Ion-surface Collisions Research Articles

Forward electron emission produced in grazing ion-surface collisions: Dependence on the surface topography

We present measurements of electron emission produced in 64-keV ${\mathrm{H}}^{+}$ grazing bombardment of Si(111) surfaces prepared with different topographies. The surfaces were initially irradiated with several fluences of ${\mathrm{Ar}}^{+}$ at normal incidence, and their topographies characterized by an atomic-force microscope (AFM). The electron energy spectra, measured close to the direction of the projectile specular reflection, show two characteristic structures: a peak at an energy ${E}_{\mathrm{CE}}=35\mathrm{eV},$ corresponding to electrons moving with the projectile velocity (convoy electrons), and a broad peak at an energy ${E}_{M}>{E}_{\mathrm{CE}}.$ Their relative intensities depend strongly on the surface roughness. A similar behavior was observed for GaAs(110) and Al(111) surfaces prepared with different in situ polishing methods. A code developed to process the AFM images allowed us to assign the electron structures at ${E}_{M}$ and ${E}_{\mathrm{CE}}$ to specific topographic features.

Investigation of the trans effect in the fragmentation of dinuclear platinum complexes by electrospray ionization surface-induced dissociation tandem mass spectrometry.

Cis and trans isomers of two dinuclear platinum complexes, [cis-¿Pt(NH3)2Cl¿2 mu-(NH2(CH2)nNH2)](NO3)2 (1,1/c,c) and [trans-¿Pt(NH3)2Cl¿2 mu-(NH2(CH2)nNH2)](NO3)2 (1,1/t,t), where the diamine was 1,4-butanediamine (n = 4) or 1,6-hexanediamine (n = 6), were studied using electrospray ionization surface-induced dissociation (ESI/SID) tandem mass spectrometry (MS/MS). The same fragment ions were observed for both the cis and trans isomers of each complex (n = 4 or 6), but the relative intensities were dependent on the isomer studied. The ESI/SID data and energy-resolved mass spectra show that the position of the chloride plays a significant role in the fragmentation of these ions. Two major fragmentation pathways were detected for the complexes. The cleavage of the Pt-N bond trans to chloride was the most favorable pathway for both isomers of the complexes following the ion-surface collision. The differences in the ESI/SID spectra between the cis and trans isomers can be explained by the trans effect, namely that the Pt-N bond trans to chloride is the most labile bond.

Measurements of absolute electron-emission spectra from grazing-incidence ion-surface collisions

Convoy electron spectra have been measured in coincidence with specularly reflected ions in grazing-incidence collisions of carbon ions with a silicon (100) surface. Absolute yields of convoy electrons have been determined experimentally and compared with the results obtained from a classical trajectory Monte Carlo simulation. By comparing the velocity dependence of the convoy yields with theory, we are able to assess the relative contribution to convoy emission from valence and core electron capture to continuum states of the projectile. {copyright} {ital 1998} {ital The American Physical Society}

Neutralization of Polyatomic Ions at Self-Assembled Monolayer Surfaces before and after Electrodeposition of Poly(phenylene oxide)

Self-assembled monolayer (SAM) films chemisorbed onto polycrystalline gold substrates have been shown to be effective targets in ion surface collision investigations. One concern has been whether localized defect sites within SAM films (pinholes) influence the amount of ion neutralization that occurs at the surface. Two molecular ion probes, benzene and DMSO-d6, are employed to measure the degree of neutralization of a selected monolayer. In this investigation, the behavior of these probes upon collision with SAM surfaces composed of CH3(CH2)3S−Au (C4), CH3(CH2)17S−Au (C18), and CF3(CF2)7(CH2)2S−Au (CF) is compared to the behavior of the same SAM surfaces after potential defect sites have been modified by electrodeposition of poly(phenylene oxide). Plots of total ion current vs time show no change in the scattered ion efficiency as a result of the chemical treatment for any of the monolayer films, even in the case of C4 where the CV electrodeposition experiments indicate exposed gold. The results indicate...

