Theoretical energy distributions of atoms sputtered from elastic collision spikes by monomer and dimer ion bombardment

The energy spectra of atoms sputtered from metal targets by bombardment with heavy monomer and dimer ions have been investigated on the basis of a standard model for elastic collision spikes. In particular we calculated the energy distributions for a Au sample bombarded by 4 keV I⁺ and 8 keV I ₂ ⁺ ions. The results were compared with the recent time of flight measurements performed by de Vries and coworkers. It was shown that the model described accurately most of the experimental observations: the count rates and the relative cascade to spike ratios for both I⁺ and I ₂ ⁺ projectiles, and the energy spectrum for dimer sputtering. The spectrum for monomer bombardment, however, was shifted towards lower energies in comparison to the experimental one. The possible source of this discrepancy is discussed.This work was supported by the Central Research Program CPBP 01.09     

The emission of correlated electrons from surfaces

This work offers a theoretical framework for the treatment of the simultaneous many-electron excitation in solids and surfaces. Starting from the transition operator, an expansion is presented that treats all involved interactions on equal footing. The method is applied to the simultaneous two-orbital excitation and the results are compared with recent experimental findings. It is concluded that such studies provide a useful tool for the investigation of inter-electronic coupling. Received: 23 March 1999 / Accepted: 25 June 1999 / Published online: 23 September 1999     

The energy distributions of atoms sputtered from HfC

The energy spectra of Hf and C atoms sputtered from an HfC target with a 6 keV Xe⁺ beam have been measured. It was found that the target constituents of widely different masses were sputtered with energy distributions of the same form. The results are compared with the collision cascade theory for compound targets.     

Formation of hollow atoms at metal- and insulator surfaces

Received: 27 March 1998/Accepted: 23 October 1998     

Angular distributions of sputtered atoms for low-energy nitrogen irradiation of silicon

We studied the angular distributions of silicon and nitrogen atoms emitted from a Si target subjected to reactive sputtering by N ₂ ⁺ ions at primary energies of 0.5 and 2keV. The composition of the deposited material does not depend strongly on the substrate position. From a comparison with nonreactive sputtering, we show that the observed shift of the Si angular distribution is mainly due to the contribution of collision events occurring in the first monolayer. Contrary to the case of noble gas ions, the sharpness of the Si distribution depends on the N ₂ ⁺ energy. The behavior of the differential sputtering yield of silicon indicates that this effect is likely to be due to a loss of recoil atoms out of the preferential direction. A possible explanation of the observed phenomena consists in assuming an anisotropic emission of Si _x N _y radicals. This hypothesis is very attractive as it could satisfactorily explain the similarity we observed between the angular distributions of silicon and nitrogen.     

Energy spectra of atoms sputtered by KeV light-ion bombardment

A calculation has been made for the sputtering of heavy targets by KeV light-ion bombardment. The calculation is based both on linear transport theory and on the assumption that only primary recoiling atoms are candidates for the sputtering process. The energy spectra calculation predicts the experimentally observed peak shift towards lower velocities than those obtained by standard linear cascade theory.     

Experimental and computer study of the spatial distributions of particles sputtered from polycrystals

The spatial distributions of the sputtered particles were simultaneously and separately studied by experiment and computer simulation for 30 keV argon ion bombardment of copper in a wide range of primary-ion incidence angles (0°–86°). The distributions were found to be similar. The discrepancies between the data obtained and the predictions of the Roosendaal-Sanders analytical theory are discussed.     

The secondary-electron yield of air-exposed metal surfaces

The secondary-electron-yield (SEY) variation of atomically clean metal surfaces due to air exposure and during subsequent heat treatments is described. As an example SEY results are presented for the case of a sputter-deposited Nb thin film. Corresponding variations in the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results, and of previously obtained SEY results for metals and metal oxides, the origin of the SEY variations is discussed. The SEY increase, which is generally observed during long-lasting air exposure of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about three for the maximum SEY of this layer is higher than that of all pure metals. Only in some cases can the air-formed oxide contribute to the air-exposure-induced SEY increase, while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposure is mainly due to an increased secondary-electron escape probability. Received: 17 June 2002 / Accepted: 25 June 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-22/767-9150, Email: Christian.Scheuerlein@cern.ch     

Detection of hydrogen isotope atoms by mass spectrometry combined with resonant ionization

