combined with a determination of the absolute yields of the resulting hydrocarbon products have allowed us to deduce the initial yields of HCO radicals and hence the absorption cross section for HCO at the monitoring wavelength. We find σ=(2.3±0.6) × 10⁻¹⁸ cm², giving k₁=(7.5±2.9)× 10⁻¹¹cm³ molecule⁻¹ s⁻¹. Our values are compared with previous results.     

Angular scattering using parameterized S matrix elements for the H + D2(v(i) = 0, j(i) = 0) → HD(v(f) = 3, j(f) = 0) + D reaction: an example of Heisenberg's S matrix programme

A neglected topic in the theory of reactive scattering is the use of parameterized scattering (S) matrix elements to calculate differential cross sections (DCSs). We construct four simple parameterizations, whose moduli are smooth step-functions and whose phases are quadratic functions of the total angular momentum quantum number. Application is made to forward glory scattering in the DCS of the H + D(2)(v(i) = 0, j(i) = 0) → HD(v(f) = 3, j(f) = 0) + D reaction at a translational energy of 1.81 eV, where v and j are vibrational and rotational quantum numbers respectively. The parameterized S matrix elements can reproduce the forward scattering for centre-of-mass reactive scattering angles up to 30° and can identify the total angular momenta (equivalently, impact parameters) that contribute to the glory. The theoretical techniques employed to analyze structure in the DCS include: nearside-farside theory, local angular momentum theory--in both cases incorporating resummations of the partial wave series representation of the scattering amplitude--and the uniform semiclassical theory of forward glory scattering. Our approach is an example of Heisenberg's S matrix programme, in which no potential energy surface is used. Our calculations for the DCS using the four parameterized S matrix elements are counterexamples to the following universal statements often found in the chemical physics literature: "every molecular scattering investigation needs detailed information about the interaction potential," and "an accurate potential energy surface is an essential element in carrying out simulations of a chemical reaction". Both these statements are false.     

Abstraction versus insertion in the reaction O(1D2)+H2→OH(υ″ = 2) + H

The product hydroxyl radical arising from the reaction O(¹D₂)+H₂→OH+H was detected by LIF following excitation of the off-diagonal transition OH(A²Σ⁺, υ′=1←X²Π, υ″=2) in the region 348–357 nm. The rotational population distribution in υ″=2 appears to be inverted and quite similar to that previously reported for υ″=0 and 1. Because rotationally cool OH was not observed, there is no evidence for the existence of an abstractive pathway in which the subject reaction occurs without the initial formation, via insertion, of a chemically activated HOH collision complex.