Theoretical study of single-electron capture in the N5+ + He and O6+ + He collisions by means of ab initio methods

Partial cross sections of single-electron capture on the n = 3 levels have been determined theoretically for the N⁵⁺ + He and O⁶⁺ + He collisions by means of a semiclassical method using ab initio potential energy curves and radial and rotational coupling matrix elements. The different behavior of these two isoelectronic systems is fairly well reproduced by our calculations.     

Resonances in collinear He + H2+ collisions

Reactive and non-reactive transition probabilities are reported for the collinear He + H₂⁺ collisions in the energy range 0.955–1.5 eV. An interesting mirror-image relation between states of the same symmetry is pointed out. The scattering resonances are interpreted qualitatively in terms of the bound states supported by the vibrational adiabatic potential in hyperspherical coordinates.     

CAS SCF/CI calculations of potential energy surfaces of He3+ and He2+

Complete active space MC SCF (CAS SCF) calculations followed by second-order configuration interaction (SOCI) calculations are carried out on the potential energy surfaces (bending surface, linear surfaces) of the ²Σ_g⁺ ground state of He₃⁺. The potential minimum for the ²Σ_g⁺ state occurs at a linear geometry with HeHe bond length of 1.248 Å. The binding energy of He₃⁺ with respect to He + He⁺ + He was calculated to be 2.47 eV at the SOCI level. The energy required to dissociate He₃⁺ (²Σ_g⁺) into He₂⁺ (²Σ_u⁺) and He(¹S) is calculated to be 0.14 eV. The same level of SOCI calculations of He₂⁺ yield a D_e value of 2.36 eV.     

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     

Collinear quantum mechanical calculations of the He + H+2 proton transfer reactions

Exact quantum mechanical results for collinear He + H⁺₂ → H + HeH⁺ reactive collisions are presented for the (total) energy range of 0.93 cV to 1.4 eV. The H⁺₂ initial vibrational states include ν = 0 through ν = 5. The diatomics-in-molecules semi-empirical surface of Kuntz is used in the computations. Except for a short range of energies, the calculated reaction probabilities of H⁺₂ (ν = 0) are larger than those of excited H⁺₂.     

Single electron capture by C3+ ions in He,Ne and Ar

Translational energy spectra have been obtained for 6 keV C²⁺ ions resulting from single-electron capture by 6 keV C³⁺ ions in collision with He, Ne and Ar. Our data for He and Ne are in good agreement with previous measurements, while data for the Ar target have not appeared in the literature. The spectrum for C³⁺ in Ar is complex and appears to contain many strong spectral features which involve capture with excitation of the target product ion in Ar⁺.     

Theoretical study of single-electron capture in He2+ —H− collision

We present a detailed theoretical treatment of single-electron transfer between He²⁺ and H^?. The total cross section is calculated using stationary molecular states which are appropriate in the energy range covered by the experiments (between 0.5 and 2250 eV in the centre of mass frame). We use an expansion on a two-electron basis built with one-electron diatomic molecule (OEDM) orbitals and including the common translation factor of Errea et al. All coupling terms are calculated explicitly. Because of the small binding energy of H^? compared to that of the ground state of He⁺, capture occurs into highly excited states of He⁺. Results obtained with a straight-line quasiclassical calculation are in good agreement with the experimental data. At low energy, He⁺ (n=5) +H(1s) is the dominant capture channel; at higher energy, the He⁺ (n=4) + H(1s) channel becomes important. The rise in the cross section below 6 eV can be attributed to the Coulomb attraction in the incoming channel. To account for this effect, a fully quantal calculation has been performed. The agreement with the low-energy measurements is then excellent.     

