Desorption Of Neutral Atoms Research Articles

Effect of surface charge on laser-induced neutral atom desorption

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges (Barth and Henry in J. Phys. Chem. C 113:247, 2009). In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures (Stoneham et al. in Appl. Phys. A 69:S81, 1999). We report here that such surface charging strongly impacts neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single-crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution.

Effect of secondary electrons on the yield of electron-stimulated desorption of neutral atoms under differently localized core-level excitations in a substrate

This paper reports on the results of a comparative analysis of the changes in the electron-stimulated desorption (ESD) yield of neutral particles from layers of alkali metal and Ba atoms deposited on the surface of a metal atop an oxygen (O/W, O/Mo) or germanium (Ge/W) film as a function of the incident electron beam energy E. The atomic yield q(E) is compared with the ionization cross sections of the core levels whose ionization potentials coincide with the ESD yield thresholds of the atoms. Three types of dependences q(E) are discussed, and the role of the secondary electrons generated in the electron-bombarded substrate for each type of the dependence of the ESD yield on E is elucidated. The analysis is based on the experimental studies performed by the authors in the recent years, starting from 1991. It is shown that the actual type of dependence q(E) is determined both by the actual localization of the atom excited by the electron beam and by the extent of localization of the core excitation resulting in ESD.

Interaction of wide-band-gap single crystals with 248-nm excimer laser irradiation. X. Laser-induced near-surface absorption in single-crystal NaCl

Ultraviolet laser-induced desorption of neutral atoms and molecules from nominally transparent, ionic materials can yield particle velocities consistent with surface temperatures of a few thousand kelvin even in the absence of visible surface damage. The origin of the laser absorption required for this surface heating has been often overlooked. In this work, we report simultaneous neutral emission and laser transmission measurements on single-crystal NaCl exposed to 248-nm excimer laser radiation. As much as 20% of the incident radiation at 248 nm must be absorbed in the near-surface region to account for the observed particle velocities. We show that the laser absorption grows from low values over several pulses and saturates at values sufficient to account for the surface temperatures required to explain the observed particle velocity distributions. The growth of absorption in these early pulses is accompanied by a corresponding increase in the emission intensities. The diffuse reflectance spectra acquired after exposure suggest that near-surface V-type centers are responsible for most of the absorption at 248 nm in single-crystal NaCl.

Interaction of wide-band-gap single crystals with 248-nm excimer laser radiation. XI. The effect of water vapor and temperature on laser desorption of neutral atoms from sodium chloride

We show that low partial pressures of water vapor (10−5Pa) dramatically increase the intensity of neutral Na and Cl emissions from cleaved, single-crystal NaCl during pulsed laser irradiation at 248 nm (KrF excimer). The time-of-flight distributions of these emissions are consistent with thermal desorption from laser-heated surfaces. Significantly, introducing water vapor lowers the particle velocities and thus the effective surface temperature during emission. Transmission measurements confirm that laser absorption is reduced in the presence of water vapor. The Arrhenius analysis of the emission intensities and effective temperatures show reduced activation energies in the presence of water vapor, which more than compensate for the vapor-induced reduction in laser absorption and surface temperature. Atomic force and scanning electron microscopy of the irradiated surfaces show evidence for accelerated monolayer-scale erosion in the presence of water vapor. A mechanism for the effect of water on these emission and erosion processes is proposed and discussed.

Experiments at the Virtual National Laboratory for Heavy Ion Fusion

An overview of experiments is presented, in which the physical dimensions, transverse emittance and generalized perveance are scaled to explore driver-relevant beam dynamics. Among these are beam merging, focusing to a small spot, and bending and recirculating beams. The Virtual National Laboratory for Heavy Ion Fusion (VNL) is also developing two driver-scale beam experiments involving heavy-ion beams with I beam∼1 A to provide guidance for the design of an Integrated Research Experiment (IRE) for driver system studies within the next 5 years. Multiple-beam sources and injectors are being designed and a one-beam module will be built and tested. Another experimental effort will be the transport of such a beam through ∼100 magnetic quadrupoles. The experiment will determine transport limits at high aperture fill factors, beam halo formation, and the influence on beam properties of secondary electron production and neutral atom desorption from the vacuum pipe wall. Research into driver technology will be briefly presented, including the development of ferromagnetic core materials, induction core pulsers, multiple-beam quadrupole arrays and plasma channel formation experiments for pinched transport in reactor chambers.

