eV Kinetic Energy Research Articles

Ultrafast electron relaxation measurements on α-SiO2 using high-order harmonics generation

Time-resolved photoemission spectroscopy, using high order harmonics, is used to measure the energy relaxation rate of hot electrons in α-SiO2 with sub-picosecond time resolution. Our results indicate that electrons of 30 eV kinetic energy in the conduction band relax at a rate which is at least two orders of magnitude lower than the one of photo-excited carriers of a few eV. As a result, we give insight in the relaxation process of hot electrons and show that impact ionization probability per unit time is only of the order of 1/40 ps−1, in very strong contrast with the much higher value generally assumed in models of optical breakdown.

Observation of a threshold in potential sputtering of LiF surfaces

A quartz-crystal microbalance technique is used for measuring total sputtering yields for LiF under impact of slow (20 eV, 100 eV, 500 eV and 1000 eV kinetic energy) singly and doubly charged ions. At low kinetic energies (⩽100 eV) potential sputtering (PS) (i.e., sputtering due to the projectiles potential energy) clearly dominates over kinetically induced sputtering. New insight into the mechanisms for PS is gained by determining the minimum potential energy necessary to induce PS. The measured potential energy threshold at around 10 eV provides evidence that PS can already be induced by the production of cold holes in the valence band of LiF via resonant neutralisation.

Ｎｉ／Ｐｄ膜構造の下地条件依存性の分子動力学法による検討

Molecular dynamics simulations of the formation process of Ni/Pd bilayer films were performed by employing the embedded atom method with high (10 eV) and low (0.1 eV) kinetic energies of incident atoms for two different crystal orientations: fcc(111) and fcc(100).The tendencies of the film formation process and the interdiffusion at the interface were similar in both the (111) and (100) cases. However, the strain at the interface of films on the (100) substrates was larger than that on the (111) substrates.In the case of Pd deposited on an Ni(100) underlayer, the Pd film grew with the (111) symmetry and partially observed Ni(100)-c(16 × 2) structure. The crystal orientation of the Pd layer varied according to the value of the strain in the Ni(100) underlayer, regardless of the surface condition of that underlayer. In the crystal orientation dependencies, the value of the strain in the magnetic layer at interfaces, which is obtained by simulation, corresponds well to the experimental value of the perpendicular magnetic anisotropy.

A novel electron spin-polarization detector with very large analyzing power

The low energy electron reflectivity from a magnetic surface depends on the relative orientation between the electron spin and the sample magnetization. This effect has been exploited for realizing a new electron spin polarization detector. As a scattering surface we have used a well ordered Fe(001)-p(1×1)O surface, which is stable against surface contamination and gives rise to very large spin dependent effects. We describe in detail the preparation in vacuum of such a target surface and show that it can be transferred in a separate system, without performance losses. The analyzing power S of the detector is given by the relative variation of its response when the target magnetization is reversed. We find extremely large S values for electrons reflected at 3–6 eV kinetic energy, with maximum above 45%, i.e., roughly 3 times better than other polarimeters presently used.

Slowing down of atoms in metals studied by the Doppler-broadened γ-ray line shapes produced after thermal-neutron capture in Fe and Cr crystals

Molecular-dynamics simulations describing the slowing down of atoms in solids are used to study interatomic potentials in metals. This analysis is achieved by observing the fine structures of Doppler-broadened \ensuremath{\gamma} rays emitted by the recoiling excited nuclei. The recoil of the atom, \ensuremath{\sim}400 eV kinetic energy, is generated by the emission of a preceding \ensuremath{\gamma} ray following thermal-neutron capture. The experiment was performed with oriented single crystals of Fe and Cr as target and high-resolution \ensuremath{\gamma}-ray spectroscopy. Two different nuclear levels for each element were studied and the best interatomic potential among many available in the literature could be deduced from the data. The construction of a different potential was also investigated with this technique. Lifetime values with a much improved precision were obtained for four excited nuclear levels.

Trapping dynamics of disilane on bare and monohydride-terminated Si(100)-2×1

The initial trapping probability of disilane on bare and monohydride-terminated Si(100)-2×1 surfaces was measured using molecular beam techniques. On the bare surface, trapping is shown to be very efficient, with near unit trapping probability at incident kinetic energies up to 0.2 eV, and nearly 50% trapping at 1 eV kinetic energy. The monohydride-terminated surface displays significantly less trapping efficiency than the bare surface. Interestingly, trapping probabilities decrease as the angle of incidence is increased, with the decrease becoming more pronounced at higher kinetic energies; this effect is discussed in terms of parallel momentum accommodation dynamics.