We examined the application of mass spectrometric methods using resonant ionization by a tunable laser and proposed its use for analyzing hydrogen isotopes. We conducted resonance ionization mass spectrometry (RIMS) to detect gas-phase hydrogen isotope atoms. The ionization efficiency was increased by more than 1000 times that obtained with conventional methods using nonresonant ionization. Resonant laser ablation mass spectrometry (RLAMS) was applied for deuterium detection in solid samples. A graphite substrate implanted with deuterium was used for ordinary laser ablation mass spectrometry (LAMS) and RLAMS. The deuterium signal was observed very clearly by RLAMS, in contrast to LAMS. Mass spectrometry combined with resonance ionization was very useful for hydrogen isotope detection, because components with equal mass numbers were resolved and the method demonstrated higher ionization efficiency. Received: 4 November 1998 / Revised version: 12 January 1999 / Published online: 7 April 1999     

(e,2e) electron spectroscopy of surfaces

Received: 21 March 1997/Accepted 12 August 1997     

A theoretical model for laser removal of particles from solid surfaces

Received: 13 November 1996/Accepted: 30 January 1997     

Angular distributions of atoms sputtered from polycrystalline FCC- and BCC-metals

When polycrystalline target materials are sputtered with high energy Ar⁺-ions at oblique ion incidence, the anisotropy of the low energy flux beneath the target surface can be estimated from the asymmetry of the angular distribution. As too it is predicted from random cascade theory, the measured emission profiles are almost symmetrical to the target normal indicating an isotropical flux of recoils inside the target. Nevertheless even symmetrical emission profiles show characteristic deviations from the cosine law. They have to be related to the low energy cascade, which strongly depends on lattice structure and surface binding. Born-Mayer repulsion, a surface atom suffers from its next neighbours when it is ejected, is proposed to generate these differences among the specific emission profiles.     

The velocity distribution of sputtered Zr atoms for irradiation at normal and oblique angle of incidence

Laser-induced fluorescence by means of a cw dye laser has been used for investigating the velocity distribution of sputtered Zr atoms in the ground state. In order to evaluate the data the excitation probability of the atoms in the observation volume has been measured. The velocity distribution of Zr atoms for irradiation with Ar⁺-ions as well as light ions at normal and oblique angle (70°) of incidence is presented.     

Trapping and energy accomodation of atoms on metal surfaces

Expressions for the trapping and sticking probability and for the energy accomodation coefficient of atoms on metal surfaces are derived by using a Gaussian form for the distribution of the energy transfer. The results are in good agreement with recent numerical simulation calculations and experiments.     

Mechanisms for the reflection of light atoms from crystal surfaces at kilovolt energies

The computer program MARLOWE was used to investigate the backscattering of protons from the (110) surface of a nickel crystal. Grazing incidence was considered so that anisotropic effects originated mainly from the surface region. The contribution of aligned scattering was studied by comparing the results with similar calculations for an amorphous target. Energy distributions of backscattered particles were investigated for incident energies ranging from 0.1 to 5keV. The structure of these distributions was explained by making calculations for several target thickness. Specular reflection was found to depend on the structure of the first few atomic planes only. The (110) rows in the surface plane were responsible for focusing into surface semichannels. Focusing in these semichannels was found to be the strongest under total reflection conditions (below about 1.3 KeV) while the scattering intensity from surface rows increased with increasing incident energy. The orientation of the plane of incidence was found to have large influence on the relative contributions of the reflection mechanisms involved. Operated by Union Carbide Corporation for the Department of Energy.     

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of = 266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at = 193 nm and their kinetic energy distribution was determined by time-of-flight measurements. For = 1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of = 266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum atE _kin = 0.16 ± 0.02 eV. For = 355 nm the non-thermal distribution is even bimodal with maxima appearing atE _kin = 0.16 ± 0.02 and 0.33 ± 0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluentes of the laser light could also be identified.     

Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.     

Spatial and angular distributions of third harmonic generation from metal surfaces

Third harmonic generation from planar and spherical metal surfaces is studied theoretically through the standard Green function method, so that the results are not affected by the uncertainty associated with previous simplified models. In general the pattern of the non-linear scattering loosely resembles the pattern of Mie scattering. The strong backward scattering is uniquely related to the non-linear process. These results differ significantly from the predictions of the surface charge model. Received: 20 May 1998 / Received in final form: 25 August 1998 / Accepted: 1st September 1998     

Theory of sum frequency generation from metal surfaces

zzz (ω₁,ω₂). Four types of metal surfaces are considered: simple metals, alkali metal overlayers, noble metals, and charged metal surfaces. Differences and similarities with respect to second harmonic generation from these surfaces are pointed out. Received: 20 September 1998