The angular behaviour of electrons inelastically scattered from the shape resonance for CO chemisorbed on Ni(110)

Recent vibrational high resolution electron energy loss experiments (HREELS) have shown evidence for molecular shape resonances in the inelastic scattering of electrons from chemisorbed molecules. Such resonances arise from the capture of the incident electron in a quasibound state of the molecule, leading to the formation of a temporary negative ion. They are manifest as an enhancement in the intensity of a specific vibrational mode at a characteristic incident electron energy. In contrast to gaseous species, the alignment which the surface provides for the chemisorbed species, can be exploited to determine the angular characteristics of the resonant state. In this work, we show evidence for a shape resonance, centred at an incident energy of 18 eV, for CO/Ni(110). The angular dependence of the scattered electron intensity has been measured for the CO stretching vibration. The results are discussed in terms of the spherical harmonic components of the resonant state, modified by vibrational broadening caused by low frequency bending modes associated with the bonding of the CO molecule to the surface.     

Time-of-flight diagnostics of wavelength-dependent CO2 laser-induced desorption from condensed layers

Time-of-flight (TOF) diagnostics have been applied to study the interaction of CO₂ laser pulses with multilayers of D₂O, CH₃OH, CH₃F, CCl₄, and C₆H₅CHO condensed on different substrates (Ge, Ag, SiO₂). Maxwellian TOF distributions were fitted to the measured distributions. The TOF temperatures and the desorption yields show the same spectral features as do the respective ir absorption spectra. This indicates resonant heating by wavelength-dependent excitation of internal modes of the condensed molecules. Semilog plots of the desorption yield versus reciprocal TOF temperature exhibit Arrhenius behavior, as expected for a thermally activated process.     

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.     

Some aspects of auger electron spectra of 3d transition metal oxides

Auger electron spectra of the transition metals Cr, Mn, Fe, Co and Ni as well as their oxides have been investigated in the energy range between 0–100 eV. In each case of the clean metal surface the observed spectrum consists essentially of one Auger line identified asM _2,3 VV transition. After oxidation a line doublet is observed revealing two transitions instead of one. Additional new Auger peaks appear in the low energy range between 0–30 eV. The “splitting” of the Auger line can be explained as resulting from aM _2,3 V _dV_d and aM _2,3 V _pV_p transition. The latter is characteristic for the compound and can in a simple way be interpreted as a cross transition.     

Reflection electron energy-loss spectra of the fullerenes C60 and C70

High purity polycrystalline samples of C₆₀ and C₇₀ were obtained and studied by the electron energy-loss spectroscopy in the reflection mode. The spectra were used for determination of the loss functions of fullerenes. Loss functions of the fullerenes were compared with those of graphite. It was established that the relative intensities of the peaks corresponding to (+)- and -plasmons depended on the primary electron energy, while the (+)-plasmon energies did not depend on the primary electron energy and were equal to 25.0 eV for C₆₀ and 24.8 eV for C₇₀. The conclusion on the space localization for plasma occilations in fullerenes was made on the base of the study of the energy dependent loss functions.     

Femtosecond laser-induced formation of spikes on silicon

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF₆. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. Received: 10 September 1999 / Accepted: 7 January 2000 / Published online: 8 March 2000     

Influence of molecular adsorbate layers on the optical spectra of small metal particles

2 , N₂O, CO₂ or N₂ on the particle surface. Depending on the molecules to which the clusters are exposed, different changes of the surface plasmon resonance in the spectra can be identified. The most essential experimental results are as follows. First, changes of the optical spectra are found if the clusters are covered by less than a molecular monolayer. Secondly, the observed variations of the spectra allow to distinguish between physisorption and chemisorption, i.e. characterize the strength of the surface chemical bond. Third, in sharp contrast to the usual shift of the plasmon frequency to longer wavelengths by change of the dielectric surrounding, a blue shift of the resonance has been observed upon dosage of CO₂. Finally, diffusion of adsorbate molecules into the bulk of the particles can be identified. Received: 11 September 1997     

High-resolution electron microscopy of Si cones formed on Ar+-bombarded Si wafers

Conical Si projections generated on Si wafers bombarded with obliquely incident Ar⁺ ions were studied by high-resolution transmission electron microscopy. The cones were composed of an [111]-oriented bulk phase covered with a disordered thin layer, but the bulk phase was not perfectly ordered, containing an amorphous domain underneath the outermost area. Such a multi-phase structure is inexplicable in terms of ion erosion, suggesting interplay of the redeposition of sputtered Si atoms on the bombarded area with the ion-erosion process so as to promote cone evolution. The cones were also characterized by the development of web-like platelets at their acute angles, an indication of a crystal growth process involved in the surface phenomenon observed.     

