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Tapping mode atomic force microscopy (TM-AFM) was used to characterize individual gold clusters deposited on a 1,4-benzenedimethanethiol layer. Au n ( n =1–500) clusters were produced with a laser vaporization source and deposited with low kinetic energy (0.4 eV/atom). The free thiol group of the 1,4-benzenedimethanethiol layer gives rise to a large attractive force that inhibits reliable AFM probing of the sample with standard silicon tips. After etching the tip with hydrogen fluoride this interaction becomes repulsive and the image quality drastically improves. Tip–surface interaction measurements combined with an analysis of the cluster sizes reveal a configuration where clusters grow on top of the dithiol layer. The clusters are slightly deformed after impact and due to a strong sulfur–gold binding the cluster mobility on the layer is negligible.

The resonance and nonresonant laser ionization of uranium atoms sputtered from thin metal films and individual micrometer-size uranium oxide particles, respectively, was studied to evaluate a new setup for the analysis of actinide-containing micrometer-size particles. Experiments using nonresonant (193-nm) ionization of atoms and molecules sputtered from micrometer-size uranium oxide particles have shown that the uranium detection efficiencies for sputtered neutral atoms are approximately 2 orders of magnitude higher than for secondary ions. In uranium particles of 0.5-microm diameter, 6 x 10(6) atoms of 235U were easily detected and the isotopic ratio of 235U/238U = 0.0048 +/- 4.6% is in excellent agreement with the certified value. The use of two-color, two-step resonance ionization of the sputtered neutral uranium atoms from thin films was investigated. Several excitation schemes were tested, and a significant population of several low-lying metastable states after ion sputtering was observed. Autoionizing states for double-resonant ionization were determined, and the high selectivity of ionization schemes involving these autoionizing states was illustrated by comparing the flight-time distributions of different sputtered species obtained both by resonance and nonresonant multiphoton (355-nm) laser postionization. Ideally, the options for resonance as well as nonresonant ionization would be combined in a single setup, to obtain a large gain in sensitivity and selectivity. Thus, information about the main components as well as specific isotopic information of a trace element could be obtained from the same single particle.

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.

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.

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.

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.

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.

Experiments have been conducted to gain insight into the processes of desorption of neutral species from surfaces covered with organic molecules due to bombardment with keV particles. The system is comprised of benzene molecules adsorbed onto Ag(1 1 1) and bombarded with 8 keV Ar+ ions. Molecular dynamics (MD) simulations of the same system have been performed. Results show that the presence of the benzene alters the yield, the kinetic energy distributions, and the angular distributions of the silver atoms. These changes of the desorption characteristics are the result of collisions between the Ag atoms and the benzene molecules adsorbed to the surface. As more benzene is adsorbed to the surface, the changes to the Ag atom desorption characteristics become more pronounced. The simulations reproduce the modifications to the Ag atom energy and angle distributions.

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.

We investigated the desorption of neutral benzene (C 6 H 6 ) molecules and silver atoms from C 6 H 6 /Ag{1 1 1} upon bombardment by 8 keV Ar + ions. Using state-selective resonant ionization spectroscopy, substrate atoms sputtered in the ground and a high-lying metastable state, and ground-state and vibrationally excited molecules could be probed separately. The silver atom yield, kinetic energy and polar angle distributions were found to be modified upon benzene dosing. From these results, it was inferred that a large fraction of the metastable silver atoms de-excite during collisions with adsorbates. Also the ejection of benzene molecules depends strongly both on the internal energy of the molecules and the degree of coverage of the Ag surface. Up to monolayer thickness, the benzene molecules are mainly ejected during collisions with departing substrate particles. Molecules with higher internal energy leave the surface with a distribution shifted towards higher kinetic energies. At multi-layer coverages, a slow desorption mechanism becomes dominant. It is suggested that only benzene molecules vibrationally excited near the benzene–vacuum interface can survive the ejection process without de-excitation.

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.

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.

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.

Laser postionization in combination with mass spectrometry was used to detect neutral molecules desorbed from self-assembled monolayers of phenethyl mercaptan and benzyl mercaptan on gold upon 8 keV Ar + bombardment. From experimentally obtained time distributions, it was determined that the majority of the desorbed neutral molecules leave the surface with low translational energies of about 0.03 eV. As the substrate temperature is reduced, the distributions become broader and shift to longer flight times. In combination with mass distributions, these results indicate that the thermal desorption process is initiated by chemical reactions in the organic layer, creating fragments of the original molecule weakly bound to the surface, prior to release.

