Vladimir A. Esaulov

UPS, XPS, and NEXAFS Study of Self-Assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.

An apparatus for multiparametric studies of ion–surface collisions

This paper describes the design and tests of an ultrahigh vacuum apparatus built for the study of particle surface interactions, with emphasis on ion scattering experiments. The system was designed to provide facilities for angle resolved electron spectroscopy, ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). It has provisions for photon spectroscopy and fixed angle time‐of‐flight (TOF) scattering and recoiling spectrometry. Mass selected ion beams in the energy range from a few eV to a few keV can be produced in a continuous or pulsed mode. Two independent, parallel plate tandem electrostatic analyzers, which can rotate around the sample are employed. The angular range spanned is analysis‐type dependent and varies from 0° to 135°. One of the analyzers was designed for low energy secondary electron spectroscopy (0–100 eV) and the other one for ISS and AES measurements in the energy range from a few eV to 5 keV. The system disposes of a...

Electron capture and loss in the scattering of hydrogen and oxygen ions on a Si surface

Abstract We present the results of a study of H − and O − formation in collisions between 0.5–4 keV hydrogen ions and oxygen ions and atoms with an Si surface. Measurements of scattered negative ion fractions are reported. These are found to be of the same magnitude as for clean metal surfaces like Al. A dynamic velocity-dependent effect in electron capture is observed. A non-resonant, velocity-dependent, charge transfer process, involving localized dangling bond surface states, is invoked to explain this observation. The production of positive ions is discussed.

Dynamics of excited state production in the scattering of inert gas atoms and ions from Mg and Al surfaces

Abstract We present results of a detailed study of the production of electronically excited states in the scattering of ionic or neutral inert gas projectiles in the keV energy range at Al and Mg surfaces. The complementary observation of scattered particles (neutrals or ions), secondary electrons and photons leads to a rather complete description of the successive stages of inelastic scattering events. Efficient neutralisation of the incident ions occurs, when they approach the surface. This is clearly demonstrated by the strong similarities between results obtained for incoming ions and incoming ground-state neutrals. The characteristics of the scattered particle distributions, the observation of scattered ions, and also of some excited states by electron and photon spectroscopy, delineates the decisive importance of short-distance binary collisions with atoms of the surface, in the production of these species. A detailed comparison with the “inverse” collisional systems in the gas phase shows that the same kind of primary excitations as described in the quasi-molecular orbital promotion model are operative and allows, for example, predictions about which of projectile and/or target atoms can be excited. Some very strong differences to gas-phase collisions are also demonstrated. They stress the importance of surface-specific effects, such as the role of resonant or Auger electron transfers between the metal surface and the receding particle in defining the final state population. In particular, an interesting surface-induced core rearrangement effect is emphasised, and different rearrangement mechanisms are presented and discussed.

Electron capture and loss in the scattering of oxygen atoms and ions on Mg, Al and Ag surfaces

We present the results of a study of collisions of 1 to 4 keV oxygen ions and atoms with Mg, Al and Ag surfaces. Formation of O- is in particular investigated. This is an interesting multichannel problem, since the ground state electronic configuration of oxygen 2p(4) corresponds to three states and electron capture processes involve three atom (P-3, D-1 and S-1)-metal continua. We report scattered ion fractions, measured in an angular range extending from 2 degrees to 40 degrees with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for a large range of collision velocities normal to the surface, thus probing the charge transfer process in different atom-surface distance ranges. The ion fractions are found to increase with increasing angle and increasing energy. Similar fractions are obtained for Al and Ag, but significantly higher ones for Mg. Ionisation processes in hard collisions with surface atoms are observed. An electron spectroscopy study was performed and did not reveal any signs of autoionising state (O**2p(2)3s(2)) production.

On Auger neutralization of He+ ions on a Ag(111) surface

Neutralization of He+ ions in grazing incidence scattering on Ag(111) is studied. A small scattered ion fraction is observed. The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulations of scattered ion trajectories are performed and the surviving ion fractions are then calculated using the theoretically estimated Auger neutralization rates. The calculations agree quite well with the experimental data and empirical estimates of the neutralization rates.

Multielectron effects in the interaction of F− ions with Ag(110)

In this paper an experimental investigation of the survival rates of negative fluorine ions scattered against an Ag(110) surface is presented. A theoretical analysis shows that the charge transfer dynamics can be attributed to resonant tunneling. We show that the results can be well described using a time-dependent Anderson model. We discuss the role of many-electron interactions and show that they must be included for an accurate modeling of the charge transfer.

H− formation in the scattering of hydrogen ions on an Al surface

Abstract We present results of an experimental and theoretical investigation of H − formation in collisions of 1–4 keV positive and negative hydrogen ions with clean Al surfaces. The scattered ion fractions were investigated in a large angular range extending from 2 to 40° with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process in a wide range of collision velocities normal to the surface, allowing us in particular to reach the domain where small atom-surface distances play an important role. Results of theoretical calculations could thus be tested in a more stringent manner than in previous high grazing-incidence experiments of Wyputta et al.

Going beyond the self-assembled monolayer: metal intercalated dithiol multilayers and their conductance

A study of the self-assembly of silver atom intercalated 5,5′-bis(mercaptomethyl)-2,2′-bipyridine (BPD; HS-CH2-(C5H3N)2-CH2-SH) and 1,4-benzenedimethanethiol (BDMT; HS-CH2-(C6H4)-CH2-SH)) dithiol (DT) multilayers on gold is presented. The bilayer of these SAMs can be obtained starting from the exposure of a DT monolayer to a concentrated silver ion solution. After grafting the silver atoms on the sulfur end group, the incubation of the resulting DT–Ag SAM in a DT solution leads to the formation of a DT–Ag–DT bilayer. This process was extended to make a multilayer structure. The corresponding changes in these self-assembled layers on Au are characterized by X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) measurements, and I–V characteristics. Our interpretation of evolution in the absorbed layer are based on changes in intensities of peaks in XPS related to S bound to substrate or Ag and in –SH groups, as well as changes in thickness and absorption features in SE measurements. The latter show the evolution in absorbance wavelength as a function of thickness and indicate a decrease in the HOMO–LUMO gap from about 4.5 eV to 4 eV. The I–V characteristics show a significant bias dependence on the number of the BPD layers and there appears to be a transition from tunneling to a hopping regime when going from the single to the multiple layers.

Electron transfer processes on Ag and Au clusters supported on TiO2(110) and cluster size effects

The results of a detailed study of Li(+) neutralization in scattering on Ag and Au clusters and thin films supported on TiO(2) are presented. A very efficient neutralization is observed on small clusters with a decrease for the smallest clusters. These results closely follow the size-effects observed in the reactivity of these systems. The energy dependence of the neutralization was studied for the larger clusters (>4 nm) and observed to be similar in trend to the one observed on films and bulk (111) crystals. A general discussion of possible reasons of the enhancement in neutralization is presented and these changes are then tentatively discussed in terms of progressive modifications in the electronic structure of clusters as a function of reduction in size and as it evolves from metallic-like to discretised states. The highest neutralization efficiency would appear to correspond to clusters sizes for which a metal to nonmetal transition occurs. The relative position of the Li level and the highest occupied molecular orbital in the molecular cluster can be expected to strongly affect the electron transfer processes, which in this case should be described in a molecular framework.