Low keV Energies Research Articles

Quantitative determination of hydrogen adsorption site on the Pt(111)-(1×1) surface by low energy ion channeling

Ion channeling using low keV energies is introduced as a sensitive probe for determining the adsorption site of hydrogen on surfaces. The technique is shown to be capable of quantitatively probing the positions of light elements on heavy substrates with analysis by simple geometrical constructs. Quantitative analysis of the Pt(111)-(1×1)–H surface using 5 keV Ne+ ions shows that the hydrogen atoms preferentially populate the fcc site with a height of 0.9±0.1 Å above the first-layer Pt atoms and a corresponding Pt–H bond length of 1.9±0.1 Å.

Charge transfer in collisions ofC2+ions with H atoms at low-keV energies: A possible bound state ofCH2+

Electron capture in ${\mathrm{C}}^{2+}+\mathrm{H}$ collisions is studied theoretically by using a semiclassical molecular representation with nine molecular states for the doublet manifold at collision energies above 10 eV. The ab initio potential curves and nonadiabatic coupling matrix elements for the ${\mathrm{CH}}^{2+}$ system are obtained from the multireference single- and double-excitation configuration interaction method. The adiabatic potential curves show no bound state for the $1{}^{2}{\ensuremath{\Sigma}}^{+}$ state, but a very shallow well in the $2{}^{2}{\ensuremath{\Sigma}}^{+}$ potential, suggesting a possibility of a bound state of ${\mathrm{CH}}^{2+}.$ The corresponding total and partial cross sections for charge transfer are found to be in a reasonable agreement with experiment in shape, but the present magnitude is found to be larger by nearly a factor of 2 at the high-energy end.

Particle-beam treatment of organosilicon gas separation membranes: A novel way of controlling their mass transport properties

Fast atom bombardment applied in the low keV energy range to organosilicon gas separation membranes was found to modify their mass transport properties in a controllable manner. In particular, asymmetric polyvinyltrimethylsilane membranes and polydimethylsiloxane-based composite ones were treated by particle beams obtained from various gases like Ar, He, H2, and NH3, with particle energies of about 1 keV and does of about 1015 particles cm−2. In each case, improvements in the component selectivities for various gas mixtures coupled with decreases in the component permeances were obtained. The extent of mod-ification of the mass transport properties increased with increase of the calculated average depth of penetration of the bombarding particles. The modification of the mass transport properties was considered to take place as a result of compaction of the surface layer due to reaction like crosslinking and loss of pendant groups. © 1996 John Wiley & Sons, Inc.

Single and double electron capture in H+ on Kr collisions at low-keV energies

Abstract Absolute differential and total cross sections for single and double electron capture were measured from H + ions on krypton in the energy range of 1.0 to 5.0 keV. Both differential cross sections exhibit well-resolved oscillations over the range of angles and energies studied. Results are discussed and compared with semiempirical models and with previous measurements, showing good agreement. 1. Introduction Electron capture processes of H ÷ ions in collisions with krypton are of great importance in the fields of thermonuclear fusion research and astrophysics. Quan- titative knowledge of atomic hydrogen negative ion formation cross sections is needed for all the important atmospheric gas species [ 1 ]. The need for collisional information on the production of both neutral hydrogen and its negative ion is of current interest for neutral beam injection into fusion plasma device and in ion source development for high energy accelerators and storage rings. Most experiments concerned with the measurement of single electron capture cross sections for H ÷ [2--4] have been performed at energies below 350 eV. On the other hand few data have been accu- mulated on H+-Kr collisions in the low keV region [5-10]. Measurements of o1_1 have been reported by Williams [9] in the energy range 2.0 to 50.0 keV, by Fogel' et al. [ 11 ] at energies between 3.0 and 65.0 keV

Transfer excitation in F6+ and Ne7+ ions colliding with He and Ne targets

An experimental study is presented, using coincident energy gain spectroscopy, of transfer excitation processes in the low keV energy range, for lithium-like F6+ and Ne7+ ions colliding with He and Ne atoms. A theoretical model is developed in a quasimolecular framework, implying the same basic interactions as invoked in non-resonant transfer excitation (NTE), a mechanism well established at large collision velocity. This model describes transfer excitation as a two-step process essentially governed by the electron-nucleus interactions.

