Electron Yield Spectra Research Articles

Analytic yield spectra for electrons on H2

Using a modified discrete energy bin method, we calculate equilibrium electron yield spectra produced by 50 eV to 25 keV incident electrons impacting upon H2. Analytic representations of this function and the probability of ionization simply explain the ion yield per energy loss, the approximate constancy of its reciprocal, the energy loss per ion pair, and other types of yields.

Cascade Auger transitions following Cs 4d−15/26p excitation and Cs 4d ionization

Auger electron and partial ion yield spectra of atomic Cs have been measured at the 4d−15/26p excitation resonance and above the 4d ionization threshold using monochromatized synchrotron radiation. The ion production obtained from the electron spectra is compared with the results of the partial ion yield spectra. The experimental findings are compared with the theoretical predictions obtained using the multiconfiguration Dirac–Fock approach and with results for the neighbouring closed-shell atom, Xe.

Inner‐shell photoabsorption spectroscopy of push–pull nitroanilines—Theoretical and experimental studies at N 1s region

AbstractOscillator strengths for N 1s excitation spectra of the gas‐phase aniline and nitrobenzene, and the solid‐phase meta‐ and para‐nitroanilines have been derived from the total electron yield and total ion yield spectra acquired by tunable synchrotron radiation. Ab initio calculations carried out within the improved virtual orbital method and equivalent ionic core virtual orbital model have been used to aid spectral interpretation. The Rydberg‐valency mixture has also been evaluated for all compounds. These results are compared to those acquired with the extended hückel molecular orbital method [Goeta et al., Chem Mater, 2000, 12, 3342]. The building block model has been used to simulate the nitroanilines spectra. The sensitivity of core excitation spectroscopy to intramolecular nitroanilines charge transfer and the N 1s → π* spectral features dependence upon the substitution pattern (meta or para) have been confirmed, as already verified for the gas‐phase inner‐shell energy loss spectra for the same molecules [Turci et al., Can J Chem, 1996, 74, 851]. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Soft x-ray photoreactions of CF3Cl adsorbed on Si(111)-7×7 studied by continuous-time photon-stimulated desorption spectroscopy near F(1s) edge

The continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was employed to monitor the monochromatic soft x-ray-induced reactions of CF3Cl adsorbed on Si(111)-7x7 near the F(1s) edge (681-704 eV). Sequential F+ PSD spectra were measured as a function of photon exposure at the CF3Cl-covered surface (dose=0.3x10(15) molecules/cm2, approximately 0.75 ML). The F+ PSD and total electron yield (TEY) spectra of molecular solid CF3Cl near the F(1s) edge were also measured. Both F+ PSD and TEY spectra show two features at the energy positions of 690.2 and 692.6 eV, and are attributed to the excitations of F(1s) to 11a1[(C-Cl)*] and (8e+12a1)[(C-F)*] antibonding orbitals, respectively. Following Auger decay, two holes are created in the F(2p) lone pair and/or C-F bonding orbitals forming the 2h1e final state which leads to the F+ desorption. This PSD mechanism, which is responsible for the F+ PSD of solid CF3Cl, is employed to interpret the first F+ PSD spectrum in the sequential F+ PSD spectra. The variation of spectrum shapes in the sequential F+ PSD spectra indicates the dissipation of adsorbed CF3Cl molecules and the formation of surface SiF species as a function of photon exposure. From the sequential F+ PSD spectra the photolysis cross section of the adsorbed CF3Cl molecules by photons with varying energy (681-704 eV) is determined to be approximately 1.0x10(-17) cm2.

Phase change observation in reflection multilayers by total electron yield and reflection spectra

Precise optical elements for use in the extreme ultraviolet region are necessary for controlling both the uppermost layer thickness and the reflection phase in a reflection multilayer system. In this study, a phase value corresponding to both the uppermost layer thickness and the reflection phase was derived from the reflectance and total electron yield (TEY) intensity. Mo∕Si multilayers were fabricated on a silicon wafer with various thicknesses of the uppermost Mo layer. Phase values obtained from the reflection and TEY measurements were found to change in accordance with increases in the thickness of the uppermost Mo layer and a concomitant change in the reflection phase.