Interaction of mass selected antimony clusters with HOPG

We have investigated the interaction of positively charged antimony clusters Sb n + with HOPG as a function of cluster size (2 < n < 13) and cluster kinetic energy (<600 eV) by tandem time of flight mass spectrometry and scanning tunneling microscopy. The processes taking place when the cluster hits the surface can be classified into three main categories according to its kinetic energy. At very low kinetic energies (⩽13 eV) soft landing should be feasible, i.e. no fragmentation of the cluster is taking place as a result of the impact. With increasing kinetic energy up to about 150–180 eV we find two competing processes. One is the fragmentation of the cluster into the most stable products (unimolecular decay), the other is neutralization. The efficiency of the second process increases with increasing particle energy from about 85% to almost 100% at 150–180 eV kinetic energy. The third category, too, is characterized by two competing processes. At about 150–180 eV collision energy the interaction leads to an ultrafast heating of the projectile. The cluster shatters immediately forming very small fragments. These fragments are ionized in part directly at or near the surface. The second process, namely implantation, starts at about 110 eV for the smallest investigated cluster, and it shows a strong threshold behavior as a function of cluster size. During this process the cluster is presumably strongly deformed and the surface atoms are displaced from their original positions, leaving the impact area in a highly amorphous state.

Very high resolution electron spectroscopy with third generation synchrotron radiation sources: the resonant Auger decay spectrum of the Xe 4d5/2−16p state

We have recorded an Auger decay spectrum of the resonantly excited Xe 4d5/2−16p state between 36.4 and 37.3 eV kinetic energy. The spectrum was obtained under resonant Raman conditions, with a bandwidth of ≈6 meV for the exciting radiation, much smaller than the natural lifetime width of 110–120 meV. The full width at half maximum of the peaks in the spectrum was 10 meV, making it possible to completely resolve structure that overlapped in earlier studies.

The structure at the interface of Ni/Pd superlattices with different crystal orientations estimated from molecular dynamics simulation

Molecular dynamics simulations of the formation process of an Ni/Pd film were performed by employing the embedded atom method with high (10 eV) and low (0.1 eV) kinetic energies of incident atoms for two different crystal orientations: fcc (1 1 1) and fcc (1 0 0). The tendencies of the film formation process and the interdiffusion at the interface were similar in both the (1 1 1) and (1 0 0) cases. However, the strain at the interface of films on the (1 0 0) substrate was larger than that on the (1 1 1) substrate. In the case of (1 1 1), lattice relaxation is found at the interface and the strain distribution of Ni layers is not uniform over the plane. From the observation of the traces of the atoms in early stage of film formation, it was found that the range of migration of adatoms on (1 1 1) surface is longer than that on (1 0 0) and the Ni nuclei as large as three atoms are mobile on Pd (1 1 1) surface.

Ion beam deposition of 107Ag(111) films on Ni(100)

Monolayer thick silver films were grown on an Ni(100) surface using direct ion beam deposition of 107Ag + ions with 20 eV kinetic energy and the substrate at 25°C. Deposition was performed by means of a mass-selected, low-energy, ultra-high vacuum ion beam system with a well-defined ion energy. The films were analyzed in situ by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and by low energy electron diffraction (LEED). Classical ion trajectory simulations were used to model the TOF-SARS spectra. Results of the TOF-SARS spectra and azimuthal scans are consistent with the growth of a monolayer Ag film with a fcc (111) structure on the Ni(100) surface. The deposited films exhibited a faint hexagonal c(2×8) LEED pattern. Molecular dynamic simulations of the deposition process are in agreement with the experimental observations. The growth of such isotopically pure metal monolayers at room temperature with controlled orientations provides the opportunity for exploring a variety of unique physical properties.

Radial and spectral profiles of atomic deuterium in front of a limiter in TEXTOR 94: Results of laser-induced fluorescence at Lyman-α

Density profiles of atomic deuterium have been recorded in the plasma edge of TEXTOR 94, in front of a limiter, during ohmic discharges for central, line-averaged densities between 1.0 × 10 19 and 3.0 × 10 19 m −3. The corresponding spectral profiles could be measured simultaneously for each radial position. They show, like previous measurements in the vicinity of the wall, the predominance of very slow atoms, below 1 eV (within the covered range of 0–10 eV kinetic energy in the radial direction). The profiles give indications about the presumed molecular origin of these slow atoms.

