Molecular Hydrogen Targets Research Articles

Electron Capture from Molecular Hydrogen by Metastable Sn2+* Ions

Over a wide and partly overlapping energy range, the single-electron capture cross-sections for collisions of metastable Sn2+(5s5p Po3) (Sn2+∗) ions with H2 molecules were measured (0.1–10 keV) and calculated (0.3–1000 keV). The semi-classical calculations use a close-coupling method on a basis of electronic wavefunctions of the (SnH2)2+ system. The experimental cross-sections were extracted from double collisions in a crossed-beam experiment of Sn3+ with H2. The measured capture cross-sections for Sn2+∗ show good agreement with the calculations between 2 and 10 keV, but increase toward lower energies, whereas the calculations decrease. Additional Landau–Zener calculations were performed and show that the inclusion of spin-orbit splitting cannot explain the large cross-sections at the lowest energies which we now assume to be likely due to vibrational effects in the molecular hydrogen target.

Electron-ion momentum vector coincidences in electron collisions with atoms and molecules

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.

Close-coupling approach to antiproton-impact breakup of molecular hydrogen

An ab initio time-dependent convergent close-coupling approach to describing antiproton collisions with molecular hydrogen or the hydrogen molecular ion has been developed. The approach accounts for all possible orientations of the molecular target. Averaging over the molecular orientations is performed fully analytically. For the molecular hydrogen target calculated orientation-averaged total cross sections for single ionization and proton production are compared with several experiments over the energy range of 1–2000 keV. Results for single ionization are in good agreement with experiment, except for the region around the experimental maximum. For proton production reasonable agreement with experiment is observed above 40 keV.

Classical-trajectory calculations of the electronic stopping cross-section for low-energy H and H+ projectiles by H2-molecules

A model that enables the classical-trajectory simulation of the interaction between an atomic particle and a target containing one or more electrons is devised. It makes use of the so-called Gaussian kernel approximation and ad-hoc potentials. In this way, the most relevant quantum properties of the electron can be preserved and, at the same time, still using classical mechanics to solve the response of the electronic system to the presence of a moving, heavy charge. As a first step to assessing the proposed model we calculate the electronic stopping cross-section for 1–20 keV Protons and Hydrogen impinging upon atomic and molecular Hydrogen targets. The results show a fairly good agreement between experiments and previous theoretical calculations over the entire bombarding energy studied in this paper.

Full 3D ab initio studies of interference effects in high-energy ion-molecule collisions

We present an investigation of the interference effects which have been observed experimentally and predicted for ionizing collisions between highly charged projectiles and molecular hydrogen targets. The present data have been obtained from a non perturbative treatment of the collision system, using the semiclassical impact parameter method and solving the time-dependent Schrödinger equation fully numerically, the scattering wavefunction being discretized in the electron position space and, for detailed analysis, on spaces of reduced dimensionality. We discuss the oscillatory structures observed in differential cross sections as function of outgoing electron energy and angle in Kr34+ – H2 collisions. Emphasis is placed on the discussion on Young-type interference pattern as well as extra high frequency oscillations which have been observed experimentally but not confirmed by theoretical calculations.

Cold-target recoil-ion momentum spectroscopy studies of capture from atomic and molecular hydrogen byO8+andAr8+

We have used cold-target recoil-ion momentum spectroscopy to study electron capture from atomic and molecular hydrogen targets by slow $(0.3<\mathrm{v}<0.95\phantom{\rule{0.3em}{0ex}}\text{a.u.})$ ${\mathrm{O}}^{8+}$ and ${\text{Ar}}^{8+}$ ions. For the atomic hydrogen target, we have performed coupled-channel calculations using an atomic orbital expansion to compare to the experimental results. The $Q$-value spectra show strong populations of both $n=5$ and $n=6$ states on the projectile, with a tendency for increasing the population of high-angular momentum states for higher projectile velocity. A strong population of $n=6$ is found, a result not previously reported but in good agreement with the present calculations.

Studies of state selective electron capture processes by helium-like ions in H and H2

The technique of translational energy spectrometry has been used to study state selective one electron capture 214–900 eVamu−1 helium-like C4+, N5+ and O6+ in atomic and molecular hydrogen. In the case C4+ and O6+ in an atomic hydrogen target electron capture occurs predominantly into C3+n=3 levels and O5+n=4 levels. Both results are in agreement with theoretical semiclassical calculations. For N5+ in the energy range capture is predominantly into n=4 levels confirming theoretical predictions. In the case of a molecular hydrogen target capture by C4+ and O6+ ions is dominated by non-dissociative one electron capture channels. In the case of N5+/H2 collisions autoionising double electron capture becomes dominant as the collision energy is reduced below 1 keVamu−1. A simple attenuation technique was used in this case to obtain cross section values for individual capture channels which are compared to recent experimental and theoretical values.

