Charge transfer mechanism in N4+ + He and B3+ + He/H2 ion–atom/molecule collisions

Ab initio potential energy curves and coupling matrix elements of the molecular states involved in the collision of the N⁴⁺(2s)²S and B³⁺(1s²)¹S multicharged ions on the helium atom or molecular hydrogen have been determined by means of configuration interaction methods. The total and partial electron capture cross-sections have been evaluated in the frame of a semiclassic approach in the 1- to 100-keV laboratory energy range and compared to experimental data. A comparative study of both targets is performed in the case of the B³⁺ ion. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Quantum trajectory method calculation of collinear collisions in the hydrogen exchange reaction H + ClH′ → HCl + H′

The quantum trajectory method was used to study the collinear reaction H + ClH′ → HCl + H′. The potential energy surface was calculated on the QCISD(T)/6-311++G(3df,3pd) level. The reaction probabilities are in good accord with the results obtained by solving the Schroedinger equation using the finite difference method. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 3, pp. 156-159, May-June, 2009.     

Analytical energy surfaces for the collinear H+H2 and Li+H2 exchange reactions

A simple four-parameter function is shown to possess adequate flexibility to fit the H + H₂ →H₂ + H and Li + H₂ → LiH + H exchange reaction energy surfaces to good accuracy along the reaction paths.     

An approximate description of the double capture process in He2+ + He collisions with static correlation

It is shown that the process of resonant double electron capture in high energy He²⁺+He collisions can be approximately described by a sum over products of one-electron CDW amplitudes. The summation coefficients are determined by stationary ground-state calculations with CI wavefunctions. Total and differential cross sections are calculated and compared with experimental values.     

Coherent excitation of H(n=2) in H+, H - He collisions

The coherent excitation of H(n=2) in H⁺, H - He collisions was investigated at incident energies of 5–25 keV. From a polarization analysis of the emitted Lyman-α radiation as a function of an external electric field, the partial cross sections for excitation to the H(2s) and the H(2p _m) magnetic substates and the real part of thes ?p ₀-coherence were extracted. For H⁺-He collisions, the measured partial cross sections are in fair agreement with previous two-electron calculations by Kimura and Lin; the agreement with one-electron calculations of Jain et al. is, particularly at the lower incident energies, less satisfactory. For both collision systems, an energy-dependent forward-backward asymmetry corresponding to a shift of the center-of-charge relative to the center-of-mass (dipole moment) was observed. In H⁺ - He collisions, the measured dipole moment was positive; it thus corresponds to an electron trailing behind the proton. The same analysis applied to the H - He system showed the electron riding in front of the proton.     

Coherence parameters for He+(2p) capture and H(2p) excitation in He2+-H(1s) collisions

Semiclassical coupled channel calculations have been carried out for the collision system He²⁺-H(1s) in the velocity range 0.15–3.0 a.u. (impact energies 0.5–225 keV/amu) in order to study capture probabilities and alignment and orientation parameters for the dominant He⁺(n=2) channels. A 14-state AO basis set calculation has been combined with an analytical treatment of the asymptotic collision region. For impact velocities about and abovev=0.6 a.u. a strong propensity for resonance capture into an oriented He⁺(2p) state with the same sense of rotation as the collisional rotation of the internuclear axis is predicted together with a very smooth behaviour of the alignment angle as function of impact parameter. Eikonal method calculations of differential capture cross sections predict that the left/right orientation asymmetry will prevail in differential scattering experiments. The resulting total cross sections for capture into specificnl-substates (n=2, 3) and the total light polarisation parameter for He⁺(2p) capture compare well with previous work. Finally we report H(2s,2p) excitation cross sections, probabilties and H(2p) alignment and orientation parameters, following the established propensity rule for orientation in H(2p) excitation.     

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.     

Distorted wave calculations of vibrational excitation in CO2+He and CO2+Ar collisions

Cross sections for the vibrational excitation and relaxation of CO₂ through collisions with He and Ar atoms have been computed using several approximate methods. They all employ the infinite-order sudden approximation for the rotational motion. The vibrational motion is treated using both exact close-coupling and distorted wave techniques. The latter approximate method permits the extension of the calculation to much higher collision energies. The validity of the distorted wave approximation is examined and is shown to be particularly good for the dominant inelastic processes in He+CO₂, leading to errors of the order of 7%. These become progressively greater for smaller cross sections. The excitation cross sections are reported for several vibrational transitions over an extended energy range up to 1.36 eV.     

Total one-electron capture cross sections for Ar q+ and I q+ ions in slow collisions on H2 and He

Argon and iodine recoil ions were produced by a 2 GeV U⁷⁵⁺ beam and total one electron capture cross sections are measured for 198 eV/q Ar ^q+ (4≦q≦15) and I ^q+ (5≦q≦27 on He andH ₂. The cross section can be approximately reproduced by 1/2 πR ² according to the classical barrier model. Theq-dependences exhibit significant fluctuations even for high charge states.     

Alignment and orientation of Ar+ in He+-Ar collisions

We have measured the alignment and orientation parameters of the² F ₇ ^2/0 and² F ₅ ^2/0 states of Ar⁺ formed in the two-electron process; He⁺+Ar→He(1s ²)+Ar⁺(3p ⁴4p′). These have been measured at a collision energy of 0.25 keV/amu and for scattering angles ranging from 0.94° to 3.75°. First, by comparing the orientation parameter for the Ar⁺[(3p ⁴[¹ D]4p′² F ₇ ^2/0 ] and the Ar⁺[(3p ⁴[¹ D]4p′² F ₅ ^2/0 ] states, we have experimentally determined the importance of the spin-dependent interactions for the present collision system, by testing the Percival-Seaton hypothesis of spin independence. If the Percival-Seaton hypothesis holds for this system, the orientation parameter should beJ-independent. Secondly, the magnitude of the orientation parameter can be interpreted as resulting from the collective circulation of the unexcited 3p ⁴ electrons and the excited 4p electron. The direction of this collective circulation is compared to the propensity rule for colliding di-atom systems.     