Laser post-ionization mass spectrometry combined with Scanning Tunneling Microscopy (STM) has been used to investigate processes of ion-stimulated erosion of self-assembled monolayers (SAM) of phenethyl mercaptan C6H5CH2CH2S (PEM) deposited on gold. Results indicate that only PEM fragments are emitted from the surface. Most of the PEM fragments (predominantly C6H5CH2CH3 with m/za 106) are emitted with thermal kinetic energies. STM images collected on 8 keV H a -irradiated surfaces with a system tuned to probe electronic states of sulfur atoms show no additional damage induced by irradiation. This indicates that sulfur atoms are not removed from the surface during hydrogen bombardment. It is proposed that the emission of SAM molecules is initiated by chemical reactions which gently break C‐S bonds. ” 1999 Elsevier Science B.V. All rights reserved.

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify LikO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of LikO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li2O)Lin clusters, clear shell closure effects are present at n = 8 and n = 20.

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.

First results obtained with a new type of on-line laser ion source based on resonant photoionization in a buffer gas cell are presented. The main advantages of this new target-ion-source system are its high efficiency, its selectivity, its short delay time, and its universality. This is demonstrated with the production of an isotopically and isobarically pure beam of $^{55}\mathrm{Ni}$ ${T}_{\frac{1}{2}}\ensuremath{\approx}200$ ms).

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

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 and skimmer 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.

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.

Using laser ionization in combination with time-of-flight mass spectrometry, we have studied ion-induced desorption of neutral particles from self-assembled monolayers (SAMs) of ω-(4‘-methylbiphenyl-4-yl) alkane thiols (CH3(C6H4)2(CH2)nSH, BPn, n = 2, 4, 6) formed on Au(111) substrates. Because BPn/Au(111) SAMs with n = even exhibit polymorphism, the effect of purely structural changes on emission yield and fragmentation pattern could be studied without interference from changes in the chemical composition. In spite of the high energy of the primary ion beam (15 keV), the mass spectra reveal a striking sensitivity of the desorption process to rather subtle changes in the structure of the layer. Depending on the SAM structure, substantial differences in the ratio between the cleavage of the molecule−substrate and the C−S bonds are observed. For applications of SAMs as resists in ion beam lithography, the results demonstrate that well-defined removal of molecules requires exact control of the SAM structure.

In this paper the interaction between organic molecules and aluminium oxide is considered. The overall study concentrates on the chemical interface which is created between an organic layer and aluminium oxide. The knowledge of the composition and structure of hydrated films on aluminium is important for the bonding between aluminium and polymers. Native surface layers on aluminium consist of an anhydrous inner layer of amorphous Al2O3 and an hydrated outer layer, where the outer layer determines the adhesion properties. The pseudoboehmite oxyhydroxide surface can be seen as a model for the chemistry found at the surface of the air-formed film after ambient exposure of the aluminium metal. XPS was used to determine the hydroxyl activity on prepared Al substrates. The studied surface treatments were vacuum evaporated, thermally oxidized, alkaline etched, acid etched and boiled in water. As roughness is important electropolished aluminium samples were used. FESEM, FEAUGER, AFM and XPS were used to see the distribution of the organic molecule over the surface. FTIR-RAIRS was used to study the nature of the interfacial bonding. In this contribution some data about phosphonic acids and carboxylic acids are reported. The bonding of the carboxylic organic molecules was correlated with the presence of OH activity on the Al surface.

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.

The probability of the ejection of a neutral atom in a specific quantum state after keV-ion beam sputtering is often interpreted in terms of the interaction between the atomic states of the escaping atom and the electronic states of the solid. In this work, we examined this interplay in the sputtering of iridium as this element has--unlike the elements employed in previous investigations--a complex atomic structure due to strong configuration interactions. Double-resonant two-photon laser ionization is used to probe the sputtered Ir atoms yielding information about the probability for an ejected atom to populate a specific atomic state and its escape velocity. The qualitative features of the corresponding population partition and state-selective velocity distributions show the influence of the excitation energy and the electronic structure of the different atomic states. A comparison is made between the experimental data and predictions from the resonant electron transfer description.

