Electronic excitations and charge exchange that occur during the interaction of low energy ions with surfaces are important in many areas of basic and applied science. Ion-surface interactions include processes such as ion neutralization, electronic excitation during scattering, and alterations of ion trajectories via interaction with the surface. In addition, ion scattering spectroscopy provides a method for the determination of surface atomic structure. A good source of information about this field is the Particle-Surface Resources web site.

We perform experiments using both electrostatic and time-of-flight (TOF) detectors. We can directly measure ion yields at high-resolution, ion fractions as a function of scattered energy, and electron emission spectra. We also have the ability to vary both the incident and scattered angles independently.

When an alkali ion, such as Li⁺, is scattered from a surface, the measured ion fraction is determined primarily by resonant charge transfer (RCT) between the alkali Si-level and the surface conduction bands. The amount of overlap, and hence the measured ion fraction, is very sensitive to the surface potential. In order to probe RCT, we employ a scattering geometry in which both the incident and scattered ion trajectories are close to the surface normal, as shown in the figure above. In this way, sharp peaks in the scattered ion spectra can be correlated with single scattering from particular surface sites. We can thereby associate the neutral fraction measured for a particular single scattering peak with the local electrostatic potential directly above an atomic site.

Our initial experiments were performed by scattering ⁷Li⁺ from clean and alkali adsorbate-covered Al(100). We compared the ion fractions for single scattering from substrate and adsorbate atomic sites as a function of coverage [1-3], as shown in the figure to the right. At low coverages, the neutral fractions are different for these two trajectories, which indicates that the surface electrostatic potential is non-uniform. This is understood by considering that, at low alkali coverages, the adsorbates behave as isolated dipoles. At higher alkali coverages, the neutral fractions for substrate and adsorbate scattering become nearly equal, which shows that the potential has become homogeneous, i.e., the alkalis form a uniform 'sheet' of charge. We are also able to quantitatively model the RCT process by modifying the calculations of J. B. Marston (Brown University) to include inhomogeneities in the surface potential [3]. We have measured similar effects for other substrates [4]. Our future plans include investigations of the RCT process for electronegative adsorbates.

Another type of electronic excitation that is important in ion-surface scattering is charge promotion (CP), which results from inner-shell excitation during a hard collision. We found that when a Li ion has a close encounter with a target Al atom, there is a finite probability for CP from the Li 1s to the Al Fermi level to occur [5-7]. CP manifests itself through a satellite peak, or inelastic loss feature (ILF), in the scattered ion spectra. Furthermore, the formation of Li with a 1s hole was confirmed via secondary electron spectra, which also serve to indicate the mechanisms whereby the hole is filled. Such inner-shell excitations can occur with other materials, and we are currently investigating the CP process for other systems.

More recently, we have investigated the emission of electrons and negative ions from ion-bombarded metal surfaces. Kinetic electron emission (KEE) from Al(100) during bombardment by 50-520 eV Li⁺ ions was measured as a function of incident ion energy and direction, and a surface electron-hole pair excitation mechanism was proposed, which was quantified with a one-electron parametric theory [8]. This was a previously unidentified mechanism that is characterized by a strong dependence on the energy and angle of incidence of the primary particle. Subsequently, we observed a similar mechanism for electrons emitted from Ru(0001) under Na⁺ ion bombardment, and also investigated the effects of Na, I, Cl and O adsorption on the electron yield [9].  Some representative spectra are shown in the figure. We found that Na, I and Cl adsorption can be largely understood in terms of the induced work function changes, while for adsorbed O contributes an additional electron emission mechanism.

1. C.B. Weare and J.A. Yarmoff, "Resonant Neutralization of ⁷Li⁺ Scattered from Cs/Al(100) as a Probe of the Surface Local Electrostatic Potential", J. Vac. Sci. Technol. A 13, 1421-1425 (1995).
2. C.B. Weare, K.A.H. German and J.A. Yarmoff, "Evidence for an Inhomogeneous to Homogeneous Transition in the Surface Local Electrostatic Potential of K-Covered Al(100)", Phys. Rev. B 52, 2066-2069 (1995).
3. C.B. Weare and J.A. Yarmoff, "Resonant Neutralization of ⁷Li⁺ Scattered from Alkali/Al(100) as a Probe of the Local Electrostatic Potential", Surf. Sci. 348, 359-369 (1996).
4. J.A. Yarmoff and C.B. Weare, "Resonant Charge Transfer during Scattering of Low Energy ⁷Li⁺ from Cesiated Surfaces", Nucl. Instrum. Meth. B 125, 262-267 (1997).
5. K.A.H. German, C.B. Weare and J.A. Yarmoff, "Inner-Shell Electron Promotion in Low Energy Li⁺-Al(100) Collisions", Phys. Rev. Lett. 72, 3899-3902 (1994).
6. K.A.H. German, C.B. Weare and J.A. Yarmoff, "Inner-Shell Promotions in Low Energy Li⁺-Al Collisions at Clean and Alkali-Covered Al(100) Surfaces", Phys. Rev. B 50, 14452-14466 (1994); "Erratum: Inner-Shell Promotions in Low Energy Li⁺-Al Collisions at Clean and Alkali-Covered Al(100) Surfaces", Phys. Rev. B 53, 10407 (1996).
7. C.B. Weare, J.A. Yarmoff and Z. Sroubek, "The Effects of Charge Promotion on the Measured Charge State of Low-Energy Li⁺ Scattered from Alkali/Al(100)", Nucl. Instrum. Meth. B 119, 352-358 (1996).
8. J.A. Yarmoff, T.D. Liu, S.R. Qiu and Z. Sroubek, "Mechanism of electron emission from Al(100) bombarded by slow Li⁺ ions", Phys. Rev. Lett. 80, 2469-2472 (1998).
9. J.A. Yarmoff, H.T. Than and Z. Sroubek, in preparation.