R.J. MacDonald

Studies of surface composition and structure of Cu3Pt(111) by low energy alkali ion scattering

Abstract The composition and structure of the Cu 3 Pt(111) surface have been investigated by low energy alkali ion scattering. The surface composition of the top two layers was determined by Li + ion scattering using selective scattering geometries with calibration measurements on reference samples of Cu(111) and Pt(111). Under thermal equilibrium conditions, these results consistently showed a surface composition of 80%Cu20%Pt in the first layer, while the second layer was 69%Cu31%Pt. The surface structure was probed by measuring the intensity of Li + ion scattering as a function of incident angle along the main azimuthal directions. The interpretation of experimental data was based on a chain model simulation. The results showed that the Pt atoms were coplanar with the Cu atoms in the first layer. A clustering effect of Pt has also been tested using 1 keV K + ion scattering. The results revealed that the Pt atoms in the first layer were not clustered by lack of the PtPt pair double scattering peak along the two long azimuthal directions.

A STM study of the effects of the ion incident angle and energy on surface damage induced by Ar+ bombardment of HOPG

Abstract Scanning tunnelling microscopy has been used to study the surface damage induced by ion bombardment of highly ordered pyrolytic graphite (HOPG). Ar ions with energies in the range from 0.5 to 3 keV were used to bombard HOPG surfaces at different angles of incidence. A low fluence ( 12 ions cm 2 ) was used to enable characterisation of single ion impacts on surfaces. It is shown that each ion impact creates a small protrusion on the HOPG surface whose average volume increases as the ion energy and the incident angle to the surface decreases in the range investigated. Statistical distributions of protrusion diameter and height are given for comparison, as well as the average diameter and height. The origin of these features on the surfaces is discussed. It is suggested that thermal spikes created by the ion in the near surface region may play a very important role in producing this surface damage.

Neutralisation of He+ and Ne+ scattered from Ag

Abstract The neutralisation of low energy He + and Ne + ions scattered from Ag has been studied by measuring the angular distribution of ions scattered into the plane normal to that containing the incident beam and the normal to the surface of the solid. Assuming that the probability of the ion surviving in a charged state is given by P =exp(− V c /V ⊥ ), the energy dependence of V c has been measured. This is the survival coefficient appropriate to the incoming and outgoing trajectories but the experiment studies changes only in the outgoing trajectory. A strong energy dependence has been observed, as well as features consistent with some recent theoretical models of the neutralisation event.

A determination of the ionization probability for aluminium secondary ion emission

Abstract Independent measurements of the angular and energy distributions of Al+ secondary ions emitted from Ar+ bombarded aluminium are presented. The secondary ion yield is found to include an inverse exponential dependence on the emission velocity of the kind usually assumed to describe the survival probability of an ion near a surface, but the characteristic velocity, A/a, is energy dependent. With this exponential dependence removed, the ionization probability is found to be R+ α E1.1, where E is the secondary ion emission energy.

The role of the electronic structure in charge exchange between low energy ions and surfaces

In this study some of the fundamental aspects of neutralisation have been addressed. The validity of the three-stage approach to neutralisation which involves treating neutralisation along the approach to and departure from the surface as an Auger-like process, and the neutralisation during the collisional phase as an atomic-like process has been justified by measurements on the interaction of He+ with Pb. Neutralisation along the exit path is shown to be independent of the oscillatory niutralisation cross section associated with the collisional phase. In order to determine the role of the work function and valence band width in determining neutralisation rates for inert gas ions, a range of elements has been studied (Ni, Cu, Zr, Pd, Ag, Au, Pb). The role of the projectile has been assessed by the use of three inert gas ions (He+, Ne+, Ar+) as projectiles. This has been extended by comparison with previously reported results involving the neutralisation of Al+ of an Al surface and Si+ off a Si surface. The conclusion of this study is that there is no experimental evidence for higher neutralisation rates in situations where a resonance process is considered to be more likely than an Auger process. A linear correlation has been found between the conduction band width and vcv1. There is also some preliminary evidence for a universal behaviour for the variation of neutralisation rates with particle velocity, not previously reported.