Scattering of low energy He2+ ions by Ne,Ar, and Kr atoms

Experimental studies of collisions of He²⁺ ions with Ne, Ar, and Kr atoms have been carried out at laboratory kinetic energies in the range 8 ? E₁ ? 10 eV. For each collision pair, relative differential cross sections for elastic scattering, and for the formation of He⁺ by single charge transfer [e.g., He²⁺ + R = He⁺ + (R⁺)^*] were measured. Information concerning the initial states of the charge transfer products was also obtained, from measurements of the kinetic energy distributions of the He⁺ + He = Ne⁺(2s 2p⁶²S) ± He⁺(²S), whereas for the other systems, transfer proceeds via a number of channels. The He⁺-ion kinetic energy measurements indicated that for He²⁺. Ar both Ar⁺ both Ar⁺ and Ar²⁺ are formed in transfer, and that for He²⁺, Kr only Kr²⁺ (and no Kr⁺) was formed.The differential elastic scattering patterns were analyzed by means of cross section calculations based on an approximate form of the optical model. These calculations indicated that the pronounced shoulders observed in the σ_el(θ) versus θ curves arose from scattering from an attractive potential well, in the presence of concurrent inelastic scattering. Using parametrized Morse potentials to represent the ground electronic states of (HeNe)²⁺, (HeAr)²⁺, and (HeKr)²⁺, the corresponding well-depth are estimated to be, respectively: 1.0 eV, 2.1 eV and 2.6 eV.     

Simultaneous electron capture and target ion excitation in collisions of C4+ and N5+ on He

Cross sections for simultaneous electron capture and target ion excitation have been measured for impact of slow He-like C⁴⁺ and N⁵⁺ ions on He. The energy of the primary ion beams has been varied over more than two orders of magnitude: 0.05–7 keV/amu. The results are discussed on basis of a slightly modified version of the dynamic classical over-barrier model for multiple electron capture. The differences in the energy dependences of the experimental results of C⁴⁺ and N⁵⁺ — He can be explained qualitatively by assuming that for N⁵⁺ binding energy sharing between the two participating electrons is of importance, particularly at the lower impact energies.     

Multiphoton association reaction He + H+ → HeH+ steered by ultra‐short laser pulse

The multiphoton association reaction He + H⁺ → HeH⁺ in the electronic ground state is investigated using the time‐dependent quantum wave packet method. It is shown that the collision pairs He + H⁺ in continuum state transfer into ν = 0 state and then produce stable molecules HeH⁺ through emission of two or three photons. The multiphoton transition takes place via intermediate states, and the transfer probability is determined by the collision energy and the intermediate states. The populations of the intermediate states and ν = 0 state can be controlled by the laser duration. The three‐photon transition is more efficient than the two‐photon transition for producing the molecule HeH⁺ in ν = 0 state. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Experimental and theoretical studies of the Bi-excited collision systems He*(23 S) + He*(23 S, 21 S) at thermal and subthermal kinetic energies

We have carried out a comprehensive experimental and theoretical investigation of the autoionizing collision systems He*(2³ S, 2¹ S) + He*(2³ S). We present high resolution electron energy spectra, obtained with a single He* beam (average relative collision energy 〈E _rel〉=1.6 meV) and with crossed He* beams (〈E _rel〉> =61 meV). The spectra show substantial structure, and under single beam conditions fast oscillations due to the interference of incoming and outgoing heavy particle waves in the entrance channels are observed. Accurate ab initio potential curves for the seven lowest He*—He*(Σ) molecular states have been obtained from a Feshbach projection scheme, and width functions for He*(2³ S) + He*(2³ S) have been derived by Stieltjes imaging. Based on these ab initio data, detailed quantum mechanical calculations of the electron spectra have been carried out and provide a thorough understanding of the experimentally observed spectral features. Good overall agreement of the calculated spectra with the experimental data is observed. The close coincidence in the positions of the experimental and theoretical peaks, especially for He*(2³ S) + He*(2³ S), underlines the reliability of the ab initio potentials. In the He*(2¹ S) + He*(2³ S) electron spectrum, the dominant peak is traced to be due to autoionization from the 2³Σ⁺ _g molecular state accessed via an avoided crossing. We also present a detailed discussion of the total ionization cross sections σ_tot and of the fraction σ_AI/σ_tot for associative ionization together with a critical comparison with previous work. The ionization probabilities for close collisions in entrance channels, from which autoionization is spin-allowed, are near unity, and therefore the absolute values and the collision energy dependence of the total cross sections simply reflect the long-range behaviour of the excited state potentials.     

Collinear quantum mechanical calculations for the reaction: He + H2+ → HeH+ + H

An integral equation reaction path technique has been used to calculate, within the collinear approximation, reaction probabilities for the reaction He + H₂⁺ → HeH⁺ + H over the energy range 0.95 <- E <- 1.19 eV. This reaction differs from those that have been studied previously within the collinear approximation in that a severe oscillatory behavior is exhibited in the energy dependence of the reaction probabilities.     

