eV Kinetic Energy Research Articles

High resolution HAXPES and status of the VOLPE project

The status of the Volume PhotoEmission (VOLPE) project, now operational at ESRF, is reviewed. Commissioning phase and test experiments confirm that high energy resolution (71 meV at 5934 eV kinetic energy) with truly bulk sensitivity is achievable in Hard X-ray Photoelectron Spectroscopy (HAXPES) experiments.

The final state interaction in 3p–3d resonance excitation of Ni(1 1 1)

The change in the photoelectron energy distribution curve caused by the excitation energy change has been measured on Ni(1 1 1) in the excitation energy region from 40 to 115 eV. The MVV super Coster-Krönig (sCK) band has four features at 49.8, 56.3, 59.5, and 60.5 eV in kinetic energy. The excitation processes leading to the final state should be treated as a single process occurring simultaneously. The final states responsible for the sCK band and the two-hole bound state band appear to be independent of each other when the two bands are separated far apart. When they occur near to each other, the spectral shapes change and eventually the two bands merge into a broad band with inseparable features. Even in this band, a peak is found around a binding energy of 6.3 eV. The complete merger occurs at an excitation energy of 68 eV. The resulting single band is broader than the independent two-hole bound state band. Apparent peak shift occurs in a way similar to the case of the post-collision interaction in atomic spectra.

Cluster-assistant generation of multiply charged atomic ions in nanosecond laser ionization of seeded methyl iodide beam

Abstract The photoionization of methyl iodide beam seeded in argon and helium is studied by time-of-flight mass spectrometry using a 25 ns, 532 nm Nd-YAG laser with intensities in the range of 2 × 10 10 –2 × 10 11 W/cm 2 . Multiply charged ions of I q + ( q = 2–3) and C 2+ with tens of eV kinetic energies have been observed when laser interacts with the middle part of the pulsed molecular beam, whose peak profiles are independent on the laser polarization directions. Strong evidences show that these ions are coming from the Coulomb explosion of multiply charged CH 3 I clusters, and laser induced inverse bremsstrahlung absorption of caged electrons plays a key role in the formation of multiply charged ions.

Structural study of 1,4-cyclohexadiene adsorption on Si( [formula omitted]) surface by low energy photoelectron diffraction

The structural properties of the subsaturation coverage of 1,4-cyclohexadiene (C 6H 8) on single domain Si(0 0 1)-2 × 1 surface have been investigated by using photoelectron diffraction (PD) at the C1s core level and at the photon energy of 330 eV. The high resolution core level study allows to determine the presence of a shifted replica of +0.9 eV in kinetic energy. The structural properties of adsorbed molecules are discussed at the light of ab initio multiple scattering calculations on several adsorption models with about the same least total energy. This quantitative experimental determination timely follows the many qualitative assessments present in literature for this important prototypical case of functional Si substrate and aims to fill the gaps between several experiments reported.

Is collision-induced dissociation of low-energy carbonyl sulfide cations adversely affected by asymmetry?

We have measured relative abundances of fragment ions resulting from collision-induced dissociation of OCS(+) ions in collision with xenon neutrals as a function of ion kinetic energy and scattering angle. The lowest energy dissociation product, S(+), dominates at all energies up to 53 eV kinetic energy studied here. Surprisingly, the second most abundant dissociation channel is CS(+) and not CO(+) even though the thermochemical threshold for CO(+) is lower than that for CS(+) and CO(+) is more abundant than CS(+) in the normal mass spectrum of OCS. We do not observe any significant abundance of CO(+) in this energy range, suggesting that collision-induced excitation and dissociation of OCS(+) is significantly different to that of symmetric triatomic ions. A possible role of asymmetry in the molecular ion's collisional activation via neutral collision is suggested for the different behavior.

Increased Ordering in the Amorphous SiOx due to Hyperthermal Atomic Oxygen.

