We present evidence for the diffraction of light keV atoms and molecules grazingly scattered on LiF(001) and NaCl(001) surfaces. At such energies, the de Broglie wavelength is 2 orders of magnitude smaller that the mean thermal atomic displacement in the crystal. Thus, no coherent scattering was expected and interaction of keV atoms with surfaces is routinely treated with classical mechanics. We show here that well-defined diffraction patterns can be observed indicating that, for grazing scattering, the pertinent wavelength is that associated with the slow motion perpendicular to the surface. The experimental data are well reproduced by an ab initio calculation.

We measure fully differential cross sections for photo-double-ionization of helium at energies 1, 6, and 20 eV above threshold. The data have been obtained by measuring in coincidence the momentum vector of the He 2ϩ ion and one of the electrons. Using time-of-flight and imaging techniques, we cover a solid angle of 25-100 % 4 of the final-state continuum of all particles. Therefore the experiment is not confined to any particular set of angles or energy sharing, and allows for a reliable absolute calibration. We present momentum distributions of the ions and a comprehensive set of differential cross sections for electron emission. The latter are on an absolute scale and cover both equal and unequal energy sharing-for both the fast and the slow electron fixed-and a wide range of polar angles. We also present the first data for noncoplanar geometry. For all energies the cross section is sharply peaked around the coplanar emission, i.e., both electrons are preferentially emitted in the plane of the recoiling ion and the photon polarization direction. For most of the geometries the shape of the cross sections is well described by fourth-order Wannier theory calculations.

Prompted by recent experimental developments, a theory of surface scattering of fast atoms at grazing incidence is developed. The theory gives rise to a quantum-mechanical limit for ordered surfaces that describes coherent diffraction peaks whose thermal attenuation is governed by a Debye-Waller factor, however, this Debye-Waller factor has values much larger than would be calculated using simple models. A classical limit for incoherent scattering is obtained for high energies and temperatures. Between these limiting classical and quantum cases is another regime in which diffraction features appear that are broadened by the motion in the fast direction of the scattered beam but whose intensity is not governed by a Debye-Waller factor. All of these limits appear to be accessible within the range of currently available experimental conditions.

Impact of 600 eV protons at grazing incidence on LiF(100) is studied with a new coincidence technique combining energy loss and electron spectroscopy. Correlation between the secondary electrons and the charge state of the scattered projectiles demonstrates the role of the H 2 ions formed on the surface as precursors for electron emission. However, the main channel for energy loss is not associated with electron emission but is interpreted as the population of surface excitons. PACS numbers: 79.20.Rf, 34.50.Dy, 34.70. + e, Since the pioneering work of Souda et al.[1] ten years ago, the study of low-energy ions interacting with large band gap insulators and the subsequent energy loss and electronic emission has attracted much interest [2][3][4][5][6][7]. Compared to metallic surfaces, the large band gap and high binding energies of the valence electrons are expected to induce profound differences from several aspects: (i) The energy loss (through electronic stopping) of ions traveling along the surface should exhibit a threshold behavior with incident energy. (ii) The ion velocity threshold for kinetic electron emission is expected to increase with respect to that of metals. (iii) The resonant electron transfer (from and to the solid) should be strongly reduced. Points (i) and (ii) are simply due to the much larger energy required to excite or ionize valence electrons. The most recent observations by Auth et al. [8] have confirmed point (i) in collisions of protons with LiF but with a threshold behavior appearing only at substantially lower energy than expected [9]. With respect to point (ii), Vana et al. [2] showed that no clear energy threshold can be observed in the secondary electron emission yield during singly charged ions (H 1 , Ar 1 ) collision on LiF, whereas a threshold of 1 keV was measured for the same projectiles on Au. As for charge exchange, the resonant neutralization of singly charged alkali ions and the resonant ionization of alkali atoms is strongly reduced because of the band gap [10]. The suppression of this electron loss channel is also partly responsible for the surprisingly large negative ion fractions, up to 60%-90% [11,12] observed for oxygen or fluorine interacting with alkali halides. The capture process was elucidated only recently as being due to a lowering of the projectile affinity level in the Madelung potential [8,13,14].The presence of a large band gap indeed controls the resonant electron capture and loss but does not seem to play the same decisive role in the energy loss or in the electron emission. This paradox has been studied in detail for the H 1 -LiF system, which may be considered as a reference. The projectile has a well-known electronic structure, and since the LiF band gap extends above the vacuum level, energy loss and electron emission should be intimately related. These studies have called for the existence of an intermediate state able to reduce the effective band gap during the collision process. On one hand, independent measurements of secondary electron yields [...

