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.     

Energy dependence of the cross section for alkali—methyl iodide reactions

The dependence of the reaction cross section for all the alkali—methyl iodide reactions on the translational energy in the range E_T = 0–40 kcal/mole has been discussed in an idealized collision model of hard-sphere interaction between colliding particles. In all reaction of the family, the cross section increases sharply with E_T showing an Arrhenius-like positive energy dependence for E_T just past threshold and then takes a maximum value. The maximum value is largest for the Cs reaction and decreases with the alkali mass except that it slightly increases from Rb to K, and the peak becomes broader as the mass decreases. In the post-maximum region the cross section decreases slowly with E_T.     

Phenomenology and scaling of electron scattering cross sections from “almost spherical” molecules over a wide energy range

Total cross sections for electron scattering on “quasi spherical” (CH₄, SiH₄, GeH₄) molecules have been analyzed phenomenologically over a wide energy range. Regions, at low and high energies can be usefully represented by simple analytical formulae. Regularities associated with characteristic points such as the Ramsauer minima have been exposed. Comparison with other simple hydrides (NH₃, H₂O, H₂S) allows the demonstration of a possible correlation between the maximum value of the total cross section and the bond length. Some points of contact with first-principles theory are noted and in particular the energy at which the maximum cross section occurs, is related to the occurrence of a partial wave resonance. In the absence of complete data for GeH₄, prediction of characteristic points in the low energy cross section proves possible via the phenomenological analysis. Similarly, in the high energy regime, predictions of the cross section for SnH₄ is made from data on the lighter molecules of the series, using non-relativistic Thomas-Fermi self consistent field scaling.     

Free-electron model for the inelastic collision of fast electrons with benzene

Using a free-electron wave function the cross section for the excitation of the first excited state and that for the ionization of benzene in the inelastic collision with fast electrons have been estimated. In the ionization process the cross section has been found to be maximum when the secondary electron moves away with a kinetic energy of about 3 eV. For incident-electron energy above 20 eV the cross section for ionization is larger than that for excitation, while below it the reverse is the case.     

Dynamical model for the “translational excitation features” in the atom—diatom reaction cross section

A collision model for the kinetic energy dependence of the reaction cross section has been obtained by considering total angular momentum limitation at the entrance (reactants) or exit (products) reactive channel in the atom—diatom scheme. The model provides a satisfactory representation of the main “translational cross section features” (i.e., Arrhenius like behaviour near threshold, maximum, post-maximum decline and minimum) found in the reaction cross section by molecular beam experiments. Maximum and minimum explanations, including some predictions as well as a comparison with previous theoretical treatments are reported. Also an approximate method to obtain dissociation energies of the product diatom from the cross-section data is presented with relative success.     

Charge transfer reactions of organic ions containing oxygen: Correlation between reaction energetics and cross sections

Cross sections for charge transfer reactions of organic ions containing oxygen have been obtained using time-of-flight techniques. Charge transfer cross sections have been determined for reactions of 2.0 to 3.4 keV ions produced by electron impact ionization of oxygen containing molecules such as methanol, ethanal and ethanol. Experimental cross section magnitudes have been correlated with reaction energy defects computed from ion recombination energies and target ionization energies. Large cross sections are observed for reacting systems with small energy defects.     

I2-1-己烯复合物电子转移振动重组能的共振拉曼强度分析

获得了I₂-1-己烯复合物的吸收横截面和绝对共振拉曼横截面.用约270 nm光激发导致复合物的I-I伸缩振动模和C-C伸缩振动模等拉曼基频、泛频及其组合频强度的共振增强.采用含时波包理论的简单模型定量确定I₂-1-己烯复合物的光致电子转移振动重组能和均质展宽.总振动重组能3 744 cm^-1分布于4个振动模,贡献最大是I-I伸缩振动v₁₃,其值为2 490 cm^-1,约占总振动重组能的2/3.其次为C-C伸缩振动v₄₆,其值为1 170 cm^-1,约为总振动重组能的1/3.剩余2%的振动重组能是由烷基CH₃和CH₂的扭转振动v₃₆和v₂₄贡献.     

