The interaction of radiation with surfaces is not only of fundamental importance, but offers a method whereby selective low temperature processing of materials is possible. One direct way of addressing the relevant physics is by studying the products evolved from the surface by an electronically stimulated process. Desorption induced by electronic transition (DIET) processes can be generated by either electrons, i.e., electron stimulated desorption (ESD), or through photon stimulated desorption (PSD). Our program consists of both ESD carried out in the laboratory and PSD studies at synchrotron radiation facilities. We have investigated a number of halogen/semiconductor systems, as well as at CaF₂ and LaF₃ surfaces.
For the PSD of F⁺ and Cl⁺ from Si, we showed that the mechanism for ion desorption is dependent on the amount of charge that is transferred in forming the original chemical bond to the surface [1-4]. For both F⁺ and Cl⁺, ion desorption occurs if an anion core hole is first produced. After an intra-atomic Auger decay fills the core hole, the anion is left in a +1 state, and Coulomb repulsion between the cation and the positively charged anion is then responsible for the ion emission. If, on the other hand, a cation core hole is formed, then an inter-atomic Auger is necessary in order to form a positively charged anion.  Cation core hole formation is illustrated in the figure, which shows PSD spectra collected at the Si 2p edge. Direct desorption occurs for Si-F, as there so much charge transferred to the F atom in forming the chemical bond that an interatomic Auger decay is the only way in which to fill the hole. This is observed by the fact that the PSD edge is shifter about 1 eV higher than the TEY edge, which represents transitions in bulk Si. This mechanism explains the extremely high desorption yield of F⁺, which is often seen from surfaces containing only trace amounts of fluorine. For the less ionic Si-Cl bond, however, there is sufficient charge still associated with the Si atom that an intra-atomic process fills the core hole, thereby quenching the formation of Cl⁺. Thus, PSD for chlorine at the Si edge is indirect, and the spectrum simply mimics the bulk transitions.

Our PSD studies of CaF₂ surfaces are particularly important, as we were the first to show how surface F-center defects are created on these ionic materials [5-6]. We demonstrated that the defects form as a result of an Auger-stimulated process that begins with the removal of a cation core-electron. The removal of anion core-electron does not induce ion desorption. This result was interpreted as arising from an inward nuclear motion that must occur prior to the Auger-decay. This is the first PSD work to discuss the role of nuclear motion in a core-excited process.

Finally, our most recent work involves angle- and energy-resolved ESD [7-8]. In these experiments, we form ions via electronic excitation, and then measure angle-resolved kinetic energy distributions as a function of emission angle. By careful analysis of the data, we can determine the details of the ion-surface interactions. In particular, we account for attraction of the ion towards the surface by its image charge, and for angle-dependent neutralization. From this information, we can determine the initial surface bond angle. Our future plans call for detailed studies of how these parameters are affected by the electronic structure of the surface. This will be accomplished by collecting data from a variety of materials, including semiconductor, insulator and metal surfaces.

1. J.A. Yarmoff, A. Taleb-Ibrahimi, F.R. McFeely and Ph. Avouris, "Chemical Selectivity in Photon Stimulated Desorption of Fluorine from Silicon", Phys. Rev. Lett. 60, 960-963 (1988).
2. J.A. Yarmoff and S.A. Joyce, "Photon Stimulated Desorption of Fluorine from Silicon via Substrate Core Excitations", Phys. Rev. B 40, 3143-3150 (1989).
3. T.D. Durbin, W.C. Simpson, V. Chakarian, D.K. Shuh, P.R. Varekamp, C.W. Lo and J.A. Yarmoff, "Stimulated Desorption of Cl⁺ and the Chemisorption of Cl₂ on Si(111)-7x7 and Si(100)-2x1", Surf. Sci. 316, 257-266 (1994).
4. J.A. Yarmoff, V. Chakarian, T.D. Durbin, C.W. Lo, D.K. Shuh, W.C. Simpson and P.R. Varekamp, "Photon Stimulated Desorption of Halogens", Nucl. Instrum. Methods 101, 60-63 (1995).
5. V. Chakarian, T.D. Durbin, P.R. Varekamp and J.A. Yarmoff, "Formation of Surface F-Centers on CaF₂/Si(111)", Phys. Rev. B 48, 18332-18335 (1994).
6. V. Chakarian, T.D. Durbin, P.R. Varekamp and J.A. Yarmoff, "Radiation Damage of Epitaxial CaF₂ overlayers on Si(111) Studied by Photon-Stimulated Desorption: Formation of Surface F-centers", J. Vac. Sci. Technol. A 12, 2159-2163 (1994).
7. W.C. Simpson and J.A. Yarmoff, "Ion-Surface Interactions in the Electron-Stimulated Desorption of Cl⁺ from Cl₂/Si(111)-7x7", Phys. Rev. B 52, 2038-2046 (1995); "Erratum: Ion-Surface Interactions in the Electron-Stimulated Desorption of Cl⁺ from Cl₂/Si(111)-7x7", Phys. Rev. B 52, 14273 (1995).
8. W.C. Simpson and J.A. Yarmoff, "Bonding Geometries of Cl and F on Si(100)-2x1", Surf. Sci. 359, 135-146 (1996).