Inelastic energy loss in low-energy ion-surface collisions

Recent measurements showed a previously unobserved sharp thresholdlike inelastic excitation occurring in low-energy ion-metal surface collisions when varying the energy of the incident ion. Using a relativistic Dirac-Fock-Slater approach we deduce that the inelastic energy loss of the ion beam can be traced to the fast diabatic promotion of specific molecular levels to the continuum.

Stopping in ion-surface collisions at high energies

Total energy loss by high-energy protons colliding with surfaces at grazing angles is investigated in terms of the collision parameters. A quantum formulation for binary collisions between the projectile and the free-electron gas is developed. Detailed results are obtained by using a first-order perturbation theory, and they are compared with two other stopping mechanisms, plasmon excitations and ionization of bound electrons. The range of importance of each mechanism is established in terms of the projectile velocity and its incidence angle. It is concluded that at high impact velocities the stopping due to binary collisions with the free-electron gas is dominant. {copyright} {ital 1997} {ital The American Physical Society}

TOF-ISS investigation of the dependence of the GaAs(110) surface derelaxation with the hydrogen exposure

We have used ion scattering spectrometry with time of flight analysis to investigate the atomic structure of a GaAs(110) surface exposed to atomic hydrogen. The backscattering intensity resulting from 6 keV Ne + ion-surface collisions changes with the H exposure. The changes are consistent with derelaxation towards a bulk terminated surface. The derelaxation processes present a very strong dependence upon H exposure until approximately one half of the surface has been derelaxed. At large exposures the Ne backscattering features reach a steady stage indicating that 90% of the surface has been derelaxed.

Negative ion formation on Al, Si and LiF

Negative ion formation is enhanced in low energy ion-surface collisions at grazing incidence. As the parallel velocity of the ion increases, there is an effective broadening of the solid's surface density of states as seen from the moving ion. In the present work, we analyze negative ion formation at grazing incidence for a gapless solid (Al), a small gap (1.1 eV) solid (Si) and a large gap (8.9 eV) solid (LiF). The electronic features of the different solids lead us to a formulation of resonant charge transfer theory which takes into account the periodicity of the surface band structure and its gaps, leaving aside the common parabolic-band and jellium-like description of solids.

Surface induced reactions of cluster ions

First results are presented from a new apparatus, consisting of a supersonic beam for generating neutral clusters, a variable energy electron gun for ionizing the clusters, and a tandem mass spectrometer set-up for studying surface induced reactions of mass and energy selected cluster ions. Rare gas cluster ions, fragment ions from SF6, benzene ions and benzene cluster ions have been investigated so far. Cluster ion dissociation, intracluster ion molecule reactions and surface reactions with adsorbed hydrocarbons have been shown to be important reaction channels for these ion-surface collision at energies ranging from a few eV to 500 eV. The surface induced fragmentation spectrum is demonstrated to be a useful tool for probing binding energy and structure of cluster ions.

X-ray, visible and electron spectroscopy with the NIST EBIT

An overview is given of recent activities at the NIST electron beam ion trap (EBIT) facility. The machine has been operational for almost three years. Important characteristics and demonstrated capabilities of our EBIT are presented. Selected results include experiments with trapped highly charged ions (X-ray and visible spectroscopy), and with extracted ions (ion-surface collision studies).

Theoretical description of fast kinetic electron emission in ion-surface collisions

We present a microscopic description of the emission of fast electrons in glancing-angle ion-surface collisions. We employ a classical trajectory Monte Carlo approach that treats the primary production of kinetic electrons in close collisions and their subsequent transport through the surface region on the same footing. Dynamic image interactions and multiple scattering are explicitly included. As an application, we analyze the ejected electron spectra over a broad range of electron energies and emission angles for 0.2--0.5-MeV/u Li ions interacting with SnTe(001) surfaces. Good agreement is found with experiment for the shape of the spectrum of forward-ejected electrons containing prominent structures such as the convoy electron peak and the binary ridge.

Bombardment Induced Electron-Capture Processes at Sodium Halide Surfaces.

