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.     

Collision energy dependence of He and Ne capture by C 60 +

The hyperfine structure and the isotope shifts in the optical transition of HfI with λ=5903 Å have been investigated using high resolution laser spectroscopy. The magnetic dipole and electric quadrupole hyperfine splitting constants of the lower 5d ² 6s ² a ³ F ₃ and upper 5d ² 6s 6p z ⁵ G ₃ ⁰ levels have been determined for the two stable odd hafnium isotopes¹⁷⁷Hf and¹⁷⁹Hf.     

Field-assisted photodesorption of He,Ne, Ar,Kr and CO ions from W

The electric field dependence of the photodesorption yield of several noble gases from a tungsten surface was studied. Above a threshold field the desorption rate of ions increased until the field reached a characteristic value, which corresponds closely to the “best image field” observed for the respective gas in field ion microscopy. At still higher fields, the rate of photodesorption decreased rapidly. The intensity of the desorption signal increased in the order Kr ⪡ Ar < Ne ≈ He. Much less influence of field strength was found with a strongly chemisorbed species, CO.     

Spin violation in double-electron capture by Ar4+ ions

Translational energy spectroscopy has been employed to study state-selective double-electron capture by 8 keV Ar⁴⁺ in He and Ne targets, and 12 keV Ar⁴⁺ in He. The capture spectra are dominated by reaction channels which violate spin conservation. The double capture spectra for Ar⁴⁺ in He contain intense peaks which involve capture from the metastable quintet ⁵S⁰ state of Ar⁴⁺, recently identified in single capture spectra for Ar⁴⁺ in He and Ne.     

Electron loss and capture to continuum from He and Ne atoms bombarded with He+-Ions

We have measured the cusp electron yield in coincidence with the transmitted charge state (He⁰, He⁺ and He⁺⁺) when³He⁺ collides with He and Ne under single collision conditions. For the first time this enables the electron capture to the continuum (ECC) yield to be directly compared with that from electron loss to continuum (ELC). While the ECC contribution is smaller than that from ELC at high projectile velocities (V _p >3 au) the data suggest that ECC will dominate belowV _p =2.8 au. The relevance of the results to the projectile velocity dependence of existing capture theories is discussed.     

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.     

Collision-induced absorption in Ne + Ar

Accurate quantum-mechanical values of the collision-induced absorption coefficient are reported for a NeAr mixture in the binary-collision approximation. Two of the known expressions for the induced dipole moment are tested and two others are proposed to obtain good agreement with experiment.     

Dispersion energy in rare gas dimers from an ab initio perturbative procedure: Ne + Ne,Ar + Ar

The dispersion energy between two neon and two argon atoms is computed from an ab initio perturbative procedure not based on the multipole expansion. A comparison with the multipole expansion provides C₆ = 5.36 for Ne and 76.6 for Ar. It is seen that one d polarization function provides the main part of the C₆R^?6 contribution, the exponent of this function probably being related to the polarizability of the molecule. The multipole expansion seems acceptable up to the van der Waals minimum but quite invalid for smaller distances, and doubtful at the van der Waals minimum itself.     

Luminescence of Ar+-ions emitted after electron capture of Ar2+-ions from K-atoms

The optical attenuation method (OAM) was used to determine the cross-sections for electron capture of Ar²⁺ ions traversing a K vapour. Different values have been found for Ar²⁺ ions in the ground state 3p ^{4 3} P:σ(³ P)=(1.61+/?0.23)·10^?14 cm² and the first metastable state 3p ^{4 1} D:σ(¹ D)=1.88+/?0.21)·10^?14 cm² at the kinetic energy of 4.1 keV applied in the experiment. The values are comparable with Matsumoto's [4] values for the system Ar²⁺+Na, although the energy chosen in the latter experiment was somewhat lower, namely 1.5 keV. At vapour densities 6 times higher than in the above experiment the decay curves cannot be represented any more by a single exponential. Still a superposition of two exponentials fits the decay curve well. We suggest that this behaviour should be attributed to secondary collisions in which excited Ar⁺ ions formed by electron capture are reionized again.     

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.     

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.     