Trajectory and molecular binding effects in stopping cross section for hydrogen beams on H2

The complex interaction of an atomic projectile with a molecular target is studied by considering the time-dependent electron-nuclear dynamics of the collision. We calculate the energy loss, charge exchange, and differential cross section for a hydrogen beam colliding with molecular hydrogen targets for projectiles energies from 10 eV/amu up to 25 keV/amu. We obtain the total, electronic, nuclear, and rovibrational contribution for the orientationally averaged stopping cross section of the molecular target when scattering over all the angles is considered. We emphasize the violation of Bragg’s rule (additivity of the atomic energy loss for the compound target) and the acceptance angle dependence of the experimental stopping cross section.

Two-center effect on low-energy electron emission in collisions of 1-MeV/u bare ions with atomic hydrogen, molecular hydrogen, and helium. I. Atomic hydrogen

We have investigated ionization mechanisms in fast ion-atom collisions by measuring the low-energy electron emission cross sections in a pure three-body collision involving bare carbon ions $(v=6.35\mathrm{a}.\mathrm{u}.)$ colliding with atomic hydrogen targets. The measurements have also been extended to molecular hydrogen and helium targets. In this paper we provide the energy and angular distributions of double differential cross sections of low-energy electron emission for atomic hydrogen targets. The Slevin rf source with a high degree of dissociation was used to produce the atomic H target. It is found that the two-center effect has a major influence on the observed large forward-backward angular asymmetry. A detailed comparison is presented with calculations based on the continuum distorted-wave (CDW) and CDW-EIS (eikonal initial-state) approximations. Both the continuum distorted-wave calculations provide a very good understanding of the data, whereas the first Born calculation predicts almost symmetric forward-backward distributions that do not agree with the data. The two-center effect is slightly better represented by the CDW calculations compared to the CDW-EIS calculation. The total cross sections are, however, in good agreement with the theories used. The results for molecular hydrogen and helium will be discussed in the following paper.

Absolute measurements of electron capture cross sections of C3+ from atomic and molecular hydrogen

Absolute measurements of single- and double-electron-capture cross sections by C3+ projectiles on atomic and molecular hydrogen targets were performed for projectile energies between 1.0 and 3.5 MeV for the single- and 1.0 and 2.0 MeV for the double-capture processes. The H/H2 cross section ratios were measured using an absolutely calibrated tungsten-tube furnace for the production of atomic hydrogen. The single-capture data are compared with calculations based on the boundary-corrected first Born approximation, the eikonal approximation and a semiclassical model, presenting a good overall agreement. Calculations for the double capture using an analytical expression, obtained within the independent electron approximation and based on the same semiclassical model, give a reasonable qualitative description of the data.

Triply Differential Study of Positron Impact Ionization ofH2

The first study of positron impact ionization with fully determined kinematics is reported. A molecular hydrogen target has been used to investigate the possible occurrence of electron capture to a continuum state of positronium. A small broad peak in the spectrum of the ejected electron has been found which provides the first clear experimental evidence for the process.

Single electron capture from molecular hydrogen targets by impact of protons and α particles

The present work deals with single electron capture processes from ${\mathrm{H}}_{2}$ targets by impact of heavy bare ions at intermediate and high collision energies. Within the distorted wave formalism, the one active electron model developed to describe collisions with multielectronic atomic targets is used for the case of ${\mathrm{H}}_{2}$ targets. Three different distorted wave models are employed: the continuum distorted wave, the continuum distorted wave--eikonal initial state, and the continuum distorted wave--eikonal final state. Using these distorted wave approximations, total cross sections for ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact are calculated employing two different representations of the initial bound molecular wave function. The variation of the total cross sections due to this change in the description of the initial wave function and due to changes of the distortion factors in the entrance and exit channels is studied. The theoretical results are compared with available experimental data.

Energy and angular distributions of electrons from ion impact on atomic and molecular hydrogen. IV. 28-114-keV He+ + H collisions.

Absolute ionization cross sections for 28--114-keV helium ion impact on atomic hydrogen, differential in energy and angle of the ejected electrons, have been obtained from crossed-beam measurements and previously measured cross sections for molecular hydrogen targets. A radio frequency discharge source produced a mixed atomic and molecular target with a typical dissociation fraction of 74%. Energy spectra were measured from 1.5 to 130 eV by an electrostatic analyzer with a resolution of 5%. The angular range was 15\ifmmode^\circ\else\textdegree\fi{}--160\ifmmode^\circ\else\textdegree\fi{}. Results are compared with calculations based on the first Born, continuum-distorted-wave--eikonal-initial-state, and classical trajectory Monte Carlo methods. Total electron yields are obtained by combining calculations that are separately performed for liberating the target and projectile electrons. Model potentials are used to represent the interparticle-separation-dependent screening of the nuclear charges experienced by the electrons. Though projectile electron emission is a negligible component of the total ionization cross section for the present collision energies, its contribution is significant for particular regions of the spectrum of ejected electrons. Through comparison with our proton-impact data [Kerby et al., Phys. Rev. A 51, 2256 (1995)], differences and similarities are demonstrated owing to the common asymptotic charge of these two projectiles but differing nuclear charges and the electron carried by the ${\mathrm{He}}^{+}$ ion. Comparisons are also made illustrating the differences between atomic and molecular hydrogen targets. \textcopyright{} 1996 The American Physical Society.