Experiments and simulations on the effect of channeling on the ranges of 1 keV deuterons in silicon

Simple extrapolation of the Lindhard analytic theory of channeling predicts very wide channeling angles (∼ 10°) at low keV energies; however, the effect is highly sensitive to the exact atomic interaction potential chosen, as shown by computer simulations. Amorphous, polycrystalline, and single-crystalline 〈110〉, 〈111〉 and 〈100〉 silicon samples have been implanted with 1 keV deuterons in channeled and random directions. The ranges are measured by the ERD biE × biB (elastic recoil detection) ion beam technique and compared to simulations with the codes MARLOWE, TRIM and BABOUM. Some significant conclusions are that the channeling angle is approximately 10° in the 〈110〉 direction, and that polycrystalline material gives significant “accidental” channeling.

Single electron capture in He + on SF 6, collisions at low keV energies

We have measured the absolute differential and total cross sections for single electron capture from He + ions on SF 6 in the energy range 1.0 to 5.0 keV. The total collisional destruction cross sections of He Rydberg atoms on SF 6 has been compared with the sum of the available free electron cross sections (for equal velocity) and the present absolute total cross sections. The results support the validity of the independent particle model for the destruction of Rydberg atoms over this range of energies.

Mutual neutralization in collisions between negative hydrogen ions and single-charged positive ions. II. He+ and Li+ projectiles at low-keV energies

Coupled-channel impact parameter cross sections are reported for mutual neutralization in He++H- and Li++H- collisions for the impact energy range between 0.07 and 25 keV amu-1. The results are obtained using a one-active-electron model and a 22-state two-centre atomic orbital basis. They are compared with available molecular orbital calculations as well as with experimental data. In a wide range of energies considered, good agreement is obtained with recently measured cross sections. Comparison of present results with neutralization cross sections for H++H- collisions reveals some differences in the neutralization curves regarding impact velocity, for different projectiles.

Development of 3He solid state detectors epithermal neutron spectrometer

A neutron spectrometer was developed for measuring the neutron energy in the range from 1–300 keV. Its fabrication was based on the 3He(n,p) 3H reaction, with two Si solid state detectors used for measuring the energy of the two charged particles. The sum of the energies of the two particles is the neutron energy E n plus the reaction Q-value (764 keV). The FWHM detector resolution is ≈3%. The detector efficiency, ≈3% for thermal neutrons, is proportional to 1 E (i.e., to the detection reaction cross section). Cooling improves the resolution. To avoid obscuring of the low keV energy range by thermal neutrons, the detector was covered with a 1 mm layer of Cd. Background due to neutron interaction with the Si detector, and with the detector construction materials and gamma rays, was removed by repeating the foreground measurement with no 3He in the detector. Neutron spectra were measured both with a moderated 252Cf source and with an epithermal beam at the Brookhaven Medical Research Reactor (BMRR). Results were in good agreement with Monte Carlo calculations using the MCNP code.

Electron Capture by C+, N+and O+Ions in Collisions with H2Molecules and He Atoms at Low keV Energies

Electron capture cross sections for C + , N + and O + ions produced by controlled electron impact have been measured in collisions with H 2 and He targets at low keV energy region. It is found that the metastable state ions, if present, change significantly the observed cross sections and, in particular, the previously determined cross sections for C + ions colliding with H 2 targets are found to be mostly due to the metastable state ions instead of the ground state ions. More significant effects of the metastable state ions are observed in He targets.

Ionisation processes in collisions of open-shell atoms. I. Fluorine atom collisions with inert gases and molecules

Results of an electron spectroscopy study of ionisation processes in collisions of F atoms with He, Ne, O2 and CO2 at low keV energies are presented. It is shown that in case of the He and Ne targets ionisation is partly due to population of singly-excited autoionising states of the 1D nl series and partly due to another mechanism which results in the production of a continuous electron energy distribution and becomes particularly important at high energies. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation must be the other F+ production channel. In the case of molecular targets the authors observe a direct ionisation channel and also dissociative ionisation via the population of 2D nl autoionising states of fragment O atoms.

Neutralization and ionisation processes in the scattering of D+, D2 + and D3 + on a Mg surface

Abstract Results of a time of flight (TOF) study of charge fraction measurements of positive and negative ion production in the scattering of D+, D2 + and D3 + by a polycrystalline Mg surface in the low keV energy range are presented. The ion fractions are found to be small, the positive ion fractions being in the range 0.02–0.2% are an order of magnitude smaller than the negative ion ones which are in the range 0.4–6.0%. The negative ions must be D−. Our results indicate that the positive ions are dominantly atomic also. The charge fractions are found to scale with the energy per nucleon whatever the incident ions. This could be related to a dominantly total dissociation of the molecules with subsequent interaction of the scattered D atoms with the surface leading to positive and negative ion production. Resonant neutralisation into repulsive excited states may be one of the main dissociative processes.