Protein tau-3R/4R isoform ratio and phosphorylation regulates binding to microtubules and, when disturbed by aging or mutations, results in diverse tauopathies and in neurodegeneration. The underlying mechanisms were studied here in three transgenic mouse strains with identical genetic background, all expressing the tau-4R/2N isoform driven specifically in neurons by the thy1 gene promoter. Two strains, expressing human tau-4R/2N or mutant tau-4R/2N-P301L at similar, moderate levels, developed very different phenotypes. Tau-4R/2N mice became motor-impaired already around age 6–8 weeks, accompanied by axonopathy (dilatations, spheroids), but no tau aggregates, and surviving normally. In contrast, tau-P301L mice developed neurofibrillary tangles from age 6 months, without axonal dilatations and, despite only minor motor problems, all succumbing before the age of 13 months. The third strain, obtained by tau knock-out/knock-in (tau-KOKI), expressed normal levels of wild-type human tau-4R/2N replacing all mouse tau isoforms. Tau-KOKI mice survived normally with minor motor problems late in life and without any obvious pathology. Biochemically, a fraction of neuronal tau in aging tau-P301L mice was hyperphosphorylated concomitant with conformational changes and aggregation, but overall, tau-4R/2N was actually more phosphorylated than tau-P301L. Significantly, tau with changed conformation and with hyperphosphorylation colocalized in the same neurons in aging tau-P301L mice. Taken together, we conclude that excessive binding of tau-4R/2N as opposed to reduced binding of tau-P301L to microtubules is responsible for the development of axonopathy and tauopathy, respectively, in tau-4R/2N and tau-P301L mice and that the conformational change of tau-P301L is a major determinant in triggering the tauopathy.

Resonance enhanced laser photoionization schemes are employed to ionize C6H6 molecules and Ag atoms desorbed from Ag{111} by 8 keV Ar+ ion bombardment. These schemes allow for probing ground state ...

This contribution deals with double-resonant photo-ionization schemes, involving auto-ionizing states, of Co, Ni, Cu, Sr, Ag and Ir metal atoms. The continuum structure around the first ionization limit of these atoms was explored, resulting in the identification of a large number of auto-ionizing states. The saturation behavior of double-resonant ionization schemes for the atoms in the ground state as well as in several metastable states was studied, and photo-ionization cross sections were derived. These highly efficient ionization schemes were applied in ion beam sputtering studies, which resulted in the identification of the dominant electronic process governing the emission of neutral atoms.

We report on our investigations on the electronic processes during keV sputtering of metals. The population partition and the state-selective velocity distributions of Cu atoms sputtered in the ground and metastable states are obtained by resonant laser ionizution. The qualitative trends in the observations are successfully interpreted assuming a resonant transfer of electrons between the bulk metal and the escaping particle to be the dominant mechanism to populate atomic states. We show that the velocity dependence of the population probabilities is in good agreement with the outcome of a rate-equation description of the resonant electron transfer process.

We have investigated the processes that lead to the ejection of excited neutral silver atoms upon sputtering of a polycrystalline silver surface under bombardment with keV rare gas ions. Resonant multiphoton ionization incombination with time-of-flight mass spectrometry was used in order to allow a state-selective detection of the sputtered species. As a result, excited atoms in both states of the metastable multiplet 4d 9 5 s 2 ( 2 D J ) with excitation energies of 3.75 eV ( 2 D 5 / 2 ) and 4.30 eV ( 2 D 3 / 2 ) have been detected. The total population of both states is determined to be of the order of 1%, which is surprisingly high in view of the large excitation energy. The velocity distribution of atoms ejected in both states reveals that the metastable atoms are ejected with lower average velocity than those emitted in the electronic ground state, thus indicating a mechanism populating this state which becomes less efficient with increasing emission velocity. The results are discussed in terms of a two-step model describing the formation of an excited neutral atom as a combination of collective excitation processes within the collision cascade followed by resonant electron transfer between the surface and the escaping particle.

We have conducted an experimental investigation into molecular desorption stimulated by 8 keV Ar+ ions. The investigated systems are comprised of aromatic molecules (benzene and phenol) adsorbed to an Ag(111) surface. Resonance-enhanced laser ionization coupled with time-of-flight mass spectrometry provide the ability to obtain quantum state resolved kinetic energy distributions of the desorbed molecules. Our results indicate that the desorption mechanisms for the molecules are dictated by the molecular coverage and determine if the molecules are desorbed in an internally excited state. We use specific mechanisms observed in molecular dynamics simulations reported in the literature to describe our experimental observations. In the low coverage regime, ballistic collisions between dislocated silver substrate atoms and the adsorbed molecules lead to the emission of energetic molecules. A collision between a single substrate atom and an adsorbed molecule leads to the emission of translationally and internall...