Probing the Ordering of Semiconducting Fluorene−Thiophene Copolymer Surfaces on Rubbed Polyimide Substrates by Near-Edge X-ray Absorption Fine Structure

The temperature-dependent alignment of semiconducting liquid crystalline fluorene−thiophene copolymer (F8T2) thin film surfaces was investigated using the near-edge X-ray absorption fine structure (NEXAFS) technique. Partial electron yield spectra were recorded over a range of temperatures in order to observe directly the surface orientation as the polymer is heated and cooled through glass, crystal, and liquid crystal phases. In addition, samples annealed under varying processing conditions and quenched to room temperature were analyzed. The NEXAFS data show that (a) in thin F8T2 films at all temperatures the polymer backbone lies in the plane of the substrate, (b) the fluorene and thiophene rings are rotated randomly about the molecular axis, (c) orientation of the polymer backbone can be controlled using a rubbed polyimide alignment layer as a template for liquid crystal orientation, and (d) under proper annealing conditions there is strong temperature-dependent alignment of the copolymer main-chain ax...

How well does total electron yield measure x-ray absorption in nanoparticles?

The possibility to quantitatively analyze total electron yield (TEY) spectra as a measure of the x-ray absorption cross section from supported magnetic nanoparticles is explored by a combination of experiments and Monte Carlo simulations. Unless the particles are very small, it is found that saturation effects are important and can lead to a severe underestimation of the magnetic moments, when the magneto-optical sum rules are straightforwardly applied to experimental data. However, unlike for planar samples the possibility to examine and correct the saturation effects by varying the x-ray angle of incidence is not given, the absorption profiles are largely unaffected in their shape. The Monte Carlo simulations, appropriately conducted for the material under investigation, provide an estimate of the saturation effects occurring in nanoparticle TEY data.

Study of neutral desorption reaction of core-excited PMMA thin film by femtosecond laser ionization

A highly sensitive detection method for neutral photoproducts by combining a femtosecond laser ionization and time-of-flight mass spectrometry has been demonstrated to investigate the photon stimulated desorption (PSD) of neutrals in core-excited poly-methylmethacrylate (PMMA) thin films. A preliminary experiment in gaseous water indicates that ionization process induced by laser irradiation is caused by non-resonant multiphoton absorption in the range of laser intensity used. Core-excited PMMA produces various photodesorbing neutrals in comparison with ionic desorption. Yield spectra of all neutrals indicate similar profiles to total electron yield spectra, while ionic desorption indicates site-selective dependence on the preliminary excitation. This result indicates that PSD of neutrals does not necessarily reflect a characteristic site-selective nature of core excitation, thus the ionic fragmentation process is important to clarify this characteristic nature.

Ar 2p excited states of argon in non-polar media

Ar 2p photoabsorption spectra for Ar solid and Ar mixture with Kr, Xe and N 2 have been measured in order to study Ar 2p excited states in solid phase. Based on difference between partial electron yield and fluorescence yield spectra for solid Ar, Ar 2p 3/2–4s and 2p 3/2–4p excitations on surface are identified. For Ar:Kr and Ar:Xe mixtures, all bands move to lower energy as the Ar concentration decreases. Compared with solid Ar, Ar:N 2 mixtures show small red shift for the Ar 2p 3/2–4s excitation and blue shift for broad bands originating from Ar 2p– nd excitations. The spectral differences between the mixtures are discussed in terms of the polarization stabilization of the ionization threshold and the exchange repulsion of the Rydberg electron by the valence electrons of the neighboring atoms/molecules.

Experimental and Theoretical Joint Study on the Electronic and Structural Properties of Silicon Nanocrystals Embedded in SiO2: active Role of the Interface Region

AbstractThe local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The comparison between total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 nm. The optoelectronic properties show the appearance of localized states not only in the Si-nc core region but also in the modified SiO2 region.

Vacant states of TiO2 with rutile structure and their reflection in different‐type x‐ray absorption spectra