Excimer laser assisted etching of AlGaAs and GaAs

Laser assisted etching of AlGaAs and GaAs by chlorine using ArF and KrF excimer lasers was investigated. The etching rate of AlGaAs was shown to be larger than that of GaAs. Quadrupole mass spectroscopy analysis of the etching products for KrF assisted etching showed intense Ga+ and As+ signals from AlGaAs. It is suggested that a laser-induced neutral atom desorption process is enhanced in the AlGaAs etching. Hall measurements of a shallow channel heterostructure sample and GaAs Schottky diode characterization demonstrated the low damage of the etching process. The etching profile was examined by scanning electron microscopy and feasibility for field effect transistor gate recess etching was shown.

Electronically stimulated desorption of neutral atoms from Ar films on Ru(001): Desorption mechanisms and energy-transfer processes derived from distributions of kinetic energy.

Desorption yields as well as kinetic-energy distributions of desorbing argon atoms were investigated for pure argon films of variable thickness (annealed and not annealed), and for Ar samples doped with ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and NO additives of (a) 1% and 5% concentration in the bulk, and (b) 1/4 to 1/2 monolayer on the surface. The preparation-dependent prevalence of different desorption processes is investigated. Bimodal energy distributions are obtained from clean, annealed samples which are drastically changed by dopants as well as lattice defects. From thick, annealed samples the yield ratio of fast to slow particles is found to be 1.6, which is much larger than reported for previous experiments. This indicates that perfect vacuum conditions are decisive for the study of such systems if mechanistic conclusions are to be drawn.

Microscopic theory of desorption of neutral atoms due to high excitonic density at the surfaces of III?V compounds

A microscopic theory of desorption of neutral atoms from the surface of crystalline GaP is presented. Derived results suggest that bonds are broken at the surface due to high excitonic density, so that a pair of excited holes can get localised on the same bond because such an excited state has much lower energy than that of a free exciton state. Any bond with a pair of holes, instead of covalent electrons, will be broken. Strong exciton-lattice interaction is assumed. It is argued that the mechanism of atomic desorption from surfaces is analogous to that of polymer ablation; and the desorption of neutral atoms increases linearly to super linearly with the increase in laser fluence. This agrees well with experiments.

Core excitation and decay in rare gas mono- and multilayers on a metal surface: screening, deexcitation, and desorption of neutrals and ions

In order to investigate the modification of excitation and decay of core holes by condensation and adsorption in the simplest possible cases, as well as the coupling to atomic motion, we have studied the Ar2p and Kr3d excitation regions in Ar and Kr mono- and multilayers on Ru(001). Using synchrotron radiation from BESSY (Berlin), total and Auger electron yields and yields of desorbing ions and neutrals, as function of photon energy, as well as decay electron spectra for specific primary excitations have been measured. The main results are:Multilayers: Energies for resonant core excitations are shifted to higher values by 0.6 to 1 eV compared to free atoms; for Kr, surface (smaller shifts) and bulk excitations can be distinguished. Autoionization and normal Auger spectra are clearly different, as in the gas phase. Besides desorption of neutral atoms as in the valence region, singly and doubly charged ions as well as ionic clusters are desorbed.Monolayers: The shift of resonant excitation energies relative to the gas phase is smaller than for condensed layers. Decay spectra for resonant and non-resonant excitations are identical, as for strongly coupled chemisorbates, proving that charge exchange with the metal is fast compared to core decay. As in the valence region, only neutral atoms desorb.The results shed light on the screening and charge transfer behaviour and on the mechanisms of stimulated desorption which are operative in them.

Unified approach to polymer ablation and desorption of neutral atoms from surfaces of non-metallic compounds

Abstract A unified microscopic theory is developed here to study processes of polymer ablation and desorption of neutral atoms from surfaces of non-metallic compounds which occur due to laser irradiation. In high excitonic density an excited state having a pair of excitons with their two holes localised on the same bond becomes energitically more favourable, and the bond gets etched. Results obtained here agree quite well with those of experiments and phenomenological theories.

Recent results from ESD and PSD on rare gas solids with low energy electron and with synchrotron radiation excitation

Abstract Desorption of neutral atoms following selective excitonic excitation with low energy electrons or with synchrotron radiation underlines the crucial role of exciton–lattice interaction in the microscopic desorption mechanisms. All rare gas solids yield desorption of ground state atoms which stem from relaxation of the molecular-type trapped excitations after their radiative decay. In addition, solid Ne and Ar yield desorption of metastables, 3P1 and 1P1 atoms. The desorption of excited atoms is a consequence of the negative work function. It is closely related to exciton trapping in an atomic type configuration with cavity formation. The combination of various experimental techniques is needed to get a detailed insight into the excitonic desorption mechanisms.