Atomic processes in antiproton-matter interactions

Atomic processes dominate antiproton stopping in matter at nearly all energies of interest. They significantly influence or determine the antiproton annihilation rate at all energies around or below several MeV. This article discusses what is known about the atomic processes which, through their effect on stopping and annihilation, significantly influence the spatial distribution of antiproton annihilations in a material. For stopping above about 10 eV the processes are antiproton-electron collisions, effective at medium keV through high MeV energies, and elastic collisions with atoms and adiabatic ionization of atoms, effective from medium eV through low keV energies. For annihilation above about 10 eV it is the enhancement of the antiproton annihilation rate due to the antiproton-nucleus Coulomb attraction, effective around and below a few tens of MeV. At about 10 eV and below, the atomic rearrangement/annihilation process determines both the stopping and annihilation rates. Although a fair amount of theoretical and some experimental work relevant to these processes exist, there are a number of energy ranges and material types for which experimental data does not exist and for which the theoretical information is not as well grounded or as accurate as desired. Additional experimental and theoretical work is required for accurate prediction of antiproton stopping and annihilation for energies and materials relevant to antiproton experimentation and application.

Time-of-flight study of medium energy range collisions of metastable helium atoms with atomic or diatomic molecular targets

Collisions of He(23S) on atomic (He, Ar) or molecular (D2, O2) targets are experimentally investigated at low keV energies (0.1-1.5 keV) by a time-of-flight technique. This method yields an overall picture of the various inelastic processes and their relative probabilities. Direct experimental evidence of the importance of the He(23P) and He(n'l') n'>or=3 excitations of the projectile with He and D2 targets is reported. With an O2 target differential cross sections for He+ production measured at various energies reveal a rainbow feature. With the Ar target, the predominance of Penning ionisation and excitation transfer processes is confirmed. The experimental results concerning the He*-He system have been compared with the predictions of available theoretical models: the important He(23P) production is qualitatively in agreement with calculations based on the c3 Sigma g+-b'3 Pi g rotational coupling. The covalent-ionic transition in the He2 molecule is proposed as a primary mechanism which can account for the Rydberg He(nl) population. For the other targets the possibility of connecting those Rydberg excitations with the existence of a transient ionic complex (He+-T-) is examined.

"R-matrix" approach to electron transfer in alpha -H collisions.

A mixed, atomic-and-molecular-state approach to heavy-particle collision processes is applied to electron transfer in ..cap alpha..-H collisions. Such collisions are difficult to treat at all impact parameters using a pure molecular-state basis without translational factors or a pure atomic-state basis (with plane-wave translational factors) at low-keV energies. It is shown that the use of translational factors in the molecular-state basis may be avoided for internuclear separations R< or =Rsub0 by transforming to an atomic-state basis at R = Rsub0. This ''R-matrix'' method is shown to be stable over a range of Rsub0, and a physical criterion is given for determining Rsub0.

Electron capture in small-angleAr2++ Ar collisions

Electron capture in ${\mathrm{Ar}}^{2+}$ + Ar is studied at low keV energies. Using ${\mathrm{Ar}}^{2+}$ + He collisions as an energy reference we show that the dominant single-electron-capture process is endothermic and of the type ${\mathrm{Ar}}^{2+}^{3}P+\mathrm{Ar}\ensuremath{\rightarrow}{\mathrm{Ar}}^{+}^{2}P+{\mathrm{Ar}}^{+}3{s}^{2}3{p}^{4}\mathrm{nl}$. Detailed studies at 2.4 keV show $\mathrm{nl}$ to be predominantly $3d$ and $4p$. Weaker exothermic processes are found and due mainly to long-lived, highly excited states of ${\mathrm{Ar}}^{2+}$. The direct scattering and double-electron-capture channels are also studied. The direct scattering occurs with no Ar-target excitation. Differential cross sections are presented for the collision processes. The single-electron-capture results are interpreted with the use of a simple model in which a strongly attractive intermediate state couples the incident and outgoing channels.