AbstractA tight‐binding linearized muffin‐tin orbital method was applied for the calculation of the densities of states (DOS) for TiO2 with the rutile structure in the wide energy region of the conduction band. The calculated partial DOS of titanium and oxygen were compared with the experimental Ti 2p total electron yield spectrum (TEY), which was measured in the present work, and the Ti 1s x‐ray absorption spectrum, O 1s TEY and bremstrahlung isochromat spectrum taken from the literature. We found that in the energy region higher than ∼6 eV above the bottom of the conduction band the fine structure of experimental absorption spectra follows the distribution of the corresponding partial DOS. In connection with this, one does not need to use any satellite mechanism for an interpretation of these spectra. In the region of vacant Ti 3d states (up to ∼6 eV), correspondence between the experimental and calculated splitting of t2g and eg subbands is observed in all spectra except Ti 2p TEY. As we showed using atomic calculations, this is connected with the Ti 2p53d1 multiplet in the final state of absorption which leads to suppression of the intensity of lower states of the t2g subband in Ti 2p TEY of TiO2. As a result, t2g–eg splitting extracted from Ti 2p3/2 and Ti 2p1/2 TEY is less than in Ti 3d DOS and other experimental spectra (Ti 1s XAS and O 1s TEY) in which an interaction of core level vacancy with photoelectron is absent owing to the delocalized character of valent states. Copyright © 2002 John Wiley & Sons, Ltd.

A core-level photoionization study of furan

We have measured the vibrationally resolved C 1s photoelectron spectrum of the aromatic heterocycle furan (C4H4O). The vibrational profile appears to be dominated by a low-frequency mode, which shows an unusually high degree of excitation for ionization of the C atoms furthest from the oxygen. Our experimental results are compared with calculations, which reveal that several different vibrational modes participate in the vibrational pattern. A well resolved C 1s satellite spectrum at a photon energy of 335 eV has also been measured, and is compared to previous experimental and theoretical results. In a separate experiment the partial electron yield (“absorption”) spectrum has been obtained in the region from just below the C 1s π* resonance up to the respective ionization thresholds. A new high intensity feature has been identified on the high-energy flank of the C 1s−1π(3b1)* resonance. With the aid of resonant Auger spectroscopy we have tentatively assigned this feature to a Rydberg resonance. We also present the C 1s and O 1s Auger spectra of furan.

FRAGMENT-ION DESORPTION FROM SULFUR-CONTAINING AMINO ACIDS BY LOCALIZED CORE-LEVEL EXCITATION

The desorption of fragment ions from sulfur-containing amino acids following the sulfur K-edge photoexcitation has been investigated using monochromatized synchrotron radiation. The samples investigated are pellets of neutral L-cystine, L-cysteine and L-methionine. It was found that only atomic S+ ions are desorbed following the S 1s → σ* resonant excitations, while various kinds of molecular fragment ions containing carbon, nitrogen and oxygen atoms are desorbed by the low-energy electron irradiation. The photon-energy dependencies of the S + yields approximately resemble those of the total electron yields (TEY's). On the other hand, the edge-jump ratios, defined as I on /I off , where I on is the intensity at the core-to-valence resonance energy and I off is that at off-resonance energy, for the S +-yield curves are by more than ten times higher than those of the corresponding TEY curves. The edge-jump ratios and the partial electron yield spectra revealed that the low-energy electrons produced following the Auger decays scarcely contribute to the ion desorption. Also, it was elucidated that the S * desorption is induced by the direct core-to-valence excitation which is followed by the S KLL Auger decay. It was concluded that the core-to-valence resonant photoexcitation and succeeding specific S–S and S–C bond scissions play a major role in the ionic fragmentation in the present amino acids.

The interaction of aromatic molecules with the acid site in zeolite H-Beta as observedthrough a newly discovered Fano resonance at the Al K edge

A new tool to study the interaction of aromatic molecules withthe acid sites in zeolites is described yielding both geometricand electronic information simultaneously. A Fano resonance (ortwo-centre/two-electron transition state) produces acharge-transfer (CT) satellite peak that is observed in the Al Kedge XANES of zeolite H-Beta when aromatic hydrocarbon moleculesare adsorbed on its acid sites. The Fano resonance isexclusively observed in the electron yield spectrum. The Fanoresonance can be recognized as a broad, featureless satellitepeak at a somewhat higher energy than the edge. During the1s-3p transition, a CT occurs from π orbitalson the aromatic molecule to the T2 (3p) orbital on the Al transferring charge from the benzene to the aluminium. Thediffuse character of this latter orbital results in an overlapwith the π* of the aromatics and enables the CT. FEFF8 FMS and DOS calculations on a large clusterrepresenting the zeolite with and without benzene and in the presence and absence of the core hole effect confirm thesatellite interpretation.The Fano resonance satellite is not observed in the fluorescencedetection mode, because its inherent excitation probability issmall, but its intensity is magnified in the electron yielddetection mode because of shake-off of the extra Al valenceelectrons. The data indicate that the adsorbates do not alterthe symmetry of the tetrahedral Al sites as indicated by bothfluorescence and electron yield.The adsorption of 1-hexene on zeolite H-Beta is investigated andonly after a heat treatment of 400 °C is the Fanotransition visible. This indicates that aromatic coke hasformed on the acid site at this temperature. In addition, sometransformation from tetrahedral Al sites into octahedral sites,or at least some geometric distortion of a small amount of tetrahedral Al sites, is now observed in the fluorescencespectrum, which might be induced by the coking process.