Kilo-electron-volt Ar–ion-induced neutral atom desorption from Rh{331}: Relation of angular distributions to surface structure

The surface structure of single crystal metals in reflected in the angular distributions of neutral atoms desorbed during keV Ar ion bombardment. Results are presented here of a study of desorption from a stepped surface, Rh{331}. Recently, classical dynamics simulations of the desorption process were shown to give excellent agreement with ejection data from Rh{111}, and the same model potential has been used to simulate desorption from Rh{331}. The agreement with experiment remains excellent despite the different coordination of the surface atoms on Rh{111} and Rh{331}. The azimuthal ejection distributions are strongly affected by the steps.

Photoluminescence of solid neon and desorption of neutral atoms

Time- and spectrally resolved VUV-luminescence of solid Ne displays various types of trapped excitons. Luminescence of surface excitons and desorption due to exciton-phonon interaction are clearly correlated and established.

Surface Analysis by Laser Ionization

AbstractAn overview is presented of a recently developed surface analysis method that combines (1) desorption of neutral atoms and molecules from a sample, typically by sputtering, (2) efficient uniform ionization close to but above the surface by an intense ultraviolet laser beam, and (3) time-of-flight mass spectrometry. This technique, surface analysis by laser ionization, or SALI, provides extremely efficient and sensitive quantitative analysis of surfaces and materials with high depth resolution. Essentially any type of material can be analyzed as evidenced by the examples presented here: the Au-GaAs system, a phosphor-silicate glass, and a bulk polymer.

Structure and energetics of potassium overlayers on ruthenium (101̄0)

The chemisorption and desorption of K at the (101̄0) surface of Ru has been investigated by LEED, Auger spectroscopy, Δφ, neutral atom and ion desorption measurements. The results reveal that substantial K-enrichment of the immediate subsurface region occurs for small potassium coverages at 430 K. Desorption of neutral K atoms is characterised by a monotonic fall in activation energy from 280 kJ mol −1 at zero potassium coverage to 180 kJ mol −1 near the monolayer coverage point. Analysis of the K + desorption data yields a value for the activation energy to atomic K desorption which agrees well with that determined directly from the neutral atom desorption spectra. Work function measurements indicate a constant adatom dipole moment of ~ 3.9 D over the coverage regime of 0 ⩽ θ(K) ⩽ 0.18. At higher coverages, the alkali overlayer depolarises (α = 30 Å 3), a near metallic overlayer being formed at completion of the first monolayer (θ(K) = 0.67). LEED shows the existence of a series of ordered phases for 0.36 ⩽ θ(K.) ⩽ 0.67. An examination of the splitting of the fractional-order beams indicates that the Ru(101̄0) surface can accommodate increasing K coverage by two distinct mechanisms: growth of c(2 × 2) domains when θ(K) ⩽ 0.5 and uni-axial compression along [1̄21̄0] when θ(K) ⩾ 0.5.

Pulsed-Laser Induced Desorption in GaAs: A Dynamic Pulse Mass Counting Study

Desorption of neutral atoms and molecules of pulsed laser irradiation of GaAs has been studied by means of a pulse mass counting technique. The translational temperature of desorbing molecules, As2 is found to be three times higher than that of As atoms. Remarkable angular dependence of translational temperature is also observed. These facts suggest that the translational temperature does not necessarily indicate the surface temperature when particles are desorbed.

Electron stimulated desorption of excited alkali atoms from alkali halide surfaces

We have measured the alkali resonance photon intensity due to electron stimulated desorption of excited particles from NaCl and KCl targets as a function of incident electron energy (20 to 1000 eV) and sample temperature (room temperature to 300 °C). The energy dependence data suggest a correlation between excited Na and K desorption thresholds and substrate core levels. These are the first experimental correlations of the desorption of neutral atoms, specifically excited neutral atoms, with the creation of core holes. The intensity of emitted radiation is observed to rise sharply and then saturate with increasing temperature.

Effect of Low Energy Electron and UV Irradiations on the Charge Condition of Etched Germanium Surfaces

AbstractUsing field‐effect and contact potential difference (c.p.d.) measurements the effect of low energy (180 to 960 V) electron irradiation on etched germanium surfaces has been studied. On all n‐ and p‐type samples examined, a loss of negative charge was found to occur from the surface as shown by an increase in surface potential and decrease in c.p.d. with electron irradiation. Changes in c.p.d. were larger than changes in surface potential, the ratio being dependent on surface preparation. The difference between surface potential and c.p.d. changes can be explained by desorption of neutral atoms from the surface causing a change in surface dipole. UV irradiation above a threshold energy of approximately 5.0 eV was found to have a similar effect. If, however, dc fields were used for band‐bending, it was found that changes in surface potential and c.p.d. were equal and hence as expected no desorption occurred.