Orientation and alignment of Mg+ (3p) states excited in 1–60 keV collisions with He and Ar

The orientation and alignment of Mg⁺ (3p) states created in 1–60 keV Mg⁺ (3s)-He, Ar collisions have been studied by the polarized photon-scattered particle coincidence technique at scattering angles corresponding to impact-parameter ranges of 0.5–1.0 a.u. (He) and 1.2–2.0 a.u. (Ar). Referring to the standard geometry, in the region of maximum excitation probability (～ 35 keV), a strong propensity for population of the Mg⁺ (3p _?1) state is observed. The propensity breaks down when going towards lower energies, and for collisions with He even a transient reversal of the angular momentum occurs. The alignment angle varies little in the entire range of impact parameters and energies investigated. These observations compare well with recent general predictions of Andersen and Nielsen.     

Polarization studies of Lyman-α radiation emitted in atom-H2-molecule collisions

A polarization study of Lyman-α radiation emitted in collisions of H⁺, H, He⁺, and He projectiles with H₂ molecules has been performed at projectile energies ranging from 1–25 keV. In H⁺?H₂ and H?H₂ collisions, the measured linear polarization is negative at low incident velocities, indicating a preferred alignment of the excited electron charge cloud perpendicular to the incident projectile direction. This is taken as significance for a rotational coupling which in these two collision systems predominantly populates the H(2p _±1) substates. In He⁺?H₂ and He?H₂ collisions, the measured linear polarization is about zero and independent of the projectile energy. In these more asymmetric systems,a′?a′ and a′?a″ couplings are now of comparable magnitude.     

Collision spectroscopy with aligned and oriented atoms

Electron capture processes in the H⁺?Na(3s) and H⁺?Na(3p) collisions are experimentally investigated in the 0.3–3 keV energy range using a crossed beam experiment. The excited Na(3p) target is produced with a well-defined alignment using laser pumping. The time of flight technique enables the identification of all the H(n)+Na⁺ channels populated in the collision. Total cross section ratios σ_3p (n=2)/σ_3s (n=2),σ_3p (n=3)/σ_3s (n=2) and σ_3s (n=3)/σ_3s (n=2) for the production of H(n=2) and H(n=3) are measured in the H⁺?Na (3s) and H⁺?Na (3p) collisions. They reveal a strong dominance of the production of H(n=2) in the H⁺?Na(3p) collision, especially for energies below 1 keV.     

Analysis of impact-excited HeI states by applying magnetic and electric depolarization techniques

The magnetic depolarization of the impact radiation at λ(4¹ D?2¹ P)=492 nm and λ(3¹ D?2¹ P) = 668 nm as well as the splitting of the magnetic depolarisation signals in electric fields were investigated for 10–25 keV H ₂ ⁺ (D ₂ ⁺ )-He collisions (only 492 nm line) and for 5-22.5 keV He-He collisions (both lines). The results are compared with analogous measurements for He⁺- He and Ne⁺ - He collisions and the dynamics of the excitation process are qualitatively discussed. In particular, we emphasize the importance of inertia for the evolution of the electron cloud during the final phase of the collision process.     

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.     

The role of an initial Na(3p) state alignment in 1 keV H++Na(3p)→H(2p)+Na+ collisions

Charge exchange in 1 keV H⁺+Na→H(2p)+Na⁺ is studied in a Lyman-α scattered particle coincidence experiment. An initial Na(3p) state is prepared by laser-optical pumping; its alignment is controlled by the laser light polarization. The results are compared to a 34 coupled-channels calculation (R. Shingal), and discussed on the basis of an asymptotic model of the charge exchange process.     

Alignment of atomic autoionizing states created by slow Na++He collisions

Ejected-electron spectra have been measured for collisions of He-atoms with Na⁺-ions, whose impact energy ranged from 1.7 to 7.0 keV. The ion impact-energy dependence of the angular-differential cross-section of the ejected electrons has been investigated for an aligned autoionizing state Na**(2p ⁵ 3s ^{2 2} P), which has been created by charge transfer from the He-atoms. The alignment of the autoionizing state Na**(2p ⁵ 3s ^{2 2} P) is discussed in relation to the scattering angles of the Na⁺-ions. A complete longitudinal alignment has been observed with respect to the quasimolecular axis.     

