Charge asymmetry effects in inner-shell alignment induced by ion-atom collisions

Charge asymmetry effects in an alignment of inner-shell vacancies induced by ion-atom collisions are studied theoretically. The corrections to the first order calculations of the alignment which depend on the sign of the projectile charge are discussed. The effect of these corrections on the differential alignment is large and can be studied in coincidence experiments with proton and antiproton beams of low energies. The calculations show that with increasing projectile energy the charge asymmetry effects decrease and become practically negligible when the projectile velocity is comparable with the velocity of the electron in the shell under consideration. The total alignment depends only weakly on the charge of the projectile. The calculated differences of the integralA g (2p ₃ ^2/?1 ) alignment produced by protons and antiprotons are not greater than 20% in the whole energy range under study.     

Orbital alignment dependence of electron transfer cross sections

his paper reports experimental results for the influence of target excitation and orbital alignment on the charge exchange process for the systems Ne⁺, Ar⁺-Na(3s,3p) for impact energies in the 1-15 keV range. The cross section parameters are found to depend sensitively on collision velocity and choice of projectile. Using earlier results by Aumayr et al (Z. Phys. D 6(1987) 145-153) for the Na(3s) target, the relative cross sections are put on an absolute scale. A strong dependence of the total electron transfer cross section on the target state is observed, most dramatically for low energy Ar⁺ impact for which the cross section for a Na(3p) target is more than 200 times larger than for Na(3s). Time-of-flight spectra show that electron transfer channels with energy defects near zero are strongly preferred. Cross section estimates based on the Demkov-Olson model account well for the major trends observed, but not for the detailed behavior.     

Orientation and alignment in charge exchange processes involving sodium atoms studied by semi-classical molecular theory

The semi-classical molecular model is applied to study the charge exchange processes in the H⁺-Na 3p and Li⁺-Na 3p systems in the keV energy range. The dependence of the charge exchange on the orientation and the alignment of the initial or final state is obtained for the transition probabilities and for the differential cross sections. The 14 state present model for the H⁺-Na system is in good agreement with the experimental differential cross sections. The alignment is explained by orientation occurring in the transfer region. The cross sections predicted with a 28 state model for Li⁺-Na exhibit similar behaviour as for H⁺-Na.     

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.     

On long range effects in non-adiabatic collisions of atoms with molecules

Long range effects in atom-molecule collisions are considered. They are shown to have an important effect on the branching ratio between states that are asymptotically degenerate or nearly degenerate. A simple model shows that depending on the initial preparation of the reagents the occupation probability for a molecular state may be quite different. A semiclassical approach is proposed to include the coriolis interaction at long range into the scattering calculations. The model is applied to quenching collisions between Na*(3p) and diatomic molecules.     

The effect of the electron spin on angular momentum transfer in Na*(32 P 3/2)+Na+ collisions — crossed beam experiment and semiclassical calculation

The angular momentum transfer between electronic and heavy particle motion has been studied for inelastic collisions of laser state-prepared Na*(3² P _3/2,M _J ) with Na⁺ leading to Na*(3² D) or Na(3² S) in the energy rangeE _cm=5?47.5 eV. The measurements are compared to semiclassical calculations employing the coupled channel method in the impact parameter approximation but including dynamics of the electron spin coupling to the heavy particle motion.     

Semiclassical exponential perturbation theory. An adiabatic approach for rotationally inelastic scattering

The exponential perturbation approximation is tested in the semiclassical perturbed stationary states frame, using spline functions for interpolation of the non-adiabatic couplings. An application is realized for the p-H₂ + ⁴He rotational cross sections.     

Propensity rules for orientation by atom impact: I. three-state description of directS ↔P transitions

P state orientation and alignment created in direct, collision-inducedS ?P transitions of a quasi-one-electron atom are analyzed in the natural coordinate frame. In the velocity region of maximum transition probability, propensity rules for the orientation are derived, and their range of validity in impact parameter and velocity is discussed. The predictions are tested and illustrated by three-state calculations for Na(3s ? 3p) transitions in Na — He collisions.     

The state dependence of the interaction of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne,Ar, Kr,Xe) with ground state sodium atoms

Using crossed beams of metastable rare gas atoms Rg*(ms³ P ₂,³ P ₀) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s ² S _1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(³ P ₂)+Na(3s), the spectra are quite similar for the different rare gases, both in width and shape; they reflect attractive interactions in the entrance channel with well depthsD* _e [meV] decreasing slowly from Rg=Ne to Xe as follows: 676(18); 602(23); 565(26); 555(30). For Rg*(³ P ₀)+Na(3s), the spectra vary strongly with the rare gas, indicating a change in the character of the interaction from van der Waals type attraction (Ne) to chemical binding for Kr and Xe with well depthsD* _e [meV] of: 51(19); 107(25); 432(30); 530(50). These findings are explained through model calculations of the respective potential curves, in which the exchange and the spin orbit interaction in the excited rare gas and the molecular interaction between the two valences-electrons in terms of suitably chosen singlet and triplet potentials are taken into account. These calculations also explain qualitatively the experimental finding that the ratiosq ₂/q ₀ of the ionization cross sections for Rg*(³ P ₂)+Na and Rg*(³ P ₀)+Na vary strongly with the rare gas from Ne to Xe as follows: 15.8(3.2); 2.6(4); 1.4(2); 1.6(4).     