A guided-ion beam study of the reactions of Xe(+) and Xe(2+) with NH(3) at hyperthermal collision energies

We have measured the absolute cross sections for reactions of Xe(+) and Xe(2+) with NH(3) at collision energies in the range from near-thermal to ～34 and ～69?eV, respectively. For Xe(+), the cross section for charge transfer, the only exothermic channel, decreases from ～200A?(2) below 0.1 eV to ～12A?(2) at the highest energies studied. The production of NH(3) (+) is the only channel observed below 5 eV, above which a small amount of NH(2) (+) is also formed. In Xe(2+) reactions, the main products observed are NH(3) (+) and NH(2) (+). The charge transfer cross section decreases monotonically from ～80 to ～6A?(2) over the studied energy range. The NH(2) (+) cross section is similar to the charge transfer cross section at the lowest energies, and exhibits a second component above 0.4 eV, with a maximum of 65A?(2) at 0.7 eV, above which the cross section decreases to ～30A?(2) at the highest energies studied. At energies above 10 eV, a small amount of NH(+) is also observed in Xe(2+) collisions. Product recoil velocity distributions were determined at selected collision energies, using guided-ion beam time-of-flight methods.     

Quantum dynamics study of Li + HF reaction

We report rigorous quantum dynamics studies of the Li + HF reaction using the time-dependent wavepacket approach. The dynamics study is carried out on a recent ab initio potential energy surface, and state-selected reaction probabilities and cross sections are calculated up to 0.4 eV of collision energy. Many long-lived resonances (as long as 10 ps) at low collision energies (below 0.1 eV) are uncovered from the dynamics calculation. These long-lived resonances play a dominant role in the title reaction at low collision energies (below 0.1 eV). At higher energies, the direct reaction process becomes very important. The reaction probabilities from even rotational states exhibit a different energy dependence than those from odd rotational states. Our calculated integral cross section exhibits a broad maximum near the collision energy of 0.26 eV with small oscillations superimposed on the broad envelope which is reminiscent of the underlying resonance structures in reaction probabilities. The energy dependence of the present CS cross section is qualitatively different from the simple J-shifting approximation, in which a monotonic increase of cross section with collision energy was obtained. Received: 8 January 1997 / Accepted: 14 January 1997     

Electron-impact ionization of silicon tetrachloride (SiCl4)

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on silicon tetrachloride (SiCl4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast-neutral-beam apparatus. The results obtained by the two different experimental techniques were found to agree very well (better than their combined margins of error). The SiCl3(+) fragment ion has the largest partial ionization cross section with a maximum value of slightly above 6x10(-20) m2 at about 100 eV. The cross sections for the formation of SiCl4(+), SiCl+, and Cl+ have maximum values around 4x10(-20) m2. Some of the cross-section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV followed by a broad second maximum at around 100 eV. This is similar to what has been observed by us earlier for another Cl-containing molecule, TiCl4 [R. Basner, M. Schmidt, V. Tamovsky, H. Deutsch, and K. Becker, Thin Solid Films 374 291 (2000)]. The maximum cross-section values for the formation of the doubly charged ions, with the exception of SiCl3(++), are 0.05x10(-20) m2 or less. The experimentally determined total single ionization cross section of SiCl4 is compared with the results of semiempirical calculations.     

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.     

Electron photoemission from charged films: absolute cross section for trapping 0-5 eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.     

Quantum Dynamics of Li+HF/DF Reaction Investigated by a State-to-State Time-dependent Wave Packet Approach

Using the reactant coordinate based time-dependent wave packet method, on the APW potential energy surface, the differential and integral cross sections of the Li+DF/HF(v=0, j=0, 1) reactions were calculated over the collision energy range from the threshold to 0.25 eV. The initial state-specified reaction rate constants of the title reaction were also calculated. The results indicate that, compared with the Li+DF reaction, the product LiF of Li+HF reaction is a little more rotationally excited but essentially similar. The initial rotational excitation from j=0 to 1 has little effect on the Li+DF reaction. However, the rotational excitation of DF does result in a little more rotationally excited product LiF. The different cross section of both reactions is forward biased in the studied collision energy range, especially at relatively high collision energy. The resonances in the Li+HF reaction may be identifiable as the oscillations in the product ro-vibrational state-resolved integral cross sections and backward scattering as a function of collusion energy. For the Li+HF reaction, the rate constant is not sensitive to the temperature and almost has no change in the temperature range considered. For the Li+DF reaction, the rate constant increase by a factor of about 10 in the temperature range of 100?300 K. Brief comparison for the total reaction probabilities and integral cross section of the Li+HF reaction has been carried out between ours and the values reported previously. The agreement is good, and the difference should come from the better convergence of our present calculations.     