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The two-dimensional velocity distribution of electrons emitted in 5 -15 keV p-He collisions has been measured for completely determined motion of the nuclei, that is, as a function of the impact parameter and in a well defined scattering plane. The electrons are emitted preferentially in the scattering plane and in the forward direction. The velocity distributions show sharp structures that vary strongly with impact parameter and projectile velocity. The results are compared to classical trajectory calculations. [S0031-9007(96)01614-6] PACS numbers: 34.50.FaIn ion-atom collisions, where the velocity of the projectile is much slower than the classical Bohr velocity of the target electrons, the dominant process which ionizes the target is electron capture to bound states of the projectile. Compared to this capture transition, the emission of an electron into the continuum is extremely unlikely [1]. While in a fast ion collision direct ionization by light projectiles is rather well understood, there has been much discussion of which mechanism promotes electrons into the continuum in slow collisions.On the grounds of classical mechanics, Olson [2] suggested a "saddle point mechanism." The potential between the projectile and the residual target ion has a point (the saddle point) where no force acts on an electron moving between them. As the projectile and target separate, the potential rises and hence electrons traveling with the velocity of this saddle could be "left stranded" in the continuum between the target and projectile [2]. The relative importance of this mechanism has been discussed within the theoretical framework of classical mechanics [2-4] as well as in various quantum mechanical approaches [5][6][7][8][9][10][11][12]. Many experimental studies have sought evidence for it in ionization [3,13-16] and excitation [17,18]. Recently Pieksma and co-workers [19] measured the velocity distribution of electrons emitted in 1-6 keV p-H collisions integrated over all emission angles and found a maximum of the cross section at the velocity of the saddle point. Kravis and co-workers [20] obtained two-dimensional images of the longitudinal and transverse velocity of the continuum electrons, using a technique similar to that used here, but without impact parameter determination, for a wide range of impact velocities of p and C 61 projectiles. Only for proton impact below 1 a.u. (atomic unit velocity) did they find most of the electrons in the saddle point region. Within the hidden crossing theory [9] ionization at these slow velocities has been explained as a multistep promotion process in the quasimolecule formed during the collision, and characteristic electron velocity distributions in the scattering plane have been predicted for a quantum mechanical analog of the classical saddle point mechanism [11,12].In this Letter we present an experimental study of 5-15 keV p-He collisions, which, for the first time give two-dimensional images of the square of the final state electron wave function in velocity space with simul...

This paper deals with a study of H − /D − negative ion surface production on diamond in low pressure H 2 /D 2 plasmas. A sample placed in the plasma is negatively biased with respect to plasma potential. Upon positive ion impacts on the sample, some negative ions are formed and detected according to their mass and energy by a mass spectrometer placed in front of the sample. The experimental methods developed to study negative ion surface production and obtain negative ion energy and angle distribution functions are first presented. Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are then investigated and compared with graphite. The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature. It is concluded from these experiments that the electronic properties of diamond materials, among them the negative electron affinity, seem to be favourable for negative-ion surface production. However, the negative ion yield decreases with the plasma induced defect density. fusion power-plant prototype producing electrical energy, targeting ∼1 GW of electrical power coupled to the grid [23,24]. In the ITER and DEMO devices, the heating of the plasma will mainly be produced by neutral beam injection (NBI). NBIs systems are key components in achieving high fusion energetic-performances. The ITER NBIs are required to inject 1 MeV beams of neutral deuterium atoms (D) into the tokamak, providing plasma heating and current drive. At such high velocities, much larger than classical electron orbit velocities of hydrogen atoms, the probability of electron capture from D + ions is too low, so that production of D relies on electron detachment from high-intensity D − beams. D − negative-ions are produced in a low-pressure plasma source and subsequently extracted and accelerated.The ITER negative ion source, currently under development at IPP Garching [7,25] in Germany, operates with a high-density, low-pressure inductively coupled plasma. Extracted D − current density of 200 A m −2 , over a large surface of 1.2 m 2 , with 5%-10% uniformity and low co-extracted electron-current (below one electron per negative ion), during long operation period (3600 s) is targeted. To reach such a high D − negative-ion current, the only up-to-date scientific solution is the use of caesium. Deuterium negative-ions are created at the extraction region by backscattering of positive ions or neutrals on the plasma grid. Deposition of caesium on the grid lowers the material work function and allows for high electron-capture efficiency by incident particles and thus, high negative ion yields. Studies conducted at IPP Garching show that the ITER negative-ion source can reach the required high current densities. However, drawbacks to the use of caesium have been identified. First, the caesium is continuously injected in the source a...

We present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along 110 . When the He normal energy decreases below 100 meV, i.e. He-surface distances Z > 2Å, the corrugation unexpectedly increases up to an impressive > ∼ 85%. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with Z. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignement properties of weakly-interacting overlayers.