M-preserving propensities for rotationally inelastic NH3-He collisions. In the kinematic apse frame

Semiclassical calculations are used to obtain integral cross sections in both the initial velocity, or collision-fixed, frame and the kinematic apse frame (defined by Khare et al.) for rotational excitation in the NH₃-He system at a collision energy of 100 cm^?. A comparison with earlier close-coupling calculations in the initial velocity frame. using the same interaction potential, shows that the semiclassical method predicts relative M-dependence quite accurately. The kinematic apse cross sections, unlike those in the initial velocity frame, are highly diagonal in the quantum number M. An examination of the M-dependent transition probabilities as a function of impact parameter indicates that the propensity for conservation of M in the kinematic apse frame is due to the dominance of backward, or high-angle, scattering in the cross section. The propensity for conservation of M breaks down for large impact parameters where the interaction is attractive.     

Spin dependence of low energy charge exchange between H 2 + and Na

The difference in charge exchange rate in collisions between spin oriented sodium atoms and H ₂ ⁺ ions has been measured at an energy of about 1 eV. H ₂ ⁺ was stored in a Penning trap and polarized by spin exchange with Na beam atoms from a hexapole magnet. The ion loss from the trap due to charge exchange was different as we depolarized the atomic beam. From the data we obtain a ratio of cross sections for singlet and triplet collisionsQ ₁/Q ₃=1.5±0.2 andQ ₃=1.2·10^?15 cm².     

Atomic alignment effect on the branching to ArCl* and CCl2* formation in the reaction of oriented Ar(3P2,MJ=2) + CCl4

Atomic alignment effects for the formation of ArCl*(C) and CCl2*(A) in the reaction of Ar((3)P 2) + CCl 4 have been measured by using an oriented Ar( (3)P2, M J=2) beam at a collision energy of 0.08 eV. The emission intensity for ArCl*(C) and CCl2*(A) has been measured as a function of the magnetic orientation field direction in the collision frame. A significant atomic alignment effect is observed for the atom transfer process [ArCl*(C) formation]. Formation of ArCl*(C) is modestly enhanced when the electron angular momentum of the Ar((3)P 2) reactant is aligned along the relative velocity vector, while the excitation transfer process [CCl2*(A) formation] shows little alignment effect.     

Depolarization and fine structure effects in half collisions of sodium-noble gas systems

Population ratios and polarization of the Na3P _J fine structure states following far wing photon excitation of Na-noble gas collision pairs are studied with respect to the underlying interaction potentials and molecular coupling schemes. For this purpose spectral profiles of these quantities, i.e. its dependencies on excitation frequency, have been measured up to ±200 cm^?1 detuning from the NaD lines for NaAr and NaHe under nearly single collision conditions. Comparing the measured population ratios with quantummechanical coupled channels calculations the Σ potential well depths differ considerably from results of model potential calculations. Large residualJ=3/2 alignment observed in the far wings disagrees with simple models assuming incoherent excitation and/or full adiabatic reorientation of the radiating dipole along straight trajectories. By comparison with Lewis model calculations, using realistic trajectories and decoupling radius, it is found instead, that realistic trajectories are constitutive for alignment after Σ-excitation (blue wing), whereas coherence between the²Π_1/2 and²Π_3/2 states determine primarily alignment after Π-excitation (red wing).     

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.     

Collisions at thermal energy between metastable hydrogen atoms and hydrogen molecules: total and differential cross sections

A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3–5 km/s). Total integral cross sections for H*(D*)(2s) + H₂(X ¹Σ _g ⁺ ,v=0) collisions have been measured in a wide range of relative velocity (2,5–30 km/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H₂ total differential cross sections have also been measured, with an angular spread of 3.6°, for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented.     

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.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Proton-impact excitation of HeI triplet levels

We investigated the excitation of the λ(1s3d ³ D?1s2p ³ P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦E _p≦25 keV. In apparent contradiction to Wigner's spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d ³ D level of HeI proceeds via 1snl states withl≧3, which populate the 3³ D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigner's rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |m _L|≦1 are excited.     

Orientation of Na(3p) states excited in Na-He collisions

The orientation of Na(3p) states created in 3–13 keV Na(3s)-He collisions has been studied by the polarised photon-scattered particle coincidence technique at scattering angles corresponding to the impact-parameter range 1–2 a.u. In the standard geometry, at large impact parameters the excitation process exhibits a very high degree of orientation with about 90% of the Na(3p) states havingm ₁=?1. Strong reduction in this propensity is observed at impact parameters smaller than about 1.3 a.u., where a molecular curve crossing causes simultaneous He(n=2) excitation. In this region, also ionisation becomes important.