Alignment of atomic autoionizing states created by slow Li++Ne collisions

Ejected-electron spectra have been measured in collisions of Ne-atoms with Li⁺-ions, whose impact energy has been ranged from 0.4 to 7.0 keV. The ion impact-energy dependence has been investigated on the angular-differential cross-section of the ejected electrons from the aligned autoionizing state Ne** 2p ⁴3s ^{2 1} D. In the highest energies studied, cross section maxima have been found at both forward and backward angles with respect to the Li⁺-ion beam direction, whereas the cross sections are maximized at around right angles below intermediate energies. The data were analyzed semi-quantitatively, and the alignment of the autoionizing state was recognized to be characterized by a representative scattering angle of the Li⁺-ions.     

Classical trajectory study of orientation and alignment effects for charge exchange processes in H+- Na*(3p m) collisions

The orientation and alignment effects for charge exchange in H⁺ + Na*(3p) collisions are studied using the classical trajectory Monte-Carlo method in the energy range from 1 to 8 keV. For Na*(3p _-1) → H*(2s, 2p _±1) transitions a large orientation effect is predicted by the probability functions, in very good agreement with semiclassical calculations. Angular differential cross sections are also calculated and interpreted using the impact parameter dependence of the proton deflection angle. They predict left-right asymmetry in agreement with semiclassical calculations or experimental results, but slightly smaller. Another geometry, not experimentally realized, is considered, where the proton velocity is parallel to the quantization axis of the p _±1 oriented states. Charge exchange from different aligned states with respect to the direction of the projectile velocity is also investigated, but the alignment effects are not as well described as the orientation effects. Total cross sections from oriented or aligned states with cylindrical symmetry around the projectile velocity direction are calculated and allow the hypothesis of velocity matching to be tested.     

Polarisation effects in electron transfer reactions with laser excited Na(3P) at medium energies

We report new experimental data for the investigation of the role of electronic orbital alignment and orientation in charge transfer processes, in the medium energy range where the collision velocityv _c and the velocity of the active electronv _e are of the same order of magnitude. The results obtained for the H ₂ ⁺ -Na(3p) and He⁺-Na(3p) collisions are discussed in comparison with the experimental and theoretical findings obtained for the H⁺-Na(3p) system. Recent time-of-flight measurements for charge transfer in Li⁺-Na(3s and 3p) collisions are also presented.     

Angular momentum coherences of helium atoms excited by proton impact

Using electric-field anticrossing techniques, we investigated the coherent excitation ofn=5–8 He I states with different orbital angular momenta by proton impact. These measurements give strong evidence that saddle dynamics of H ₂ ⁺ -like systems are well suited for describing the final phase of 12.5 keV H⁺-He collisions. We conclude that, besides electron promotion via the 2pσ orbital, the collision system undergoes diabatic 1sσ–3dσ transitions during the close encounter and present an explanation of the electric dipole moments measured for excitation of H(n=2) states by H⁺-He and H-He collisions.     

Intra- and inter-atomic auger processes in collisions of low energy He++, He+, He* (23,S, 21 S) and Li+ with Li covered W(110) surfaces

Electron spectra from He⁺⁺, He⁺ and Li⁺ (10 to 1500 eV) ions colliding under grazing incidence with Li covered W (110) surfaces are reported. The results are compared with those obtained from thermal collisions of (2³ S; 2¹ S) metastable He atoms. In these collisions 1s vacancies are either produced during the collision event (energetic He⁺ (Li ⁺) collisions) or are brought into the collision (slow He⁺⁺ (He⁺, He*) collisions). Population of the 2s orbitals by two electrons produces states which decay by intraatomic Auger processes: we observe autoionization of He** (2s ²) and Li** (1s 2s ²) as well as autodetachment of He^?* (1s 2s ²). Alternatively the 1s-holes in the projectile or target (Li) can be filled by Auger processes involving one or two surface electrons. The processes leading to electron emission are studied as a function of the Li coverage in the submonolayer region (0≦Θ_Li≦1Ml) and as a function of the projectile energy. It is concluded that with one or two 1s vacancies present in the projectile the double capture of two surface electrons constitutes an important process responsible for electron emission of low work function surfaces.     

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.     

