Enhancement effect in photon-stimulated ion desorption for benzene adsorbed on silicon surfaces observed using angle-dependent technique

We have investigated photon-stimulated ion desorption from deuterated benzene (C₆D₆) adsorbed on Si(1 0 0) and Si(1 1 1) surfaces following C 1s core excitation. Using time-of-flight mass spectrometry combined with angle-dependent technique, we measured the dependences of mass-spectra of desorption ions on photon energies and on incident angle (θ) of synchrotron beam. We have found the ion yields for adsorbate-derived fragments of CD⁺ and CD₂⁺ are enhanced in very small angles of incident X-rays. Moreover, molecular orientation effect appeared in excitation energy dependences of D⁺ ions from the Si(1 0 0) and Si(1 1 1) surfaces; that is, ion yield spectra measured at θ = 10° are different from that at θ = 65°. Furthermore, it was found that desorption ion yields increase greatly with decreasing incident angles. The angular dependences are consistently similar for all ion species, excitation energies, and indexes of substrates. Possible desorption processes are described on the basis of the observations.     

Ab initio absorption spectra of 3-tert-butylcyclohexene

We present an ab initio investigation of the optical properties of 3-tert-butylcyclohexene in both its conformers. The optical spectra, here the photoabsorption cross section, have been obtained within density-functional theory at the independent-particle level, and within time-dependent density-functional theory. The optical spectra of the two conformers show small but visible differences, hence suggesting that optical absorption experiments can discriminate among the two molecular geometries. To cite this article: K. Gaál-Nagy et al., C. R. Physique 10 (2009).     

Catalytic reduction of NO in the presence of benzene on a Pt(3 3 2) surface

The catalytic reduction of NO in the presence of benzene on the surface of Pt(3 3 2) has been studied using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). IR spectra show that while the presence of benzene molecules at low coverage (e.g., following an exposure of just 0.25 L) promotes NO-Pt interaction, the adsorption of NO on Pt(3 3 2) at higher benzene coverages is suppressed. It is also shown that there are no strong interactions between the adsorbed NO molecules and the benzene itself or benzene-derived hydrocarbons, which can lead to the formation of intermediate species that are essential for N₂ production.TDS results show that the adsorbed benzene molecules undergo dehydrogenation accompanied by hydrogen desorption starting at 300 K and achieving a maximum at 394 K. Subsequent dehydrogenation of the benzene-derived hydrocarbons then begins with hydrogen desorption starting at 500 K. N₂ desorption from NO adlayers on clean Pt(3 3 2) surface becomes significant at temperatures higher than 400 K, giving rise to a peak at 465 K. This peak corresponds to N₂ desorption from NO dissociation on step sites. The presence of benzene promotes N₂ desorption, depending on the benzene coverage. When the benzene exposure is 0.25 L, the N₂ desorption peak at 459 K is dramatically increased. Increasing benzene coverage also results in the intensification of N₂ desorption at ∼410 K. At benzene exposures of 2.4 L, N₂ desorption develops as a broad peak with a maximum at ∼439 K.It is concluded that the catalytic reduction of NO by platinum in the presence of benzene proceeds by NO decomposition and subsequent oxygen removal at temperatures lower than 500 K, and NO dissociation is a rate-limiting step. The contribution of benzene to N₂ desorption is mainly attributed to providing a source of H, which quickly reacts with NO-derived atomic O, leaving the surface with more vacant sites for further NO dissociation.     

Photoabsorption and desorption studies on poly-3-hexylthiophene/multi-walled carbon nanotube composite films

Photon stimulated ion desorption (PSID) and Near-edge X-ray absorption fine structure (NEXAFS) studies have been performed on poly-3-hexylthiophene and nanocomposites thin films made of poly-3-hexylthiophene/multi-walled carbon nanotubes (MWNT) filled with iron/iron-oxide. The experiments were performed at the Brazilian Synchrotron Light Source (LNLS) operating in a single-bunch mode following sulphur K-shell photoexcitation and using time-of-flight mass spectrometry for ion analysis. Both PSID mass spectra show great similarity and exhibit desorption of the polymer fragments only. This result seems to be in accordance with previous morphological studies on these materials, which suggested that the nanotubes are highly dispersed and involved by the polymer. Although similar, the spectra present shifts in the direction of greater time-of-flights in the case of the poly-3-hexylthiophene/multi-walled carbon nanotube composite. This behavior may be related to the donation of electronic charge between the polymer matrix and the carbon nanotubes. In both cases, S⁺ desorption seems to be suppressed, which may be due to the hexyl side-chains. Relative desorption ion yield curves have been determined as a function of the photon energy, which reproduced the photoabsorption spectrum. These results are discussed in terms of the indirect XESD (X-ray induced electron stimulated desorption) process.     