Discrete features observed in the energy distribution of electrons emitted from ion-bombarded sodium halide surfaces can be attributed to a new type of collisional deexcitation mechanism. Such a mechanism involves sodium atoms in bombardment-excited autoionizing states that are the result of cascade collisions within the crystal lattice. This deexcitation process, in contrast to that for a metal, is not simply a consequence of the inner-shell lifetime of the initial collisionally excited sodium Na+* ion. Rather, the deexcitation consists of a sequence of lattice collisions during which the excited Na+* ion captures an electron to form the inner-shell-excited Na0* states responsible for the observed transitions. The formation of such autoionizing Na0* states is described within the framework of a new model in which excitation processes and localized collisional electron-transfer mechanisms are taken into account. These localized electron-transfer processes make possible new channels for electronic deexcitation, chemical dissociation, and defect production; they are critical for understanding inelastic ion-surface collisions in solids.

Transport of fast electrons near surfaces

We have developed a microscopic model to study emission of fast electrons is glancing-angle ion-surface collisions using a classical trajectory Monte Carlo approach. Our model accounts for dynamic image interactions and multiple scattering near the surface. We show that a sizeable fraction of the convoy peak is produced by ionization of electrons transiently bound to the impinging ion. Properties of the collision kernels near the surface are discussed. Good agreement is found with our recent coincidence experiments for 0.2–0.5 MeV/u Li ions interacting with SnTe(001) surfaces.

Processes in low-energy ion-surface collisions: preferential sputtering, defect and adatom formation

Emphasizing impact energies in the low- and sub-keV range, current advances in elucidating the interaction of ions with solid surfaces are highlighted by means of selected examples. Specifically, the following topics are addressed: (i) The influence of mass differences on the magnitude of preferential sputtering effects is investigated by monitoring, via mass spectrometric techniques, the angle- and energy-differential fluxes of the isotopes of an element. (ii) The modification and gradual amorphization of crystalline semiconductors under ion bombardment is studied for various impact conditions (energy, fluence and current density) by electron spectroscopy and electron diffraction. (iii) The formation of adatoms as observed in recent experiments and computer simulations is discussed and the respective yields are related to those of ejected species.

Surface-induced dissociation: an effective tool to probe structure, energetics and fragmentation mechanisms of protonated peptides.

The utility of surface-induced dissociation (SID) to probe the structure, energetics and fragmentation mechanisms of protonated peptides is discussed and demonstrated. High internal energy deposition provided by low-energy (eV range) ion-surface collisions yields extensive fragmentation of protonated peptides, allowing relatively uncomplicated and rapid sequence analysis of oligopeptides. SID of multiply protonated peptides is illustrated for peptides with molecular mass of up to approximately 5000 u. It is also illustrated that SID combined with electrospray ionization (ESI) provides a distinctive experimental technique to determine the energetics and mechanisms of peptide fragmentation. The relative position of ESI/SID fragmentation efficiency curves (plots of percentage fragmentation vs. laboratory collision energy) for peptides can be utilized to estimate relative energetics of peptide fragmentation and even to predict proton localization sites. The observed trends support the essential role of the mobile proton model in understanding peptide fragmentation by low-energy tandem mass spectrometry.

Surfaceinduced Dissociation: An Effective Tool to Probe Structure, Energetics and Fragmentation Mechanisms of Protonated Peptides

The utility of surface-induced dissociation (SID) to probe the structure, energetics and fragmentation mechanisms of protonated peptides is discussed and demonstrated. High internal energy deposition provided by low-energy (eV range) ion–surface collisions yields extensive fragmentation of protonated peptides, allowing relatively uncomplicated and rapid sequence analysis of oligopeptides. SID of multiply protonated peptides is illustrated for peptides with molecular mass of up to ∽5000 u. It is also illustrated that SID combined with electrospray ionization (ESI) provides a distinctive experimental technique to determine the energetics and mechanisms of peptide fragmentation. The relative position of ESI/SID fragmentation efficiency curves (plots of percentage fragmentation vs. laboratory collision energy) for peptides can be utilized to estimate relative energetics of peptide fragmentation and even to predict proton localization sites. The observed trends support the essential role of the mobile proton model in understanding peptide fragmentation by low-energy tandem mass spectrometry.