Study of theD++H2collision in the small-angle, low-keV energy range

A quasielastic scattering scaling law for the most probable energy loss has been derived for ion-molecule scattering, and verified for Ne/sup +/ and Ne/sup 0/ on H/sub 2/, D/sub 2/ in the small-angle, low-keV energy range. This scaling is shown to break down for the more fundamental collision D/sup +/+H/sub 2/. The neon and D/sup +/ results portray the importance of the valence forces in scattering, and shows a breakdown of the concept of a two-center potential for ion-molecule scattering. A single electronically inelastic channel is found for this collision. The inelastic excitation of H/sub 2/ by D/sup +/, in the same energy and angular range, is reported to yield a sharp universal reduced cross-section plot, similar to the He/sup +/+H/sub 2/ collision. This supports the existence of a quasimolecule that is formed during close encounters of the collision. No strong direct dissociation of the target H/sub 2/ molecule is found.

Ion-molecule charge exchange:He+andAr+onH2andN2

Charge exchange in ${\mathrm{He}}^{+}$ and ${\mathrm{Ar}}^{+}$ collisions with ${\mathrm{H}}_{2}$ (${\mathrm{D}}_{2}$) and ${\mathrm{N}}_{2}$ is studied at small angles. This work extends our earlier studies which primarily emphasized the direct inelastic scattering at low keV energies. Using time-of-flight techniques for energy analysis of the scattered atoms, it is directly shown, for the first time, that in ${\mathrm{Ar}}^{+}$ + ${\mathrm{H}}_{2}$, ${\mathrm{Ar}}^{+}$ + ${\mathrm{N}}_{2}$, and ${\mathrm{He}}^{+}$ + ${\mathrm{N}}_{2}$ the dominant small-angle charge-exchange process is quasiresonant ($Q\ensuremath{\approx}0$). In ${\mathrm{He}}^{+}$ + ${\mathrm{H}}_{2}$ there is no available quasiresonant channel, and here the dominant final channels are associated with energy losses from 13 to 18 eV. A simple model is proposed.

Inelastic processes in ion-molecule collisions:Ar+andHe+onN2at low-keV energies

The ${\mathrm{Ar}}^{+}$ + ${\mathrm{N}}_{2}$ collision is studied at energies from 1.0 to 3.0 keV and in an angular range between 0 and 4.0 deg. The energy-loss spectra of the scattered ions demonstrate the presence of elastic scattering and inelastic processes arising from vibro-rotational excitation of the ground electronic state of ${\mathrm{N}}_{2}$, electronic excitation of the target with concomitant vibro-rotational excitation, and target ionization. Studies of the angular dependence of these processes suggest that (a) the collisions involve the entire molecule and are thus not adequately described in terms of a binary collision model, (b) electronic and vibrational excitation occur via mechanisms which exhibit negligible coupling, and (c) electronic excitation of ${\mathrm{N}}_{2}$ is due predominantly to the $B^{3}\ensuremath{\Pi}_{g}\ensuremath{\rightarrow}X^{1}\ensuremath{\Sigma}_{g}^{+}$ transition as evidenced by observation of the onset of dissociation at larger scattering angles. The probability of charge exchange in ${\mathrm{Ar}}^{+}$ + ${\mathrm{N}}_{2}$ and ${\mathrm{N}}^{+}$ + ${\mathrm{N}}_{2}$ collisions is also investigated and shows an angular behavior very similar to that found in ${\mathrm{He}}^{+}$ + ${\mathrm{N}}_{2}$. Inelastic scattering results for ${\mathrm{He}}^{+}$ + ${\mathrm{N}}_{2}$ are reported.

Collisions between rare-gas atoms at low-keV energies. II. Asymmetric collision systems: He-Ne, -Ar, -Kr, -Xe

Elastic and inelastic collisions in the asymmetric He-$X$ ($X=\mathrm{N}\mathrm{e},\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{K}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{X}\mathrm{e}$) systems are studied at low-keV energies (0.8-3.0 keV). The experimental method involves energy-loss measurements on the scattered particles using either a time-of-flight technique (for neutrals) or electrostatic energy analysis (for ions). Differential cross sections are reported and the results interpreted in a quasimolecular treatment of the collision. Of particular significance is the very low excitation and ionization of the helium projectile relative to excitation of the target. An understanding of this requires a modification of the usual diabatic correlation rules, and implies the promotion of the outer filled molecular orbital. Predictions made from the modified correlation diagram, and calculated potential-energy curves are found to agree with the experimental results.