Further studies of the N2+ + N2 → N4+ association reaction

Data are presented which strongly suggest that stabilisation of the excited intermediate (N₄⁺)* complex in the reaction (1) N₂⁺ + 2N₂ (rate coefficient k₁) occurs via N₂ switching whereas for (2) N₂⁺ + N₂ + He (rate coefficient k₂) it occurs via superelastic He collisions. This explains the differing temperature variations of k₁ and k₂ previously obtained for these reactions. Drift tube data are also presented which show how k₁ varies with N₂⁺/N₂ centre-of-mass energy as compared with thermal energy.     

Calculation of Ionization,Excitation and Electron Capture Cross Sections for Be4++H(2s, 2p) Collisions

A computational study of Be⁴⁺+H(2s, 2p) collisions has been carried out employing the Classical Trajectory Monte Carlo (CTMC) method for the impact energy range from 20 keV/u to 1000 keV/u. The integral n partial cross sections for H(n) excitation and Be³⁺(n) electron capture and, the total ionization and electron capture cross sections are calculated and compared to recent semiclassical results. A general good agreement is observed for the n partial and total electron capture and ionization cross sections. The comparative study of the three inelastic processes show no significant differences between both excited targets.     

Collision spectroscopy with aligned and oriented atoms

Neutralisation processes in 0.15–1.5 keV collisions of H^? with Na atoms in the 3s ground state or in the excited 3p state have been investigated by means of time-of-flight analysis of the neutral H atoms produced. The H^? - Na(3p) system, investigated here for the first time, is particularly interesting since the entrance channel is embedded in the [H - Na(3s)] +e ^? continuum, enabling Penning detachment to occur. The measured relative neutralisation cross section ratios σ(3p)/σ(3s) decrease from 3 to 1.6 with increasing energy. Based on earlier published results for σ(3s), σ(3p) total cross sections exceeding 100 Ų are estimated.     

Partial differential cross sections for inelastic He++Ne collisions at low energy

We report differential cross section measurements with high angular resolution for different channels of the inelastic processes He⁺+Ne→He⁺+Ne* and He⁺+Ne→He*+Ne⁺, for collision energies between 100 and 200 eV. For the Ne states (2p ⁵3s)^1,3 P ₁, which decay optically, we determined the fraction with the alignment at right angles to the scattering plane. The results are used to discuss the mechanism of the processes and the influence of the spin-orbit interaction upon the collision.     

Penning ionization electron spectrometry of hydrogen iodide

The energy spectra of electrons released in thermal energy ionizing collisions of metastable helium and neon atoms with hydrogen iodide have been measured with high resolution and low background. The electron spectra, obtained for a mixed He(2¹ S, 2³ S) beam, a pure He(2³ S) beam, and a mixed Ne(3s ³ P ₂,³ P ₀) beam, are all characterized by the formation of theX ²Π _i andA ²Σ⁺ states of HI⁺. For both He(2¹ S) + HI and He(2³ S) + HI the spectra exhibit some broad features in the medium electron energy range which are attributed to ionization from an additional charge exchanged potential surface (He⁺ + HI^?) in the entrance channel. For the first time, we have detected the low energy electrons in the He(2¹ S, 2³ S) spectra due to autoionization of I** atoms which result from energy transfer to highly excited, dissociative HI** Rydberg states. The HI⁺ (X)²Π_3/2:²Π_1/2 fine-structure branching ratios vary significantly with the ionizing agent in a similar way as for the isoelectronic, atomic target case xenon.     

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.     

The lifetime of the (μHe) 2S + metastable state in helium gas

The ion-clustering mechanism of the quenching of the metastable 2S-state of the muonic helium ion (μHe) _2S ⁺ in gaseous helium is studied on the basis of quantum-chemical calculations of clusters He _n (μHe)⁺. It is shown that the quenching rates do not depend on the cluster ordern atn ≥ 2. In the helium gas at the pressure 0.1 ?p(atm) ? 10 the quenching of (μHe) _2S ⁺ proceeds, mainly, at the vibrationally excited levels of He(μHe) _2S ⁺ cluster, while atp ? 10 atm, at the ground vibrational state of the cluster He₂(μHe) _2S ⁺ . Atp ≥0.1 atm the calculated quenching rates agree with the recent experimental data.