ABSTRACTWe had studied the effects of hyperthermal (5.1eV) atomic oxygen (AO) on the structural characteristics of the silica layer and Si/SiOx interface formed by the oxidation of Si-single crystal by a variety of microcharacterization techniques. A laser detonation source was used to produce atomic oxygen with 5.1eV kinetic energy. High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) demonstrated that the silica layer formed on Si(100) by atomic oxygen is thicker, more homogeneous, and less amorphous, compared to the oxide layer created by molecular oxygen (MO). High spatial resolution Electron Energy Loss Spectroscopy (EELS) study confirmed that the Si/SiOx interface created by atomic oxygen is abrupt containing no suboxides as opposed to the broad interface with transitional states formed by molecular oxygen. SAED technique was used to observe sharper diffraction rings present in the diffraction pattern of Si(100) oxidized by reactive atomic oxygen as opposed to the diffused haloes present in the diffraction pattern of Si(100) oxidized by molecular oxygen. Radial Distribution Function (RDF) analyses were performed on the SAED patterns of Si(100) oxidized in atomic and molecular oxygen, indicating that a more ordered oxide is formed by atomic oxygen. Initial Fluctuation Electron Microscopy (FEM) results confirmed an increased medium range ordering in SiOx formed by atomic oxygen when compared to the non-regular arrangement present in the amorphous oxide formed by the oxidation of Si(100) in molecular oxygen.

Surface-induced dissociation of acetone cations from self-assembled monolayer surface of fluorinated alkyl thiol on Au (1 1 1) substrate at low collision energies

We have studied the dissociation of acetone molecular cations to acetyl cations following collision with a monolayer surface of fluorinated alkyl thiol (FC 12) self-assembled on Au (1 1 1) substrate at 13, 25.2 and 49.6 eV kinetic energies. Three energetically distinct dissociation processes contribute to total dissociation in this energy range. At all energies there is a common dissociation pathway involving loss of nearly all of the parent ion’s kinetic energy in the collision process. Fragment ions resulting from this dissociation mechanism are scattered over a wide range of angles. The second pathway, observed at 25.2 and 49.6 eV kinetic energy is delayed dissociation of collisionally excited acetone cations after only a small fraction of the ion’s kinetic energy is lost in the collision process. Fragment ions resulting from this unique dynamics feature are scattered close to the surface parallel. These dissociations take place after the excited ions have passed through the collision region and the energy analyzer and prior to their entering the mass analyzer. At 49.6 eV kinetic energy, a small intensity fragment ion peak appears at intermediate kinetic energy spectra between the low energy loss and the highly inelastic scattering peaks.

Protonation and methylation of thiophenol, thioanisole and their halogenated derivatives: mass spectrometric and computational study

The protonation and methylation of thiophenol (C 6H 5SH), thioanisole (C 6H 5SCH 3) and 4-bromo derivatives have been studied using both tandem mass spectrometric techniques and ab initio quantum chemical calculations. Protonated 4-bromo thio-compounds produced by chemical ionization (CI) are found to be collisionally dehalogenated in an rf-only quadrupole collision cell in the low (20–30 eV) kinetic energy regime giving essentially thiophenol or thioanisole radical cations. This is indicated by MS/MS/MS experiments performed in an hybrid sector-quadrupole-sector mass spectrometer. B3LYP and CCSD(T) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of either (bromo)thiophenol or thioanisole takes place on the ring; the C 4-protonated thiophenol lies about 13 kJ mol −1 below the S-protonated isomer. However, under similar conditions, protonated thioanisole is also readily demethylated generating thiophenol radical cation, but no isomer has been detected. On the other hand, experimental and theoretical results reveal a regiospecific cationization (methylation) at the sulfur atom of the title compounds. The proton and methyl cation affinities are estimated as follows: PA (thiophenol)=812±10 kJ mol −1, MCA (thiophenol)=397±10 kJ mol −1, PA (thioanisole)=839±10 kJ mol −1, and MCA (thioanisole)=454±10 kJ mol −1. The available experimental value of 873 kJ mol −1 for PA (thioanisole) appears to be overestimated. Calculated PAs at various sites of halogeno (F, Cl and Br) thiophenols ( o, m, p) are also reported.

The use of the time–energy dispersion in an electron energy analyzer

The time–energy dispersion (TED) characteristics of a spherical electrostatic mirror are used to design an unusual electron energy analyzer. It is shown that TED of a retarding spherical field in an electron mirror configuration is positive and it increases with the electron kinetic energy. This is due to an increasing penetration of electrons with high kinetic energy in the retarding field and provides the basis for developing a new type of time-of-flight spectrometer (TOF). Two limiting cases for the geometry of the proposed spectrometer are considered and for both of them the energy resolution of the TOF device depends on the combined time resolution of electron detector and readout electronics. In the case of a 10-cm inner sphere radius (22 cm outer sphere radius) for the electrostatic mirror, the spectrometer is expected to have an energy resolution of about 0.5 eV/ns and an acceptance solid angle of about 2.2 srad for electrons of 75 eV kinetic energy. A position sensitive detector allows retrieving the energy and the emission angle on the basis of the measured time-of-flight and detection point position. In the second case the inner sphere radius is 6 cm and the outer spherical segment has a radius of 120 cm. The energy resolution is expected to be about 160 meV/ns at the electron kinetic energy of 1 keV and the acceptance solid angle is 0.1 srad. In both cases changing the retarding potential can easily change the energy range within which the TOF analysis is performed.