Binary-electron emission in grazing ion-surface collisions.

We study the binary-electron production due to single-electron excitation by the direct screened Coulomb interaction between the projectile and the surface. We analyze the energy spectra of electrons emitted during the interaction of ions with surfaces under grazing incidence conditions. For high ion energies (\ensuremath{\gtrsim}100 keV/amu), the binary-electron peak is shifted to energies lower than those expected in ion-gas collisions. The calculations of the electron energy distributions for different projectile charges, surfaces with different Fermi energies and work functions, and several electron observation angles are in qualitative agreement with the experimental data. For lower ion energies (30--100 keV), the present model gives a better description at large observation angles. This indicates that the ``shifted convoy'' structure, which dominates the electron emission spectrum around the direction of the specular reflection, cannot be ascribed to a binary-collision mechanism. \textcopyright{} 1996 The American Physical Society.

Examination of Sputtered Ion Mechanisms Leading to the Formation of C7H7+ during Surface Induced Dissociation (SID) Tandem Mass Spectrometry (MS/MS) of Benzene Molecular Cations

Recent work reported in the literature has shown new evidence of a sputtered ion mechanism for associative ion surface reactions occurring during surface induced dissociation (SID) tandem mass spectrometry (MS/MS). In the context of a sputtered ion mechanism, we have studied selected routes to the formation of C7H7+ during the low-energy collision (ca. 30 eV) of benzene molecular ions with surfaces covered with a hydrocarbon overlayer. A key approach utilized in this work is the use of gas phase ion molecule chemistry to model reactive ion surface collisions. Benzene, 2H6-labeled benzene, and 13C6-labeled benzene have been examined by SID and collision activated dissociation (CAD). The CAD experiments have been used to investigate the fragmentation of hydrocarbon adducts of benzene and labeled benzene formed by methane and isobutane chemical ionization (CI) as models of those ions which may be formed via a sputtered ion mechanism during SID. In addition, the thermodynamics of many of the possible sputtered ion routes to the formation of C7H7+ have been compared using experimental heats of formation as well as total energies and zero-point vibrational energies (ZPVE) obtained by ab initio (MP2 6-31G*//HF 6-31G*) calculations. While there are several seemingly viable sputtered ion routes to the formation of C7H7+ during SID of benzene molecular ions, the combination of thermodynamic considerations and experimental results suggest that a likely sputtered ion route involves the reaction of neutralized benzene ions with C3H5+ ions (sputtered from the hydrocarbon overlayer) followed by the loss of ethene.

On the spectral lineshapes, lifetimes and deexcitation processes of autoionizing states produced in inelastic ion-surface collisions

The natural lineshapes of peaks in electron spectra due to decay of autoionizing states of Ne, formed in collisions of Ne with Mg and Al solid targets, are modelled. The modelling takes into account the level shifts of the excited and ionic states of the atom in front of the surface and various resonant and Auger capture and loss processes. It is shown that rather complex lineshapes may be expected. The electron spectrum will generally possess a peak at energies in the vicinity of that corresponding to autoionization of a free atom, as well as a high-energy tail and secondary maximum. It is suggested that a structure observed at 22.2 eV in the experimental Ne** electron spectrum is just such a secondary maximum associated with the decay of the Ne** 3P 3S2 near the metal surface. The effects of varying the model lifetimes, electron capture and deexcitation rates are discussed.

Capture of externally produced ions in an ICR cell by ion-surface collisions

A new technique for the capture of external ions by an FT-ICR ion trap is suggested. Ions produced from atoms of low ionization energy collide with one of the trapping electrodes of the ICR cell, lose energy without neutralization and are captured. The surface of the trapping electrode must have a high enough work function. The probability of ion capture was investigated as a function of the ion energy and cell geometry parameters.