Theoretical study of spin-dependent bremsstrahlung from ferromagnetic Fe(110)

By means of a nonrelativistic one-step model Green function formalism of photoemission, spin-up- and spin-down-induced ultraviolet bremsstrahlung spectra have been calculated for ferromagnetic Fe(110) for several angles of incidence. Comparison with recent experimental data shows good agreement with regard to the existence and E(k_∥) despersion of both minority and majority spin features. Calculation of the corresponding bulk band structures and k_∥-resolved layer-by-layer quasi-particle densities of states permits a physical interpretation: most bremsstrahlung features can be understood in terms of bulk interband transitions, while one is due to surface resonance.     

Isotope effects in the kinetics of simultaneous H and D thermal desorption from Pd

The kinetics of simultaneous hydrogen and deuterium thermal desorption from PdH_xD_y has been investigated. A novel experimental approach for the study of the transition state (TS) characteristics of the surface recombination reaction is proposed based on the analysis of the H and D partitioning into H₂, HD and D₂ molecules. It has been found that the hydrogen molecular isotopes distribution is determined by the energy differences of the corresponding TS of the atom-atom recombination reactions. On the other hand, the mechanisms and activation energies of the desorption process have been obtained. At 420 K, the desorption reaction changes from a surface recombination limiting mechanism during desorption from β-PdH_xD_y to a reaction limited by the rate of β to α phase transformation during the two phase coexistence. Surface recombination reaction becomes again rate limiting above 480 K, due to a change in the catalytic properties of the Pd surface. TS energies obtained from the kinetic analysis of the thermal desorption spectra are in good accordance with those obtained from the analysis of the H₂, HD and D₂ distributions. Anomalous TS energies have been observed for the H-D recombination reaction, which may be related to the heteronuclear character of this molecule.     

Inner shell excitation and dissociation of condensed formamide

Carbon and nitrogen K-shell excitation and dissociation of condensed formamide at 96 K were studied by near edge X-ray absorption fine structure (NEXAFS) recorded by total electron yield (TEY), total ion yield (TIY) and photon-stimulated desorption (PSD) yield of H⁺ measurements. It was found that electronic transitions from the C 1s or the N 1s to the σ¹ (CH and/or NH) enhanced ion yields of H⁺ from the CH and/or NH functional groups. This selective dissociation indicates that the corresponding unoccupied molecular orbital has an antibonding character of the CH and/or NH. To investigate the molecular orientation of condensed formamide, incidence-angle-dependent TEY-NEXAFS spectra were measured. The CNO plane of the adsorbed formamide molecule is determined to be tilted away from the surface by an averaged angle of about 42°.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

Assignments of several groups of iodine (I2) lines in the B-X system

The spectra obtained by means of Fourier spectroscopy and the assignments of the B-X lines of I₂ in the vicinity of two argon ion laser lines (5145 and 5287 Å), three krypton ion laser lines (5208, 5308, and 5683 Å), and one HeNe laser line (6119 Å) are given. A detailed comparison, in the vicinity of the argon ion laser line (5145 Å), between the iodine wavenumbers calculated by means of the two sets of molecular constants previously published [Wei and Tellinghuisen, J. Mol. Spectrosc.50, 317–332 (1974); Barrow and Yee, J. C. S. Faraday II. 69, 684–700 (1973)] with those calculated from Fourier spectroscopy data, is presented.     

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD_x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.     

Chlorinated ethenes at Cu(1 1 0): adsorption, dissociation and reactions of trans-1,2-dichloroethene

The adsorption and reactions of trans-1,2-dichloroethene on Cu(1 1 0) have been investigated using reflection absorption infrared spectroscopy, temperature programmed desorption and molecular beam adsorption reaction spectroscopy. These data reveal that the behaviour of the system is critically affected by temperature and can be described in terms of three main regimes: Regime I, which occurs over 85-165 K, represents molecular adsorption in the monolayer and the multilayer where the trans-1,2-dichloroethene adsorbs with its molecular plane oriented largely parallel to the metal surface; Regime II, extending over the temperature range 170-280 K, is dominated by desorption/dechlorination events which are first initiated at 171 K, then poisoned rapidly and re-initiated at the much higher temperature of 263 K, culminating in the creation of adsorbed acetylene and Cl atoms on the surface; Regime III, spanning the temperature range of 280-500 K, is governed by the behaviour of the C₂H₂ molecular intermediate at the surface which, if created at low temperatures, trimerises to form benzene that subsequently desorbs. However, when created at high temperature, direct desorption of C₂H₂ competes effectively with the trimerisation process.     