Highly charged ion beams at eV kinetic energies

An experimental setup that furnishes slow highly charged ion beams is described. A 14.5 GHz CAPRICE electron cyclotron resonance ion source provides highly charged ions at a kinetic energy of 10 to 20 keV per charge. The kinetic energy is reduced in a deceleration system, which consists of two stages, each of which is built up by two electrostatic zoom lenses. The ion beam can be decelerated down to a kinetic energy of 1 eV/charge. Typical beam currents, measured at the experiment with an emittance of (20 π mm mrad) are about 100 pA, even for the highest measured charge states (Ar16+).

Electron stimulated molecular desorption of a non-evaporable Zr–V–Fe alloy getter at room temperature

Electron stimulated molecular desorption (ESD) from a non-evaporable getters (NEG) St 707 ® (SAES Getters ™) sample after conditioning and after saturation with isotopic carbon monoxide (cf. nomenclature in Handbook of Chemistry and Physics, CRC Press, 1994), 13C 18O, has been studied on a laboratory setup. Measurements were performed using an electron beam of 300 eV kinetic energy, with an average electron intensity of 1.6×10 15 electrons s −1. The electrons were impinging on the 15 cm 2 target surface at perpendicular incidence. It is found that the desorption yields η (molecules/electron) of the characteristic gases in an UHV system (hydrogen, methane, water, carbon monoxide, carbon dioxide) for a fully activated NEG as well as for a NEG fully saturated with 13C 18O are lower than for OFHC copper baked at 120 °C. A small fraction only of the gas which is required to saturate the getter surface can be re-desorbed and thus appears to be accessible to ESD.

MULTIPLE SCATTERING THEORY OF PHOTOELECTRON ANGULAR DISTRIBUTIONS FROM ORIENTED DIATOMIC MOLECULES

We use multiple scattering photoelectron diffraction (MSPD) theory to calculate the angular patterns of electrons photoemitted from the K-shell of CO and N 2 gas-phase oriented molecules, as recently measured by several groups. For low (E < 50 eV ) kinetic energies of the photoemitted electron, the electron scattering cannot be adequately represented by spherically symmetric potentials. We thus include nonspherical scattering potentials in our formalism through nondiagonal scattering matrices. We show that intramolecular scattering and interference are responsible for the experimentally measured patterns. This MSPD approach represents a more accurate and versatile method for dealing with such angular distributions as compared to prior calculations of these effects.

A new analyzer for spin resolved electron spectroscopies

We have developed a new instrument which performs simultaneously energy and spin-polarization analysis of an electron beam, to be used in spin resolved electron spectroscopies from magnetic surfaces and thin films. The device consists of a novel polarization detector with high efficiency and analyzing power coupled to a large commercial hemispherical energy analyzer (HEA). The spin polarimeter is based on the spin dependence of the low energy (4–6 eV kinetic energy) reflectivity from a magnetic target, namely a Fe(0 0 1)- p(1×1)O surface. We describe reliable target preparation procedures and the design of the electron-optical system transferring the electrons from the HEA exit slit to the polarimeter target.

Surface core-level shifts ofGe(100)−2×1

Core-level photoemission spectra of the buckled $\mathrm{Ge}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface taken at various energies and emission angles are analyzed, using the inelastic mean free path (IMFP) to constrain the intensities of the contributions to the $3d$ signal of the first two surface layers and the bulk. This ansatz successfully represents data from the clean Ge(100) surface at all emission angles and photon energies. The surface core-level shifts of the up-dimer atom, down-dimer atom, and subsurface atom are then found at -442, -183, and +93 meV, respectively. They are in good agreement with the final-state theoretical calculations by Pehlke and Scheffler [Phys. Rev. Lett. 71, 2338 (1993)]. The IMFP is $3.5\ifmmode\pm\else\textpm\fi{}0.2\AA{}$ at 26 eV kinetic energy. Furthermore, we found that bulk and surface atoms have distinct spin-orbit splittings of 594 and 561 meV, respectively.

Counterintuitive alignment of H2(+) in intense femtosecond laser fields.