Transient adsorption or skipping motion of a 200–2000 eV Si+ beam on a Cu(111) surface

The experimental observation of transient adsorption or skipping motion of 200–2000 eV Si+ beams incident at 1–16° to a Cu(111) surface is reported. We propose that trapping occurs on an attractive excited state of the SiCu interaction potential. This state may be formed by an adiabatic charge rearrangement, or more generally, as a consequence of inelastic energy loss of the projectile to electronic degrees of freedom of the metal. Up to five vibrational oscillations in this potential could be detected, with an associated energy loss of 20% of the incident beam energy. The absolute energy losses, and their charge state dependences provide information about the charge changing interactions, the charge state of the projectile at the surface, and the time taken for the collision complex to achieve charge equilibrium.

Low energy ion beam mixing of metal-copper multilayers

Abstract The efficiency of ion beam mixing of W, Pb, Pt, Ta, Ni, In, Ag, Nb, Mo, Ti and Au with Cu during 4 and 5 keV Ne + sputter depth profiling has been determined by LEIS and SIMS. Values of the mixing efficiency, ( Dt / φF d ), were found by modelling the sputtering and mixing processes by a diffusion approximation to the transport equation. Values of mixing efficiency range from 25 A 5 /eV for Ta in Cu to 350 A 5 /eV for Ag in Cu. These values are larger than those obtained in high energy mixing experiments in Cu, but scale in the same way with thermal diffusion constants. This indicates that the same processes are responsible for low and high energy ion mixing.

Neutralisation contributions in low energy ion scattering

Abstract Further studies of the contribution to neutralisation of the outgoing trajectory in low energy ion scattering are reported. The neutralisation model assumed to apply is that due to Hagstrum, with the probability of survival in an ionised state given by P+ = A exp(− v c v ⊥ ). The dependence of vc on incident ion energy and angle of exit to the surface show the probability P+ is a function of vT, and vc itself is also a function of vT. Thus the actual dependence of P+ on vT is less than that suggested by Hagstrum. Values of vc suggest the contributions to neutralisation of the ingoing trajectory, the collision and the outgoing trajectory are approximately the same.

Local electrostatic potential determination of CsCu(111) surfaces by negative ion spectroscopy

Abstract The local electrostatic potential change of Cs-covered Cu(111) as a function of Cs coverage has been studied with negative ion spectroscopy produced by low-energy Li + ion impact. The results indicate that at low coverages, the valence electron of Cs is transferred to the substrate, while a considerable occupancy of the alkali state occurs at a coverage ⩾0.35 ML. With higher coverages, the surface shows a laterally uniform electrostatic potential. The results demonstrate that site-specific negative ion spectroscopy can provide direct information on atomic-scale changes in the surface electrostatic potential.

Au-segregated dealloying and Pd-induced clock reconstructing of Cu(001)

The structure and growth of ultrathin Au and Pd films on Cu(001) have been studied by low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED). Due to the existence of a kinetic pathway for intermixing, both Au and Pd are incorporated into the Cu(001) surface at room temperature, forming a surface alloy at a coverage of 0.5 monolayer (ML). At Au coverage near 1.2 ML, the surface layer is pure Au; this is caused by Au-induced segregation from the underlying layer to the surface. No second- and/or third-layer Au is evident. In contrast to this, for the Pd/Cu(001) case a clock reconstruction is formed after deposition of 1 ML Pd. The LEIS data, through comparisons with 3-D computer simulation, reveal a 0.25 A lateral clockwise - anticlockwise displacement of the first-layer Pd atoms. A simple model for this clock reconstruction is proposed. The driving force behind the dealloying for the Cu - Au alloy and Pd-induced reconstructing for the Cu - Pd surface is also discussed.