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.

For the analysis of actinide containing micron-size particles, a new experimental setup has been evaluated which uses sputtered neutral atoms instead of secondary ions. The atoms are post-ionized with lasers, either non-resonantly or resonantly. First experiments on non-resonant ionization of micron-size uranium oxide particles have shown that the sputter yields for secondary neutral atoms are much higher than for secondary ions, and uranium particles

Resonance ionization mass spectrometry (RIMS) was used to obtain state selective information on uranium atoms released from thin films by keV ion beam bombardment. First experiments were conducted to evaluate double-resonant laser ionization schemes. The high selectivity of such ionization schemes is further illustrated by comparing the flight-time distributions of the sputtered particles obtained both by resonant and non-resonant multiphoton (355 nm) laser post-ionization.

The electronic processes governing keV ion-beam sputtering of metals are studied by double resonance laser ionization mass spectrometry of the ejected metal atoms. We present the population partitions over the ground and metastable states of sputtered Cu and Sr atoms and their state-selective flight-time distributions. These results support the model indicating resonant electron transfer from the metal to the sputtered particle as a key mechanism to populate the electronic states.

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

One-color two-photon ionization spectroscopy was used to probe state-selectively neutral benzene molecules desorbed from a benzene overlayer physisorbed on a Ag{111} surface upon 8 keV Ar+ bombardment. Time distributions were measured for benzene molecules ejected in the zero level of the molecular ground state and in the first state of the ν6″ vibration. These distributions are found to show a strong dependence both on the internal energy of the ejected molecules and the degree of coverage of the Ag surface. Up to monolayer coverages, benzene molecules are ejected by direct collisions with Ag particles sputtered from the underlying substrate. Molecules with higher internal energy leave the surface with a distribution shifted towards lower flight times. At multilayer coverages, a second, thermal-like ejection mechanism gains significance. It is suggested that only molecules excited near the benzene-vacuum interface, survive the ejection process without being deexcited.

We report the measurement of state specific kinetic energy distributions of ground and metastable state Ni atoms ejected during 15 keV. Ar+ beam sputtering of clean polycrystalline Ni targets. A very sensitive experimental procedure based on double resonant laser ionization was used to measure a nearly complete set of kinetic energy distributions of the populated states. The state specific kinetic energy distributions and their weighted sum using the measured population partition provide further evidence for resonant electron tunneling during the emission process and its dependence on both the atomic electronic configuration and the bulk electronic structure.

We present the development of a sensitive method for the study of atomization processes via the metastable state population and kinetic energy distributions of sputtered atoms, including atoms on high-lying metastable electronic states. The experimental procedure is based on two-colour two-step resonant photoionization. We report on the population distribution of Ar ion sputtered Ni and Co atoms. The measurements show unexpectedly high populations on metastable states with energies above 1.5 eV. The kinetic energy distributions of Ni atoms on such states support the importance of bandstructure effects in the excitation mechanism of atoms after ion beam sputtering.

First results obtained with a new type of an element‐selective laser ion source for an on‐line mass separation are presented. The concept of the ion source is based on laser resonance ionization of nuclear reaction products thermalized in a high‐pressure noble gas. The fast and universal thermalization of reaction products in combination with high selectivity and high efficiency of resonance photo ionization allows to develop an element‐pure source of short lived exotic nuclei.

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.

Two-step resonant photoionization of free Ni atoms is studied by one-color and two-color processes, using pulsed lasers with photon energies around half the ground-state ionization energy. Two-color two-step resonance ionization from the ground state a${\mathrm{}}^{3}$${\mathit{F}}_{4}$ and the first metastable state a${\mathrm{}}^{3}$${\mathit{D}}_{3}$ via the triplet states z${\mathrm{}}^{3}$${\mathit{G}}_{3,4,5}^{0}$ was used to probe the autoionizing states just above the ionization limit. Several strong lines in the one-color ionization spectrum are explained as direct two-photon transitions to autoionizing states. Photoionization cross sections were deduced from saturation measurements. The large cross sections observed for specific one-color two-step transitions are due to the near double resonant excitation into autoionizing states. \textcopyright{} 1996 The American Physical Society.

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.