Target and ion charge dependences of double capture processes in A q+ (1s 2) + He, H2 ( q = 4 to 8) collisions, at [formula omitted

An investigation of the double capture into 1s 23131′ states of C 2+, N 3+, O 4+ and Ne 6+ Be-like ions has been undertaken at 0.4 a.u. collision velocity using electron spectroscopy. By fitting simultaneously two electron spectra emitted at angles symmetrical with respect to 90° (emitter frame) we obtain the first realistic populations of these states. The data obtained with helium and molecular hydrogen targets are compared and the main tendencies observed for the double capture are briefly discussed.

Electron capture from H2 targets by H+ and He2+ ions. Dependence of the cross sections on the orientation of the molecular axis

We study the electron capture process from molecular hydrogen targets by proton and alpha particle impact at high collision velocities. The dependence of the differential cross sections on the orientation of the internuclear axis of H2 is evaluated within a two effective center approximation for capture to selective final states of the projectile. In particular, the molecular B1B model is applied in the calculations. The molecular orientation effects observed are due to the interference between the scattering amplitudes from the two centers of the target.

Theory of projectile ionization by molecular hydrogen targets

The study of events in which ionization of a projectile electron occurs together with the transition of a target electron offers an opportunity to explore the dynamics of electron-electron interaction. Experimental and theoretical investigations have isolated an effect due to the interaction of electrons associated with two different (projectile and target) centers. For a molecular hydrogen target, the role of two-center electron-electron interaction is further complicated by the two-center nature of the target. The influence of the two-center nature of the target on the two-center projectile-target electron-electron interaction is illustrated through a calculation of cross sections for projectile ionization with target molecular hydrogen being promoted into its first excited state.

Differential cross sections for muonic hydrogen scattering on hydrogen molecules

The results of first calculations of the differential cross sections for muonic hydrogen scattering on hydrogen molecules are presented. They are functions of the initial and final kinetic energy of the system and the scattering angle. These calculations are based on the respective set of cross sections for muonic hydrogen scattering on hydrogen nuclei, obtained within the framework of the adiabatic method. The Fermi pseudopotential method is used to estimate the molecular binding effects. The influence of electrons on the cross sections under consideration is described in terms of the effective screening potential. Rotational and vibrational transitions are taken into account. The calculated molecular differential cross sections show a strong angular dependence. This effect is very significant for the electronic contributions to the cross sections, e.g. for collision energies above approximately 0.1 eV only the cross sections of small scattering angles are influenced considerably by the screening. Since these differential cross sections give detailed information about the final energies and complicated angular distributions of the scattered muonic atoms they are the proper basis for calculations concerning the deceleration of muonic hydrogen atoms in molecular hydrogen targets and for Monte Carlo simulations of different experiments in muonic physics.

Partial-wave formulation in the calculation of atomic and molecular ionization cross sections.

A formulation for calculating the distorted-wave Born approximation cross sections for both atomic and molecular ionizations using the partial-wave-expansion method is introduced. The ionization cross sections of molecular and atomic hydrogen target with positron impact are calculated in the intermediate collision velocity region with this formulation. The results are compared to the updated experiment data. This comparison has offered an alternative evaluation for the present model of ionization process.

Projectile electron loss with a molecular hydrogen target.

We examine the effect of a molecular hydrogen target, as contrasted with an atomic target, on one-electron-projectile electron loss within the framework of the plane-wave Born approximation. Two different approaches are explored: (1) the use of an approximate molecular wave function due to Weinbaum, and (2) the use of a modified form factor for the two atoms in the ${\mathrm{H}}_{2}$ molecule. The resulting cross-section expressions are compared with experimental cross sections for electron loss by various one-electron projectiles. We find that for a projectile atomic number larger than 5, molecular effects are negligible in total cross sections and then the electron-loss cross section per molecule is just equal to twice the total cross section per atom.

Investigation of double capture in Ne8++He, H2, by electron spectroscopy at 80 keV. II. Experimental results

For pt.I see ibid., vol.24, no.7, p.1695-72 (1991). A thorough analysis of electron spectra measured at 80 keV in collisions between Ne8+(1s2) ions and helium and molecular hydrogen targets is presented. The 1s23lnl' and 1s24lnl' lines are identified up to n=9. With helium, a fitting procedure for 1s23lnl' lines (all the lines for n=3, 4, and the first 3s5l ones), observed for the first time at different angles, make it possible to compare the authors' experimental results with two different theoretical data sets: their own theoretical values for positions, autoionization and radiative probabilities obtained by the AUTOLSJ method, which are given in a companion paper, and those recently published by Bachau et al. using a Feshbach method with a model potential. This allows the authors to test both theoretical positions and lifetimes of (3,n) states. The relative weight of the n=3, 4 and the first n=5 states is often angle dependent. The apparent, angle dependent, branching ratio for autoionization into the 2p and 2s continua of the (3,3) and the first (3,4) states is determined for several cases and discussed. Finally, differential and total cross sections are given and compared with other available experimental results.