The multiphoton ionization of H2 has been studied using laser pulses of 266 nm wavelength, 250 fs duration, and 5x10(13) W/cm(2) peak intensity. Dissociation of H2(+) via one-photon absorption proceeds through two channels with markedly different proton angular distributions. The lower-energy channel (2.6 eV kinetic energy release) is produced in the bond softening mechanism, which generates parallel alignment. The higher-energy channel (3.5 eV) originates from population trapping in a light-induced bound state, where bond hardening generates orthogonal, counterintuitive alignment.

Elastic scattering of low-energy electrons by randomly oriented and aligned molecules

Abstract Elastic scattering of low (10–50 eV) kinetic energy electrons from free diatomic molecules is studied using a single-center expansion of the full molecular potential. Dynamic exchange and polarization are included in a local form. The calculated elastic differential scattering cross-sections (DCS) for electron impact on CO and N2 are in good agreement with available experimental data. The importance of using the full molecular potential instead of a two-center potential approach is pointed out. These corrections are small for energies above 50 eV, but they become increasingly important at lower energies. When discussing the angular distributions of elastically-scattered electrons from oriented molecules (like surface adsorbates), we show that these corrections are particularly significant. The results have implications for other electron scattering problems such as those encountered in low-energy photoelectron diffraction from both core and valence levels.

Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment

ABSTRACTUsing classical molecular-dynamics simulations, we worked out a simple model of Ion Beam Assisted Deposition (IBAD) of silicon dioxide on an amorphous silica substrate, in view to in- vestigate the modifications of the structural and vibrational properties induced by medium-energy bombardment. Atoms are assumed to interact via the two- and three-body potential developed by Nakano et al.[1]. Analysis of the films grown with increasing ratio, R, of medium-(30 eV) to low- (1 eV) kinetic energy SiO2 particles shows that the density rises rapidly, from 1.3 g/cm3 for R =0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes occurring at an intermediate length scale (4-10 Å), as it manifests itself by changes in the so-called first sharp diffraction peak (FSDP), the finger-print of medium range order (MRO) in a-SiO2 glass [2, 3]. We found also that the densification results in a significant decrease of the number of “soft” vibrational modes, occurring in the 0.5-3.5 THz frequency range.

Penetration depth of energetic F atoms from F2 dissociation in layered rare gas samples

Penetration depths of atoms with kinetic energy provided by photodissociation of parent molecules in the top layer of a multilayer sample are determined from the probability to cross a spacer layer of thickness d and to arrive at the interface to a substrate. Top layer growth up to a final thickness s corresponds to a continuous increase of the effective spacer layer thickness. Modeling of growth and comparison with sample-to-sample variation of d allows us to determine separately and in a consistent way the precursors’ dissociation cross section q⋅σ and the mean penetration depth d0 of the fragments together with elimination of contaminated samples. For F atoms with 4.3 eV kinetic energy from F2 dissociation values of q⋅σ=3×10−17 cm2 and d0=2.1 nm (8 to 9 monolayers) are derived for Ar spacers. A strong increase of d0 in the case of unintentional multistep excitation of F fragments is demonstrated.

Electron- and photon-stimulated desorption of atomic hydrogen from radiation-modified alkali halide surfaces

The desorption yields of excited hydrogen atoms from the surfaces of KCl, KBr, NaCl, NaF, and LiF have been measured as a function of incident photon and electron energy and flux, time of irradiation, dosing pressure of ${\mathrm{H}}_{2}$ and sample temperature. As these surfaces are exposed to ${\mathrm{H}}_{2}$ gas during electron or photon bombardment, the fluorescence from excited hydrogen atoms ejected from the surface is monitored. The desorption yields are found to be contingent upon surface damage induced by the incident particle radiation, leading to dissociative adsorption at surface sites containing an excess of alkali metal. A desorption mechanism is presented in which incident electrons or photons induce a valence excitation to a neutral, antibonding state of the surface alkali hydride molecule complex, leading to the desorption of hydrogen atoms possessing several eV of kinetic energy.

Ab initio molecular dynamics simulation for the insertion process of Si and Ca atoms into C 74

A large-scale ab initio molecular dynamics simulation for the insertion process of silicon and calcium atoms into C 74 is carried out for the first time by using the all-electron mixed-basis approach, where a one-electron wave function is expressed by superposing plane waves and numerical atomic orbitals. The present numerical results show that a silicon atom with more than a 40 eV kinetic energy can be inserted into C 74 through the center of a six-membered ring with a very short relaxation time of about 40 fs, and a calcium atom can be inserted with a 120 eV kinetic energy with a rather long relaxation time (>540 fs).