Resonant formation measurements of \(dt\mu \) via time of flight

Solid hydrogen in the form of an inhomogeneous layered target offers several experimental advantages when compared with liquid or gas. Beams of non-thermalized muonic hydrogen atoms allow us to explore resonant molecular ion formation processes near eV kinetic energies. Isotopically specific layers make it possible to separate competing and confusing interactions and to employ the time of flight for comparison with predictions based on theoretical energy dependences. Unambiguous charged fusion product detection simplifies absolute intensity measurements.

MBE Growth and Ultrahigh Temperature Processing of High-Quality AlN Films

AbstractMolecular beam epitaxial growth of AlN on sapphire and 6H-SiC has been performed utilizing mono-energetic activated nitrogen ion beams (2-80 eV kinetic energies). The growth temperature of AlN in MBE is found to be limited by the sticking coefficient of incident reactants. The combination of elevated growth temperatures (1050-1150°C), high kinetic-energy reactive nitrogen (&gt;40 eV) and post-growth thermal processing (1150-1350°C) produces high-quality AlN thin-f ilms with narrow rocking curve widths (&lt;2 arcmin) and low dislocation densities (&lt;∼3×108cm−2). In contrast, the use of in-situ step anneals during synthesis did not achieve similar quality materials.

The structure at the interface of Ni/Pd bilayer films with different deposition sequences estimated from molecular dynamics simulation

Molecular dynamics simulations of the formation process of Ni/Pd bilayer films were performed by employing the embedded atom method with high (10 eV) and low (0.1 eV) kinetic energies of incident atoms for two different crystal orientations: fcc [111] and fcc [100] of Ni or Pd substrate. The tendencies of the film formation process and the interdiffusion at the interface were similar in both the [111] and [100] cases. However, the strain at the interface of films on the [100] substrate was larger than that on the [111] substrate. At the case of Pd deposited on Ni [100], the Pd film grew with the [111] symmetry and partially observed Ni[100]-c(16/spl times/2) structure. The crystal orientation of Pd layer varied from the value of the strain in Ni [100] under layer. In the crystal orientation dependencies, the value of strain in the magnetic layer at interfaces derived from simulations corresponds well to the experimental value of perpendicular magnetic anisotropy.

Studies in organic mass spectrometry. Part 23.a Role of the aroyl group on the competitive fragmentation reactions of the molecular ion of aroylanilides

The 70 eV and mass-analysed ion kinetic energy (MIKE) spectra of some thiophenecarboxanilides and benzoylanilides (1–10) have been compared in order to investigate the role of the aroyl (or heteroaroyl) moiety on the abundance of the competitive fragmentation reactions occurring in their molecular ions (amide–bond cleavage and phenol radical ion formation). It has been shown that the electron ionisation induced decompositions with high (70 eV) and low (MIKE) internal energy excess are qualitatively similar, but remarkable quantitative differences have been observed that can be accounted for in terms of the different effectiveness in the transmission of electronic effects of substituents in the thiophene and benzene rings.

Magnetic linear dichroism in Gd 4f and 4d photoemissionof magnetic interfaces

We present a combined experimental and theoretical study of magnetic linear dichroism in the angular distribution (MLDAD) of Gd 4f and 4d core level photoemission. Expressions are given for the spectral shape and photon energy dependence by using the method of fundamental spectra. The results are compared with experimental data for a thin Gd layer on Fe(100) and for a Fe/Gd/Fe interface. Compared to magnetic circular dichroism (MCD) in 4f photoemission, the MLDAD is much smaller and undergoes a sign change around 80 eV kinetic energy. We show that for 4f and 4d emission the spectral shapes of the MLDAD and MCD are in principle different, but in practice these differences are too small to observe.