Probable nuclear reactions to produce proton rich rhenium radionuclides

The crux of the present work is to explore the various channels leading to the production of proton rich rhenium radionuclides, ^181–186Re, for different applications. The possible production routes encompass both light and heavy ion induced reactions up to a maximum 100 MeV projectile energy starting from threshold. The nuclear reaction model codes ALICE91 and PACE-II were employed in this endeavour. Excitation functions of the rhenium radionuclides have been calculated using the aforesaid nuclear reaction model codes and compared with the measured data wherever available. The contributions of preequilibrium and equilibrium reaction mechanisms to the total reaction cross section were analysed. For the first time, this study talks about the possibility of light-heavy ion (^6,7Li and ⁹Be) induced production of proton rich rhenium radionuclides.     

Formation of negative hydrogen ion: Polarization electron capture and nonthermal shielding

The influence of the nonthermal shielding on the formation of the negative hydrogen ion (H(-)) by the polarization electron capture are investigated in partially ionized generalized Lorentzian plasmas. The Bohr-Lindhard method has been applied to obtain the negative hydrogen formation radius and cross section as functions of the collision energy, de Broglie wave length, Debye length, impact parameter, and spectral index of the plasma. The result shows that the nonthermal character of the plasma enhances the formation radius of the negative hydrogen, especially, for small Debye radii. It is found that the nonthermal effect increases the formation cross section of the negative hydrogen. It is also found that the maximum position of the formation cross section approaches to the collision center with an increase of the spectral index. In addition, it is found that the formation cross section significantly decreases with an increase of the Debye length, especially, for small spectral indices.     

Experimental and theoretical studies of the reaction Ba+(D2, D)BaD+: sequential impulse model for endothermic reactions

The sequential impulse model for direct reactions of Mahan, Ruska and Winn is extended to include endothermic reactions. The model is outlined and used to predict the variation in cross section with kinetic energy for heavy atom—light homonuclear diatom reactions. The results are found to agrees well with experimental data for the reaction Ba⁺(D₂, D)BaD⁺. The bond dissociation energy of BaD⁺, 2.5 ± 0.1 eV, and the proton affinity of Ba, 250 ± 3 kcal/mol, are derived.     

Quasi-Classical Trajectory Study on O++DH (v=0, j=0)→OD++H Reaction at Different Collision Energy

The quasi-classical trajectory calculations O⁺+DH (v=0, j=0)→OD⁺+H reactions on the RODRIGO potential energy surface have been carried out to study the isotope effect onstereo-dynamics at the collision energies of 1.0, 1.5, 2.0, and 2.5 eV. The distributions of dihedral angle P(φr) and the distributions of P(θr) are discussed. Furthermore, the angular distributions of the product rotational vectors in the form of polar plot in θr and φr are calculated. The differential cross section shows interesting phenomenon that the reaction is dominated by the direct reaction mechanism. Reaction probability and reaction cross section are also calculated. The calculations indicate that the stereo-dynamics properties of the title reactions are sensitive to the collision energy.     

On trends in total cross sections for electron (positron) scattering on atoms and molecules at intermediate energies

An examination of total electron and positron scattering cross sections in a large variety of atoms and molecules reveals a trend related to ground state target dipole polarizability. Some correlations of total cross section with diamagnetic susceptibility and the number of target electrons have also been noted. Experimental positron and electron cross sections between 100 and 500 eV can be reasonably well described by a simple formula based on polarizability and energy correlation.     

S(1S) production following electron impact on thiophosgene (Cl2CS)

A special xenon matrix detector has been used to study the production of S(1S) following controlled electron impact on thiophosgene (Cl2CS) targets over an electron energy range from threshold to 400 eV. Time-of-flight spectroscopy has been used to measure S(1S) fragment kinetic energies. Fragments with energies in excess of 1 eV have been observed. The absolute cross section for S(1S) production reaches a maximum of [1.05+/-0.35] x 10(-18) cm2 at approximately 125 eV impact energy. Two different fragmentation processes, involving triplet and singlet excited states of the parent Cl2CS molecule, have been identified.     

Low temperature quantum rate coefficient of the H + CH+ reaction

In this paper we report the first theoretical study of the title reaction. A global, single-valued model of the ground-state potential energy surface has been obtained by fitting to an extensive set of high-level ab initio calculations. The surface is found to be attractive apart from linear geometries where energy barriers appear due to conical intersections. This model was then used to calculate the reactive reactant state selected cross sections for collision energies ranging from threshold up to 4000 cm(-1). These calculations were performed using our version of the Baer's approach of the RIOSA-NIP method which is based on the use of a negative imaginary potential. We find that the reaction probability is extremely oscillatory as a function of kinetic energy as it is a case for insertion reactions with a low exoergicity. The resulting reaction rate coefficient is found to first increase slowly as a function of temperature up to a broad maximum around 20 K and then to decrease slowly when temperature keeps increasing.