Phonon focussing in crystals

It is shown that by evaluating the Jacobian of the transformation between the polar angles corresponding to the group velocity vector S and the polar angles of the wave vector k, the phonon magnification factor can be evaluated for any general direction of propagation inside a crystal.     

Polymer Surface Functionalization Using Plasma,Ultraviolet and Synchrotron Radiation

Polyurethane (PU) and polystyrene (PS) films were functionalized by ultraviolet (UV) or selective synchrotron radiations (SR) in the presence of reactive gases. The UV-PU results were compared with lowpressure plasma treatments of the same films. Oxygen or acrylic acid vapours (AA) were used as reactive gases. X-ray photoelectron spectroscopy measurements of UV modified films in the presence of oxygen or AA matched the RF-plasma treatments results. It is shown that COO and C=O functional groups were incorporated at the polymer surface efficiently with both methodologies. In addition, near-edge X-ray absorption fine structure showed that a thin film of poly(acrylic acid) is formed over the PU and PS films during the UV irradiation in the presence of AA vapours. These results resemble previous AA low-power plasma treatments. PU and PS films were also selectively functionalized by SR using oxygen as reactive gas. Surface concentrations of COO and C=O functional groups were enhanced by C_1s → σ* _C–C excitation after irradiation and oxygen introduction. This efficient surface functionalization was clearly observed in PS films which do not have CO and COO groups in their molecular structure. Excitations involving transitions to π* orbital (π*_C=C, π*_C=O) led to much lower functionalization efficiency. The SR results can be explained by taking into account previous photon stimulated ion desorption studies of polymers. SR results may open new ways to functionalize polymer surfaces selectively and efficiently.     

Spectroscopic study of plasmons in ion-irradiated single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes was experimentally investigated using x-ray photoelectron spectroscopy, reflection electron energy-loss spectroscopy, and Auger electron spectroscopy. A shake-up satellite structure observed near the C 1s core-level lines in the x-ray photoelectron spectra at high binding energies in the range 284–330 eV due to excitation of π and π + σ plasmons was studied. The effect of irradiation by 1-keV argon ions on the shape of the spectra was analyzed. The shape of the C 1s satellite spectra was found to be sensitive to Ar⁺ irradiation in the electron energy loss range 10–40 eV corresponding to excitation of π + σ plasmons. Auger spectroscopy revealed the presence of argon on the surface of ion-irradiated samples. The argon content increased to ～4 at. % with increasing irradiation dose. An analysis of the results obtained and their comparison with the data available in the literature led to a qualitative conclusion that the bond angles of the carbon atoms making up the walls of single-walled carbon nanotubes are distorted at sites exposed to Ar⁺ irradiation.     

Mechanism of nuclear spin-lattice relaxation and its field dependence for ultraslow atomic motion

The contribution of ultraslow self-diffusion of polycrystalline benzene molecules to the spin-lattice relaxation of protons is studied as a function of effective magnetic field H ₂ in a doubly rotating frame (DRF). Proton relaxation time T _1ρρ is measured by direct recording of NMR in a rotating frame (RF). The effective fields have a “magic” orientation corresponding to angles arccos(1/√3) in the RF and π/2 in the DRF so that the secular part of the dipole-dipole interactions of protons is suppressed in two orders of perturbation theory, while the nonsecular part becomes predominant. It is found that the diffusion contribution of benzene molecules to proton relaxation time T _1ρρ is a linear function of the square of field H ₂ and exhibits all peculiarities typical of the model of strong collisions generalized to only fluctuating nonsecular dipole interactions in fields exceeding the local field. This means that the model can also be employed in the given conditions. It is shown that perfect agreement with such a dependence can also be obtained in the model of weak collisions if we take into account the concept of the locally effective quantization field, whose magnitude and direction are controlled by the vector sum of field H ₂, and the nonsecular local field perpendicular to it.     

Adsorption of gold on oxidized tungsten

The kinetics of adsorption and desorption of gold atoms from the surface of a thin (<2 nm) oxide film grown on a textured W ribbon with the preferred emergence of the (100) face is studied using termal desorption spectrometry in a wide range of coatings. A single desorption phase is observed in the spectra of termal desorption of Au atoms from oxidized W. The activation energy of desorption of Au atoms from tungsten oxides is lower than the gold sublimation heat (it amounts to E = 3.1 eV for the concentration of adsorbate atoms on the surface corresponding to coverage θ _s = 2.38). The gold film on oxidized tungsten at room temperature grows in the form of 3D islands. The sticking coefficient for gold atoms at T = 300 K is close to unity in the coverage range 0.5 < θ _s < 2.5.     