Excitation and charge transfer in He++H collisions

For the conflictive case of He⁺+H collisions, we present a norm-method optimization of the parameters included in the (often used) two-electron translation factor of Errea et al. As surmised in a previous publication, a strong cut-off is needed at short internuclear distances to prevent the translation factor from marring the properities of the molecular expansion there. With a basis of 16 molecular states, we present the first calculations including translation factors, of total and partial charge exchange and excitation cross sections in He⁺+H collisions, as well as the alignment parameter A₂₀ for hydrogen excitation. Good agreement with experiment is reached up to the energy range where ionization and charge exchange cross sections are comparable.     

Classical trajectory study of rotational excitation in low energy HeCO and HeH2 collisions

Classical trajectory calculations for the rotational excitation of CO and H₂ by collision with He have been carried out and compared to the accurate quantum mechanical calculations of other workers. The agreement is reasonably encouranging, although some inherent limitations of this strictly classical approach are observed and discussed.     

Inelastic collisions between 4He and H2: Study of dynamical approximations

We report a detailed study of the convergence and accuracy of HeH₂ rotationally and ro-vibrationally inelastic cross sections, determined within both the coupled states (CS) and effective potential (EP) formalisms. Two different potential surfaces were used. CS total cross sections appear insensitive to the specific choice of centrifugal barrier. Although the CS results are more accurate, the EP method reproduces the important qualitative features of the various inelastic processes. In addition, with the counting-of-states correction, the EP cross sections for ro-vibrationally inelastic transitions out of low-lying rotational levels agree with the CS values to within a factor of two, with only few exceptions.     

Quantum approaches for the insertion dynamics of the H+ + D2 and D+ + H2 reactive collisions

The H(+)+D(2) and D(+)+H(2) reactive collisions are studied using a recently proposed adiabatic potential energy surface of spectroscopic accuracy. The dynamics is studied using an exact wave packet method on the adiabatic surface at energies below the curve crossing occurring at approximately 1.5 eV above the threshold. It is found that the reaction is very well described by a statistical quantum method for a zero total angular momentum (J) as compared with the exact ones, while for higher J some discrepancies are found. For J >0 different centrifugal sudden approximations are proposed and compared with the exact and statistical quantum treatments. The usual centrifugal sudden approach fails by considering too high reaction barriers and too low reaction probabilities. A new statistically modified centrifugal sudden approach is considered which corrects these two failures to a rather good extent. It is also found that an adiabatic approximation for the helicities provides results in very good agreement with the statistical method, placing the reaction barrier properly. However, both statistical and adiabatic centrifugal treatments overestimate the reaction probabilities. The reaction cross sections thus obtained with the new approaches are in rather good agreement with the exact results. In spite of these deficiencies, the quantum statistical method is well adapted for describing the insertion dynamics, and it is then used to evaluate the differential cross sections.     

Box quantization and collinear reactive collisions

The method of box quantization is applied to the two collinear reactive systems H + H₂ and H + MuH. The resonances are easily identified on the graphs giving the energies as a function of box size. It is shown that such graphs can serve as indicators of the possibility of performing for a given system an accurate adiabatic separation of variables.     

Semiclassical S matrix theory for a compound state resonance in the reactive collinear H + H2 collision

The broad compound state resonance found in quantum mechanical calculations of the collinear H + H₂ reaction is shown to correspond semiclassically to multiple collisions of the atoms within the collision complex. The quantitative agreement between QM and SC results is quite reasonable.     

Vibrational and rotational cooling of NO+ in collisions with He

A quantum mechanical investigation of the vibrational and rotational deactivation of NO(+) in collisions with He atoms in the cold and ultracold regime is presented. Ab initio potential energy calculations are carried out at BCCD(T) level and a new global 3D potential energy surface (PES) is obtained by fitting ab initio points within the reproducing kernel Hilbert space method. As a first test of this PES the bound state energies of the (3)He-NO(+) and (4)He-NO(+) complexes are calculated and compared to previous rigid rotor calculations. The efficiency of the vibrational and the rotational cooling of this molecular ion using a buffer gas of helium is then investigated by performing close coupling scattering calculations for collision energy ranging from 10(-6) to 2000 cm(-1). The calculations are performed for the two isotopes (3)He and (4)He and the results are compared to the available experimental data.     

Rovibrational product state distribution for inelastic H+D2 collisions

Experimental measurements of rovibrational product state distributions for the inelastic scattering process H + D2(nu=0,j)-->H + D2(nu' = 1,2,j') are presented and compared with the results of quasiclassical and quantum mechanical calculations. Agreement between theory and experiment is almost quantitative. Two subtle trends are found: the relative amount of energy in product rotational excitation decreases slightly with increasing collision energy and increases slightly with increasing product vibrational excitation. These trends are the reverse of what has been found for reactive scattering in which the opposite trends are much more pronounced.     

Low-energy H+ + H2 reactive collisions: mean-potential statistical model and role of permutation symmetry

Statistical theory based on a mean isotropic potential deduced from a full potential energy surface is applied to the complex-forming, reactive H(+) + H(2) system in the interval of collision energies E(c) = 10(-3) to 0.5 eV. We present expressions for the reaction probabilities that incorporate the full permutation symmetry of the protons and compare our results with other statistical models and full quantum mechanical approaches that take account this symmetry correctly, approximately, or erroneously for the exchange rearrangement mechanism of the reaction.