Theoretical study of Ng--NiN(2) (Ng=Ar,Ne,He)

It is shown that NiN(2) and noble gas atoms, Ar, Ne, and He, combine with the binding energy of 11.52, 4.06, and 7.37 kcal/mol, respectively, by the multireference perturbational (CASPT2) method. By the density functional theory calculations using MPWPW91 functionals, the Ni-N-N bending frequency in NiN(2) and Ar-NiN(2) is estimated as 310.7 and 358.7 cm(-1), respectively, the latter of which is in good agreement with the corresponding experimental frequency, 357.0 cm(-1), determined for NiN(2) isolated in solid argon.     

The binding energies of NO-Rg (Rg = He, Ne, Ar) determined by velocity map imaging

We report velocity map imaging measurements of the binding energies, D(0), of NO-Rg (Rg = He, Ne, Ar) complexes. The X state binding energies determined are 3.0 ± 1.8, 28.6 ± 1.7, and 93.5 ± 0.9 cm(-1) for NO-He, -Ne, and -Ar, respectively. These values compare reasonably well with ab initio calculations. Because the ?-X transitions were unable to be observed for NO-He and NO-Ne, values for the binding energies in the ? state of these complexes have not been determined. Based on our X state value and the reported ?-X origin band position, the ? state binding energy for NO-Ar was determined to be 50.6 ± 0.9 cm(-1).     

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.     

CDW and CDW-EIS investigations in an independent electron approximation for the resonant double electron capture by swift He2+ in helium

The CDW and CDW-EIS approximations are used to study the resonant double electron capture by He²⁺ in helium at intermediate and high impact velocities. Calculations of the total cross sections as a function of the impact energy are performed in an Independent Electron Approximation. A very good agreement with recent experimental data for capture into all states of the projectile is found although neither angular nor dynamical electron correlations are included in the calculations.     

M + Ng potential energy curves including spin-orbit coupling for M = K, Rb, Cs and Ng = He, Ne, Ar

The X(2)Σ(1/2)(+), A(2)Π(1∕2), A(2)Π(3∕2), and B(2)Σ(1/2)(+) potential energy curves and associated dipole matrix elements are computed for M + Ng at the spin-orbit multi-reference configuration interaction level, where M = K, Rb, Cs and Ng = He, Ne, Ar. Dissociation energies and equilibrium positions for all minima are identified and corresponding vibrational energy levels are computed. Difference potentials are used together with the quasistatic approximation to estimate the position of satellite peaks of collisionally broadened D2 lines. The comparison of potential energy curves for different alkali atom and noble gas atom combinations is facilitated by using the same level of theory for all nine M + Ng pairs.     

Cross sections for radiative charge transfer of Li+ with He,Ne, Ar and Kr targets below 4 keV

Cross sections were measured for radiative charge-transfer processes between the Li⁺ ion and inert-gas targets below 4 keV. Cascading transitions to the 2p ²P state of Li from the nd ²D states were measured; the effect was significant in the case of a Ne target.     

Exchange interaction during electron capture in Ar2+, K-collisions

During electron capture of Ar²⁺ from alkali atoms prevailingly Ar⁺ 4p states are occupied. While decaying they give rise to a spectrum which consists almost exclusively of 4p ? 4s and 4p ? 3d lines. The intensity of the lines turns out to be dependent on the Ar²⁺ projectile energy. At energies as low as 1 keV lines which belong to the Ar⁺ 3p ⁴(¹ D)n, l-system are by far the strongest. When increasing the projectile energy to 6 keV and above this system fades away and lines of the 3p ⁴(³ P)n, l-system appear. Considering that the primary beam consists constantly of 65% Ar²⁺ 3p ⁴ (³ P) and 35% Ar²⁺ 3p ³(¹ D) the strength surprises at which the 3p ⁴(¹ D)n, l system evolves from the electron capture. It is suggested that exchange interaction of the valence electron and one 3p-electron changes the 3p ⁴-core. Relative matrix elements for direct and exchange interaction of the valence electron and the 3p ⁴-core are calculated and used in a multi channel Landau Zener calculation. With this concept the measured spectra could be fitted by adopting only two parameters, namely one line strength and the ratio of the reduced matrix elements of direct and exchange interaction.