X-Ray Photoelectron Diffraction in the Backscattering Geometry: A Key to Adsorption Sites and Bond Lengths at Surfaces

For a well-characterized s2 3 1d-1O adsorbate phase of oxygen on rhodium (111) we have identified photoelectron interference fringes in the backscattering geometry for O 1s adsorbate emission at 723 eV kinetic energy. Although these features are weak, they show up as characteristic rings covering large regions of the full hemispherical photoelectron diffraction pattern. They represent the situation of a simple two-atom electron interferometer and yield directly the most relevant information on the adsorbate geometry such as the adsorption site, the adsorbate-substrate bond length, and the bond angle. [S0031-9007(98)06936-1]

Theoretical and Laboratory Studies on the Interaction of Cosmic‐Ray Particles with Interstellar Ices. III. Suprathermal Chemistry–Induced Formation of Hydrocarbon Molecules in Solid Methane (CH4), Ethylene (C2H4), and Acetylene (C2H2)

Methane, ethylene, and acetylene ices are irradiated in a ultra high vacuum vessel at 10 K with 9.0 MeV α-particles and 7.3 MeV protons to elucidate mechanisms to form hydrocarbon molecules upon interaction of Galactic cosmic-ray particles with extraterrestrial, organic ices. Theoretical calculations focus on computer simulations of ion-induced collision cascades in irradiated targets. Our experimental and computational investigations reveal that each MeV particle transfers its kinetic energy predominantly through inelastic encounters to the target leading to electronic excitation and ionization of the target molecules. Here electronically excited CH4 species can fragment to mobile H atoms and nonmobile CH3 radicals. The potential energy stored in Coulomb interaction of the CH+4 ions release energetic H and C atoms not in thermal equilibrium with the 10 K target (suprathermal species). Moderated to 1-10 eV kinetic energy, these carbon atoms and those triggered by the elastic energy transfer of the MeV projectile to the target are found to abstract up to two H atoms to yield suprathermal CH and CH2 species. C and CH, as well as CH2, can insert into a CH bond of a CH4 molecule to form methylcarbene (HCCH3), the ethyl radical (C2H5), and ethane (C2H6). HCCH3 either loses H2/2H to form acetylene, C2H2, rearranges to ethylene, C2H4, or adds two H atoms to form ethane, C2H6. C2H5 can abstract or lose an H atom, giving ethane and ethylene, respectively. C2H2 and C2H4 are found to react with suprathermal H atoms to form C2H3 and C2H5, respectively. Overlapping cascades and an increasing MeV ion exposure transforms C2Hx (x = 2, ..., 6) to even more complex alkanes up to C14H30. These elementary reactions of suprathermal species to insert, abstract, and add in/to bonds supply a powerful pathway to form new molecules in icy grain mantles condensed on interstellar grains or in hydrocarbon rich bodies in our solar system even at temperatures as low as 10 K.

GaAs microcrystal growth on semiconductor surfaces by low energy focused ion beam

A position controlled or density controlled GaAs microcrystal growth from Ga droplets was attempted using a low energy focused ion beam system. Ga ions from a liquid Ga ion source were accelerated up to 10 kV to produce a focused ion beam. Subsequently the ions were given a positive bias to reduce their kinetic energy by a four-element retarding lens system. The Ga ions with a 30 eV kinetic energy softly landed on a S-terminated GaAs surface and formed a series of Ga droplets or Ga droplets matrix. After initiating the As molecule supply, GaAs microcrystals were found to grow from these Ga droplets. The low energy focused ion beam was also applied to create nucleation sites of Ga droplets on an As-terminated Si (001) surface at 100 eV. Ga droplets with high density were formed on a region where As atoms were removed partially on the Si surface. From these results, the low energy focused ion beam was found useful for position control or density control of GaAs microcrystals on semiconductor materials.

Dissociative photoionization study of O2

Abstract Dissociative photoionization of O2 is studied by an energy–mass analyzing system and synchrotron radiation. The kinetic energy distribution spectra of the O+ ion fragments and the dissociative photoionization efficiency curves for the O+ fragments with zero and 0.8 eV kinetic energies are observed at photon energies from 18.5 to 24.7 eV. Two vibrational progressions are observed in the zero kinetic energy O+ spectrum and identified as Rydberg series nsσg (n=4 and 5) converging to the 32Πu ionic state of O2. The spectroscopic constants for these Rydberg states are obtained. Post dissociative autoionizations are observed for the O*(2P)nd″(n=3∼5) atomic states in the zero kinetic energy O+ spectrum. A shape resonance structure for the B2Σ−g ionic state is observed in the 0.8 eV kinetic energy O+ spectrum. The related dissociation mechanisms are discussed.