E.W. Thomas

Backscattering of excited hydrogen from Mo in the presence of oxygen

Abstract Impact of 20 keV H + ions on molybdenum surfaces gives rise to atomic line emission from backscattered excited H atoms. It is shown that observed intensity is a function of adsorbed oxygen on the surface; oxygen coverage causes a decrease in the rate of radiationless de-excitation with a result that intensity increases with oxygen partial-pressure in the range 10 −9 –10 −6 torr. Surface coverage of oxygen is a function of both projectile beam current and ambient oxygen pressure; consequently one observes transient behavior of light intensity after changes to either ion beam current or ambient oxygen pressure. The dependence of Balmer-β line intensity on pressure and time is modelled and filted to the data. By this procedure we derive the coefficient for sputtering of oxygen adsorbed on a Mo surface; for 20 keV H + impact we determine values of 0.0029 and 0.0012 (atoms/ion) at incidence angles of 60° and 15° respectively to the target surface normal.

Excited state formation as a function of initial energy of H+ and He+ ions scattered from metal surfaces☆

Abstract Impact of 10–30 keV H+ and He+ ions on polycrystalline metal surfaces gives rise to emission from backscattered excited atoms. The intensity of this emission has been quantitatively measured in the case of copper and molybdenum targets. The dependence of intensity on incoming projectile energy is predicted and shown to be governed primarily by radiationless de-excitation of atoms while close to the surface. Parameters governing de-excitation may be inferred from the data and are in excellent agreement with results achieved by analysis of the Doppler broadening of the spectral line.

Scattering of 10–30 keV H+, H2+ and H3+ from surfaces: Excited state composition and scattered H+ flux☆

Abstract An experimental study has been made of H and H+ scattering from metal surfaces under 10 to 30 keV H+, H2+ and h3+ bombardment. Light emission from the point of ion beam impact on the surface includes Doppler broadened lines from excited backscattered neutral projectiles. Slow recoils suffer radiationless de-excitation at the surface so that only fast recoils survive to emit photons; the coefficient for radiationless de-excitation is determined by analysis of the line shape. Using the measured de-excitation coefficient, we have predicted the excited backscattered flux as a function of incident projectile energy, and this agrees well with experiment. In a separate experiment we have measured directly the angular distribution of H+ scattered from a surface when an H+ beam is incident at an angle of 70° to the surface normal.

Small angle scattering of hydrogen from surfaces

Abstract Scattering of H + or H − and H 0 as a result of H + , and D + impact has been studied for targets of Be, C, Cu, Nb, Au, and stainless steel. Projectiles are incident at 69° from the surface normal and scattered particles are studied at near grazing emergence angles. Charge state distribution of the scattered particles is a function only of recoil energy and not impact energy nor of emergence direction. This indicates that the charge states are governed by bi-particle collision events rather than interactions with band structure of the surface. Comparison with previous experiments shows that as the incidence angle increases the average recoil energy gets larger. As an indication of absolute magnitudes we find that for 15 keV H + incident on Au at 69° from the surface normal, there are 0.081 protons, and 0.0072 H − , and 0.263 neutral hydrogen atoms scattered per incident proton and per steradian at an angle of 9.4° from the surface.

Re-emission of implanted deuterium from the back side of a stainless steel membrane

A magnetically analyzed beam of Dt ions of energy 2.5-20 keV is directed onto a type 301 stainless steel target of thickness 0.0025 cm. The implanted deuterium atoms which are not trapped can diffuse to both the front and back surfaces of the target where they can combine with other deuterium atoms and be re-emitted as neutral D, molecules. The present experiment monitors the re-emission of D, from the back side of the target by following the rise in the partial pressure of mass 4 within the chamber behind the target using a residual gas analyzer (RGA). At the projectile energies of this experiment, the projectile range is insignificant compared with the target thickness. Prior to bombardment, the target was outgassed at or above 100°C for several minutes and then allowed to stabilize at the desired temperature. Recent work by Thomas et al. [l] has shown that trapped deuterium is largely released at a temperature of 95’C, so this outgassing procedure should be sufficient to remove trapped D in the target. The lack of detectable mass 18 (DO) and mass 20 (D,O) pressures during implantation suggested the surface of the target did not have significant oxygen contamination. Temperature measurement of the target showed no significant beam heating of the target. In fig. 1 we show a series of RGA traces of the deuterium re-emission from the back surface taken with 10 keV D’ incident on a target at 1OO’C. There is no significant time delay between commencing bombardment (t = 0) and the onset of re-emission. Conventional diffusion theory [2] suggests that the time for the deuterium to diffuse through a distance I of a target where the diffusion coefficient is D should be given by