Erno Vandeweert

Sputtering of atoms in fine structure states: A probe of excitation and de-excitation events

The electronic mechanisms leading to the formation of excited atoms from ion-bombarded metal surfaces have been examined in light of recent experimental observations. Specifically, populations and kinetic energy distributions are compared for metastable fine structure states of In, Rh, Ni, Co and Ag. The comparison shows that the populations of these depend strongly on the electronic configuration of the departing atom and its correspondence with the metallic band structure. Current hypotheses about fundamental processes are discussed. Missing parts of our understanding of these processes are enumerated, and a number of new experiments aimed toward filling in these gaps are proposed.# 1998 John Wiley & Sons, Ltd. Atomic and molecular desorption from ion bombarded surfaces is initiated not only by classical momentum transfer between colliding species but also by various processes. The electronic processes are particularly important in controlling the degree to which the desorbing species leave the surface in excited states or as positive or negative ions. An improved fundamental understanding of the basic mechanisms associated with these electronic events may indeed lead to more effective strategies for enhancing the ionization efficiency of desorbing species and to improve the prospects for mass spectral-based surface analyses. In general, excited atoms may be classified into two categories. Atoms in short-lived states, on one hand, are easy to detect by their radiative decay and, therefore, a wealth of experimental information on atoms sputtered in such states can be found in the literature. 1 The interpretation of these data, however, is extremely complicated due to the

On the internal energy of sputtered clusters

We have used laser ionization and time-of-flight mass spectrometry to investigate the internal excitation of neutral clusters that were generated by sputtering from a solid indium surface under bombardment with 15 keV Xe+ ions. More specifically, single photon ionization of the clusters is accomplished by a tunable, frequency doubled laser and the photoionization efficiency (PIE) curves are collected with the photon energy varied around the ionization energy in a range between 4.2–6 eV. The results are compared with the PIE curves of supposedly cold indium clusters which were produced by a supersonic nozzle expansion using a laser vaporization source and investigated under otherwise similar conditions. As a result, the sputtered clusters show a distinctive broadening and shift of the PIE curve in the threshold region, thus illustrating the influence of the sputtering process in increasing the internal energy of the ejected clusters. The experimental PIE curves are interpreted in terms of a simple model using the internal temperature and ionization energy of the cluster as fit parameters. Depending on the cluster size, temperatures between 3850 and about 1000 K are found for sputtered Inn clusters.

Emission of ground and metastable state Ni and Co atoms during ion-beam sputtering: Quantitative measurements of population and kinetic energy distributions

Abstract We report the application of a sensitive experimental procedure for the quantitative measurement of population partitions and kinetic energy distributions of metastable atoms ejected during ion-beam–solid interactions. The method, based on double resonant two-colour two-step laser ionization, was applied to study the emission of neutral atoms during 12 keV Ar + ion-beam sputtering of clean polycrystalline Ni and Co. The combined data provide evidence for resonant electron transfer during the emission process and its dependence on the correspondence of the atomic electronic configuration with the bulk electronic structure.

Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers

Due to the ease of their preparation and their relatively high stability, self-assembled monolayers (SAMs) are promising candidates to be used in the development of micro- and nanostructured materials with various functionalities. [1] The formation of SAMs is mainly driven by a combination of molecule– substrate and intermolecular interactions. So far, most of the fundamental studies of SAMs structures have been performed on simple alkanethiols chemisorbed on the Au(111) substrate. [1, 2] The importance of alkane length odd-even effects in SAMs has been recently reviewed. [3] In particular, the odd-even effect was observed in the reaction of low-energy pyrazine and [D6]benzene molecular ions with the terminal group (e.g. CH3 or CF3) of aliphatic SAMs/Au(111). [4–6] Thus, a possibility of using low-energy molecular ion beams for analysis of the molecule–vacuum interface in SAMs was clearly demonstrated. More recently, aromatic thiols have moved into the focus of interest mainly due to their potential use in molecular electronics. [7–9] However, the stress which originates from the misfit between the structure preferred by the aromatic moieties and the structural template provided by the Au(111) substrate usually results in higher defect concentration in SAMs of aromatic thiols on Au(111). [10, 11] One way to overcome this problem can be realized by introducing an alkane spacer chain between the thiol head group and the aromatic moiety, as demonstrated in previous studies on the model system of biphenyl-substituted alkanethiols BPnS [CH3(C6H4)2(CH2)nSH, n = 1–6] on a Au(111) substrate. [10] By addition of the alkane spacer the individual thiolates forming these SAMs have additional degrees of freedom through which stress is reduced without breaking up the film structure. However, insertion of the flexible alkane

