Jeroen Bastiaansen

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.

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.

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.

Electronic processes during the sputtering of atoms from metallic surfaces studied by resonance laser ionization spectroscopy

Abstract We demonstrate how resonance laser ionization spectroscopy can be applied to investigate the electronic processes that lead to the sputtering of neutral atoms in different quantum states upon keV-ion bombardment of metallic surfaces. Key information about the electronic processes is contained in the population probability of a specific atomic state and the state-specific escape velocities of the emitted particles. State-selective laser probing allows one to measure the population partition and velocity distributions of sputtered atoms, which is exemplified for Ni. It is demonstrated that the trends in these experimental observables can be qualitatively interpreted within the framework of the resonant electron transfer model. Furthermore, we show that our data are in good quantitative agreement with the outcome of a rate-equation approach based on this model.

Sputtering of metals: Probing and modelling the emission of atoms in different electronic states

Abstract This paper deals with the investigation of electronic processes involved in the emission of neutral atoms during keV ion-beam sputtering of metals. We first present an experimental study of Ar + ion-beam sputtering of clean, polycrystalline metals. Atoms emitted in the ground and metastable states are probed using double-resonant laser ionization. These efficient probing schemes allow the quantitative determination of population partitions as well as state-specific kinetic-energy (KE) distributions. Experimental data on the sputtering of Co, Ni, Cu, Sr and Ag are reported. In the second part of this article it is shown that the emission of neutral atoms during sputtering of metals can be modelled as follows: as a result of the collision cascade created by the incoming particles, a sputtered particle escapes from the surface as a positive ion and becomes neutralized by resonant transfer of an electron from the valence band of the metal. A qualitative description of this multichannel electron transfer process is given, explaining the main observations of these sputtering studies.

Resonant electron transfer in the emission of ion-beam sputtered metal atoms studied by double resonance laser ionization

Abstract We report the measurements of population partitions and state-specific kinetic energy distributions of ground and metastable state Sr, Co and Ni atoms ejected during Ar + ion-beam sputtering of clean polycrystalline targets. To obtain this information we use a very sensitive and generally applicable method based on resonance laser ionization mass spectrometry. The experimental results show a strong dependence on the correspondence of the electronic configuration of the atomic state with the bulk electronic structure and are discussed in view of resonant electron transfer as a mechanism for the emission of metal atoms in excited states.

Resonant electron transfer during ion-beam sputtering of metals studied by double-resonant laser ionization

Abstract This paper deals with the investigation of electronic processes by resonance ionization spectroscopy during keV ion-beam sputtering of metals. We report the measurement of the population partition over the ground and metastable states of sputtered Sr atoms and its interpretation involving sputtering data for Co and Ni. The observations indicate that the population of a particular state strongly depends on the correspondence of the electronic configuration of the atomic state with the bulk electronic configuration. The combined interpretation of the sputtering results provides strong evidence that the electronic states are predominantly populated by resonant electron transfer from the metal to the sputtered particle.

Even-parity auto-ionizing states of iridium I observed by resonance laser ionization spectroscopy

Abstract Auto-ionizing states of neutral iridium were observed in the continuum structure near the first ionization limit using one-color and two-color two-step resonance laser ionization spectroscopy. The total angular momentum of 20 even-parity auto-ionizing states could be determined from a combined analysis of the two-color spectra obtained with ionization schemes using intermediate states with different total angular momentum. Double-resonant ionization schemes were evaluated by fluence-dependence measurements, and photo-ionization cross-sections for resonant ionization transitions were determined. We could also identify several high-lying members of n s, n p and n d Rydberg series converging to the first ionization limit of the atom.

The detection of Sr sputtered from metallic and biological matrices by double-resonant photoionization mass spectrometry

Resonance ionization mass spectrometry (RIMS) was used to obtain isotope and state selective information on Sr sputtered from metallic and biological matrices. In exploratory experiments Sr atoms were sputtered from bulk metal upon impact of 15 keV Ar+ ions, and probed by stepwise resonant ionization using two-color schemes. Efficient ionization schemes were selected to excite ground-state originating atoms into autoionizing states. Cross sections for photoionization were found to be up to the order of 10−15 cm2. The Sr content in bone fragments was probed utilizing these schemes. Even with minimal sample preparation, a detection limit of ±50ppm Sr in the hydroxiapatite-matrix of the bone was demonstrated, with isotope specificity. While this is inferior to detection limits normally associated with RIMS, these preliminary experiments were carried out for sputtering from untreated, non-conducting matrix materials.