Yarmoff Research

Ion-Surface Interactions Electronic excitations, charge exchange and electron emission that occur during the interaction of ions with solids is important in many areas of basic and applied physics, such as surface analysis, semiconductor processing, ion implantation, nanomaterial fabrication, etc. In addition, ion scattering spectroscopy provides a method for the determination of surface atomic structure. We investigate ion-surface interactions via experimental measurements performed in the lab at UCR, in conjunction with calculations that support the analysis of our data. In addition to detailing specific mechanisms that are relevant to ion-surface interactions, we have developed the use of ion scattering as a probe of surface electronic properties. A focus of our work has been on the charge exchange between atomic particles and solid surfaces. Much of this has involved the interaction of low-energy (0.5-5.0 keV) alkali and other metal ions with surfaces.   As a consequence of the overlap between the ion valence levels and the surface bands, these ions resonantly exchange electrons while in the vicinity of a surface, which leads to a certain fraction of the scattered ions being neutralized. Here are some specific examples of our work: Adsorbates.   For alkali metals adsorbed on metal and semiconductor surfaces, the neutralization is enhanced for scattering from the adsorbate due to the minimum in the surface local electrostatic potential (LEP) at the adatom site. This indicates that neutralization during ion scattering is way in which to measure the LEP. More recently, we showed that ion-surface charge exchange depends on the internal charge density distribution of an adatom. This technique can experimentally "image" the internal electron density distribution of a surface adatom, which is not possible with any other technique. In particular, we showed that the electron density of adsorbed halogens is internally polarized, leading to an anomalous LEP above the adatom.   Metal ion-surface interactions. Systems in which the projectile and target quantum states are matched are fundamentally the most interesting, technologically important and potentially useful, yet they have been largely unexplored. We are investigating the interactions of low energy metal and semiconductor ions with similar solid surfaces. We first used noble gas ion to remove Al surface atoms, and investigated the charge exchange between the emitted Al and the surface, and found that the charge distribution can be described with the same theoretical treatment that is used in scattering if the surface electronic temperature is adjusted.  We have also used Si projectiles to impact Al surfaces, and demonstrated that a unique inner-shell excitation occurs during a close collision with an Al substrate such that fast multiply charged Al is emitted. Kinetic Electron Emission.   We measured electrons emitted from Al and Ru surfaces under Na+ ion bombardment, and interpreted the data by a kinetic electron emission (KEE) process involving the formation of electron-hole pairs.  We had initially identified this process using Al surfaces, then we extended the experiments to a d-electron metal (Ru) and investigated the effects of adsorbates. We unambiguously showed that our model is applicable to more complex material systems, and that it can also explain the effects of adsorbate-induced modification of the surface electronic states.   Ion scattering from nanomaterials. Nanometer-scale materials have unique properties due to their quantum-confined states. We have shown that the neutralization of Na+ scattered from Au nanocrystals, grown on TiO2, depends the confined quantum states.   This work will enable the development of a new technique for nanomaterial characterization, through which a monolayer-level understanding of the physics of nanostructures will become possible. We have recent data in which nanomaterials were produced by bombarding a thin film of Au. This demonstrates that ion scattering can monitor the presence of confined states, even if the material is disordered.