Electron stimulated desorption of H3O from 316L stainless steel

Surface ions generated by electron stimulated desorption from mass spectrometer ion source grids are frequently observed, but often misidentified. For example, in the case of mass 19, the source is often assumed to be surface fluorine, but since the metal oxide on grid surfaces has been shown to form water and hydroxides, a more compelling case can be made for the formation of hydronium. Further, fluorine is strongly electronegative, so it is rarely generated as a positive ion. A commonly used metal for ion source grids is 316L stainless steel. Thermal vacuum processing by bakeout or radiation heating from the filament typically alters the surface composition to predominantly Cr₂O₃. X-ray photoelectron spectral shoulders on the O 1s and Cr 2p_3/2 peaks can be attributed to adsorbed water and hydroxides, the intensity of which can be substantially increased by hydrogen dosing. On the other hand, the sub-peak intensities are substantially reduced by heating and/or by electron bombardment. Electron bombardment diode measurements show an initial work function increase corresponding to predominant hydrogen desorption (H₂) and a subsequent work function decrease corresponding to predominant oxygen desorption (CO). The fraction of hydroxide concentration on the surface was determined from X-ray photoelectron spectroscopy and from the deconvolution of temperature desorption spectra. Electron stimulated desorption yields from the surface show unambiguous H₃O⁺ peaks that can be significantly increased by hydrogen dosing. Time of flight secondary ion mass spectrometry sputter yields show small signals of H₃O⁺, as well as its constituents (H⁺, O⁺ and OH⁺) and a small amount of fluorine as F⁻, but no F⁺ or F⁺ complexes (HF⁺, etc.). An electron stimulated desorption cross-section of σ⁺ ∼ 1.4 × 10⁻²⁰ cm² was determined for H₃O⁺ from 316L stainless steel for hydrogen residing in surface chromium hydroxide.     

Autoionization resonances in the photoabsorption spectra of Fe<Superscript><Emphasis Type="Italic">n</Emphasis>+</Superscript> iron ions

The photoabsorption cross sections of a neutral iron atom, as well as positive Fe⁺ and Fe²⁺ ions, are calculated in the relativistic random-phase approximation with exchange in the energy range 20–160 eV. The wavefunctions of the ground and excited states are calculated in the single-configuration Hartree–Fock–Dirac approximation. The resultant photoabsorption spectra are compared with experimental data and with the results of calculations based on the nonrelativistic spin-polarized version of the random-phase approximation with exchange. Series of autoionization resonance peaks, as well as giant autoionization resonance lines corresponding to discrete transitions 3p → 3d, are clearly observed in the photoabsorption cross sections. The conformity of the positions of calculated peaks of giant autoionization resonances with experimental data is substantially improved by taking into account additionally the correlation electron–electron interaction based on the model of the dynamic polarization potential.     

X-ray photoabsorption in metallic aluminium,polycristalline and amorphous Al2O3

The K photoabsorption spectrum of Al in pure Al, amorphous and γ Al₂o₃ have been studied and compared with corresponding L spectra and with theoretical data. The results show that the Al photoabsorption curve is strongly influenced by the density of the empty allowed states on about 30 eV from the edge. For Al₂O₃, an interpretation of differences observed between K and L spectra is proposed.     

Photoelectron (He(I), He(II) and X-ray) spectroscopy of γ-pyrone and its related sulphur derivatives: valence and core ionization energies and shake-up satellites

The photoelectron He(I) and He(II) spectra of 4H-pyran-4-one (1), 4H-thiopyran-4-one (2), 4H-pyran-4-thione (3) and 4H-thiopyran-4-thione (4) are reported. The assignments are based on experimental evidence, taking into account the results of theoretical calculations. The outermost orbital sequences proposed (X = O, S) are n_X (σ), 3b₁ (π), 1a₂(π) for 1, 2 and 4; and 3b₁(π), n_X(σ) and 1a₂(π) for 3. The shifts of the core-ionization energy values for 1–4 are ascribed to a drift of π-charge from the intracyclic heteroatom towards the carbonyl or thiocarbonyl group. Low-energy shake-up satellites (up to 25% with respect to the main line) are observed in the various energy regions of the XPS spectra. They are qualitatively reproduced by CNDO/2 calculations. The most important satellites derive from the transition in the ion corresponding to the 3b₁ (π) → 4b₁(π*) transition in the UV-visible spectra of the neutral molecules. Charge rearrangements accompanying this transition lead to charge depletion of the core-ionized atom.