Excitation of He—41 D-substates by He+- and Ne+-projectiles investigated by level-crossing techniques

Collisions of He⁺- and Ne⁺-projectiles with He at impact energies between 90 keV and 800 keV were investigated. Relative excitation cross sections for magnetic sublevels of He—4¹ D were determined using level-crossing techniques. Absolute excitation cross sections σ _m of the Zeeman-sublevels are given using He—4¹ D cross sections from earlier measurements. The results show strong variations of the cross sections σ₀ and σ_±1 with a quasi-oscillatory behaviour. σ_±2 is much smaller than σ₀ and σ_±1.     

Mobilities of H+3 and HeH+ ions in helium gas and rate coefficient for HeH+ + H2 → H+3 + He

The mobilities of mass-identified H⁺₃ and HeH⁺ ions in helium and the reaction rate coefficient for HeH⁺ + H₂ → H⁺₃ + He have been measured by a drift-tube quadrupole mass spectrometer at 300 K. The zero-field reduced mobilities of H⁺₃ and HeH⁺ ions, corrected to 273 K, are 31.0 ± 0.8 and 23.4 ± 0.6 cm² V^?1 s^?1 respectively. The reaction rate coefficient was found to be (1.26 + 0.16) × 10^?9 cm³s^?1 and was observed to be independent of the mean ion kinetic energy in the range from 0.04 to 0.3 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.     

Quantum mechanical scattering calculations on a spline-fitted ab initio surface: the He + H+2 (ν = 0, 1, 2) → HeH+ + H reaction

All-channel time-dependent quantum mechanical reaction probabilities are reported for the collinear He + H⁺₂(ν = 0, 1, 2) → HeH⁺ + H reaction at a total energy of 1.2 eV on previously reported diatomics-in-molecule (DIM) and spline fitted ab initio (SAI) surfaces. These results are in agreement with the previous quasiclassical trajectory results in that there is vibrational enhancement of the reaction probability on the SAI surface but not on the DIM surface. This agreement lends support to our previously drawn conclusion that small differences in the potential-energy surface can lead to substantially different dynamic results.     

Long-Range interaction coefficients between H,He+, He,and Li+

Using the new expression for the dynamic multipole polarizabilities in terms of effective oscillator strengths and effective transition energies derived from the variational procedure based on the total energy expansion in terms of induced multipole moments, we have calculated the long-range dispersion force coefficients and the leading relativistic correction to the long-range potential between H, He⁺, He, and Li⁺. We have in addition calculated the oscillator strength sum rules for the above systems. Our results are comparable with those obtained using both a hydrodynamic model to quantum mechanics and double perturbation theory.     

Recoil-ion momentum distribution for He(e,2e)He+-and He(e,3e)He++-reactions

Using Recoil-Ion Momentum Spectroscopy (RIMS) we have measured the momenta of recoiling target ions in He(e,2e)He⁺- and He(e,3e)He⁺⁺-reactions at impact energies between 270 ev and 3200 eV. The recoil-ion momentum reflecting the sum momentum of all outgoing electrons was determined for the first time in all three spatial dimensions separately for single and double ionization by electron impact. The data are compared to results of a nCTMC-calculation.     

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.     

Atomic and molecular impurities in4He clusters

Recoil-ion charge distributions produced in single collisions of 8 MeV/u Kr¹³⁺ and Kr³²⁺ projectiles with Xe atoms have been measured using time-of-flight spectroscopy. The post-collision charge states of the projectile ions were determined by magnetic dispersion onto a position sensitive microchannel plate detector. The recoil-ion distributions for ionization accompanied by electron loss from Kr¹³⁺ projectiles were bell-shaped with averages that ranged from 7.4 for 1-electron loss to 13.9 for 5-electron loss. The recoil-ion distributions for ionization accompanied by electron capture to Kr³²⁺ projectiles were also bell-shaped, but had much higher average charges that ranged from 20.4 for 1-electron capture to 28.5 for 4-electron capture. The large difference in the average charges produced in the two types of collisions is mainly attributable to charge magnification by Auger decay. A simple model quantitatively explains the variation of the capture-ionization charge distribution width and average charge as a function of the number of captured electrons.