Double-resonant photoionization spectroscopy of SrI

Abstract We report on the investigation of the double-resonant photoionization of Sr atoms out of the ground state 5s2 1S0 and the metastable triplet states 5s5p 3PoJ. Specifically, the saturation behavior of both the excitation steps to intermediate states, and the ionization steps to auto-ionizing states have been studied, and the photoionization cross-sections from the intermediate levels into auto-ionizing states were determined. The ionization schemes were applied in saturation conditions to estimate the ratio of atoms sputtered in the ground and metastable states during Ar+ ion-beam sputtering from a polycrystalline strontium target.

Two-step resonant photoionization spectroscopy of Co I and Ni I : continuum structure near the first ionization limit

We explored the continuum structure of Co I and Ni I near the first ionization limit using one-color and two-color two-step photoionization spectroscopy. The ionization spectra of both elements show a large number of autoionizing features. The use of different excitation schemes involving intermediate states with different angular momentum allowed the assignment of the angular momentum for part of the detected autoionizing states. Several of these states could be identified as members of different Rydberg series.

Resonance ionization in a gas cell: a feasibility study for a laser ion source

Abstract A laser ion source based on resonance photo-ionization in a gas cell is proposed. The gas cell, filled with helium, consists of a target chamber in which the recoil products are stopped and neutralized, and an ionization chamber where the atoms of interest are selectively ionized by the laser light. The extraction of the ions from the ionization chamber through the exit hole-skimmer setup is similar to the ion-guide system. The conditions to obtain an optimal system are given. The results of a two-step one-laser resonance photo-ionization of nickel and the first results of laser ionization in a helium buffer gas cell are presented.

In situ observation of particle-induced desorption from a self-assembled monolayer by laser-ionization mass spectrometry

We studied particle-stimulated desorption processes of highly ordered, self-assembled monolayers of biphenyl-based thiols covalently bound to Au/mica substrates with laser postionization in combination with mass spectrometry. Direct evidence was obtained that large molecular fragments are removed from these monolayers during impact of electrons with a kinetic energy of 1 keV. The damage that accumulates in the self-assembled monolayer with increasing electron dose was measured using ion-beam, sputter-initiated laser probing. Our results show that electron-induced desorption competes with the gradual erosion of the monolayer by the formation of a carbonaceous residual layer on the substrate.

Internal excitation of sputtered neutral indium clusters

Abstract The internal energy distribution of neutral indium clusters Inn sputtered from a polycrystalline In surface by 15 keV Xe+ ions is investigated by means of laser post-ionization and time-of-flight mass spectrometry. In particular, the neutral species are ionized by a tunable frequency doubled dye-laser and the photoionization efficiency (PIE) curves were collected with the photon energy varied around the ionization potential. This way, internal energy distributions of sputtered clusters in the size range n=2–35 could be determined using photon energies of 4.2–6 eV. The results are compared with the PIE curves of cold indium clusters which were produced by a supersonic nozzle expansion using a laser vaporization source and investigated under otherwise similar conditions. As a result, the sputtered clusters show a distinctive broadening of the PIE curve in the threshold region, thus illustrating the influence of the sputtering process in increasing the internal energy of the ejected clusters.

Modeling the multichannel electron transfer during the sputtering of Co atoms

Modeling of the electronic processes during sputtering of metals is presented in the framework of the resonant electron transfer (RET) model. The population partition and the state-selective velocity distributions of Co atoms sputtered in the ground and metastable states from polycrystalline foils are used for the interpretation. The experimental observations are in qualitative agreement with the trends predicted by the RET model. Furthermore, we show that the data on Co are in quantitative agreement with the outcome of a multilevel rate equation approach of the RET model.