Sputtered neutral mass spectrometry (SNMS) as a tool for chemical surface analysis and depth profiling

An experimental system for mass spectrometry of supttered neutral particles involving a hf plasma operated in Ar at several 10⁻⁴ Torr is described. The potentialities of the method for quantitative surface analysis are reasoned. Depth profiling by sputtered neutral mass spectrometry is demonstrated for anodic oxide layers on Nb and Ta.     

Organic chemistry at interfaces studied by optical second-harmonic and IR-vis sum-frequency generation

Three selected systems are presented which demonstrate the versatility of second-order nonlinear techniques for interface analysis and thin film monitoring. With second-harmonic generation processes at buried interfaces can be monitored in real time as is shown by thiol adsorption on polycrystalline gold from solution. The time dependent second-harmonic signal shows a strong wavelength dependence due to resonances. In situ monitoring of thin-film formation by second-harmonic generation can differentiate between the reaction at the interface and the subsequent film growth. The chemical vapor deposition of polyamic acid is presented as example. Hydrogen adsorption on diamond C(111) is analysed by sum-frequency generation. It is demonstrated that sum-frequency generation has the sensitivity and specificity to trace chemical reactions at surfaces and to infer on the orientation of chemical bonds.     

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.     

Dynamics of C 60 + surface impact: Rolling,deformation, disintegration,and deposition on HOPG graphite

Cluster surface collisions of C⁺ ₆₀ (Buckminsterfullerene) ions with pyrolytic graphite are studied by time of flight mass spectroscopy in the energy range of 100 eV to 1500 eV at oblique angles of incidence extending our previous studies with steep angles of incidence. Analysis of the velocity dependence of the scattered cluster ions reveals that the normal and tangential component of the ion velocity have different significance for the collision dynamics. The tangential component is partially transformed into rotational energy of the C⁺ ₆₀ during the interaction with the surface as may be explained by an intuitive and astonishingly simple rolling ball model. In contrast, the normal component of the velocity appears to determine the amount of energy transferred into vibrational and deformational energy of projectile and target. At high normal energies the C⁺ ₆₀ disintegrate significantly and can also be partially deposited onto the surface.     

Contribution to time-resolved enhanced chemical etching and simultaneous annealing of ion implantation amorphized silicon under intense laser irradiation

Surfaces of single-crystal silicon wafers are amorphized by high-dose phosphorous ion implantation. These surfaces of the wafers, immersed in concentrated KOH, are laser-chemically etched by pulse irradiation of a ruby laser. Simultaneously, the remaining parts of the amorphous layer are annealed. The time dependence of the etching process enhanced during pulse irradiation is recorded and analysed. Reasons for the etching rates which differ between amorphous and single-crystal silicon are given on the basis of experimental and numerical results.     

Bubble nucleation and pressure generation during laser cleaning of surfaces

with a pressure pulse width of . Additionally, the phase of an acoustic pulse is observed to change upon reflection at the liquid–solid interface if bubbles are present, providing a direct proof for laser-induced bubbles. Received: 5 December 1996/Accepted: 6 January 1997     

Cluster size dependence of SiO2 thin film formation by O2 gas cluster ion beams

Cluster size effects of SiO₂ thin film formation with size-selected O₂ gas cluster ion beams (GCIBs) irradiation on Si surface were studied. The cluster size varied between 500 and 20,000 molecules/cluster. With acceleration voltage of 5 kV, the SiO₂ thickness was close to the native oxide thickness by irradiation of (O₂)_20,000 (0.25 eV/molecule), or (O₂)_10,000 (0.5 eV/molecule). However, it increased suddenly above 1 eV/molecule (5000 molecules/cluster), and increased monotonically up to 10 eV/molecule (500 molecules/cluster). The SiO₂ thickness with 1 and 10 eV/molecule O₂-GCIB were 2.1 and 5.0 nm, respectively. When the acceleration voltage was 30 kV, the SiO₂ thickness has a peak around 10 eV/molecule (3000 molecules/cluster), and it decreased gradually with increasing the energy/molecule. At high energy/molecule, physical sputtering effect became more dominant process than oxide formation. These results suggest that SiO₂ thin film formation can be controlled by energy per molecule.     

The measurement of energy spectra of neutral particles in low energy ion scattering

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×10¹⁰ ions/cm²) is much smaller than the dose needed for an ESA-spectrum (6×10¹³ ions/cm²). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV⁴⁰Ar⁺ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.     

Core electron excitation spectra of diamond,graphite, and glassy carbon

Low-energy electron energy loss spectra in theC(1s) core electron excitation region have been measured in diamond, graphite and glassy carbon in the non-differentiated form. The background subtractedN(E) spectrum has been proved to reflect the density-of-states (DOS) of the conduction band well, and the conduction band structures of these materials have been elucidated. The energy positions of the symmetry points in their conduction bands have been determined from the second-derivative spectra, which were obtained by numerical differentiation of the measuredN(E) spectra.