Isolated-core excitations in strong electric fields. II. Experimental results in magnesium

We report on the electron yield spectra of doubly excited magnesium atoms in electric fields in the region 50--1600 V/cm. One electron is excited to a stationary Rydberg state around $n=16,$ either a nonmixing state with low angular momentum or a Stark state. Subsequently the second isolated-core electron is excited with a narrow-band nanosecond laser pulse. The isolated-core excitation induces a shake-up which is measured as a function of excitation frequency. An initial low-angular-momentum state leads to a much broader shakeup than an initial Stark state, i.e., many more autoionization peaks are observed for an initial low-angular-momentum state than for an initial Stark state. Full quantum calculations introduced in the preceding paper [F. Robicheaux, preceding paper Phys. Rev. A 61, 033406 (2000)] show excellent agreement with the recorded spectra. We discuss the implications of this experiment for dielectronic recombination and compare the frequency spectra to previously reported time-dependent measurements on magnesium [J.B.M. Warntjes et al., Phys. Rev. Lett. 83, 512 (1999)].

Photolysis of CF3Cl adsorbed on Si(111)(7×7) surface by monochromatic synchrotron radiation

We present an analysis of the photolysis of an adsorbed molecule by monochromatic synchrotron radiation. The system studied was CF3Cl adsorbed on Si(111)(7×7) at 30 K, and the techniques employed were photoemission spectroscopy (PES) and photon-stimulated desorption (PSD). The valence-level photoemission spectra, under various photon exposures, show that the photolysis effect induced by the incident monochromatic synchrotron radiation photons (hν=98 or 110 eV) occurs on this adsorbed gas–solid system. Variations of the F− and F+ PSD ion yields were measured as a function of monochromatic (110 eV) photon exposure at three adsorbate coverages (the lowest dose=0.3×1015, the medium dose=0.8×1015, and the highest dose=2.2×1015 mol/cm2). For the lowest CF3Cl-dosed surface, the photon-exposure dependencies of the F− and F+ yields show the characteristics: (a) at early stages of photolysis, the desorption of F− yields is mainly due to dissociative attachment (DA) and dipolar dissociation (DD) of the adsorbed CF3Cl molecules induced by the photoelectrons emitting from the silicon substrate, while at high photon exposure the F− formation by electron capture of the F+ ion is the dominant mechanism; (b) the F+ ion desorption is associated with the bond breaking of the surface SiF which is formed by reaction of the surface Si atom with the neutral fluorine atom or F− ion produced by scission of the C–F bond of CF3Cl, CF2Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of the lowest CF3Cl-dosed surface. Based on this model and the variation rates of the F−/F+ signals during 110 eV photon bombardment, a photolysis cross section of ∼1.9×10−17 cm2 was determined. The photolysis cross section as a function of incident photon energy in the range 98–110 eV, near Si(2p), was also obtained, which shows a threshold at around 100 eV, corresponding to the same rise as the total electron yield (TEY) spectrum of the clean silicon substrate. This edge-jump indicates an enhancement of the photolysis efficiency by indirect excitation of the substrate core-level [Si(2p)] electrons, and confirms the substrate-mediated excitation as the photolysis mechanism for the lowest-dosed surface. However, for the medium- and the highest-dosed surfaces both the direct photodissociation and the substrate-mediated dissociation of the adsorbed CF3Cl occur at early stages of photolysis, while only the substrate-mediated dissociation was observed at higher photon exposure. For photolysis of all three surfaces the SiF is the only fluorosilyl product observed on the surface.

Partial electron yield spectrum of N 2: doubly excited states at the K-shell threshold

The partial electron yield spectrum of N 2 has been measured at a kinetic energy corresponding to the Auger decay of doubly excited core–hole states. The spectrum reveals previously unresolved double excitations just below and above the K-shell ionization threshold. Their excitation energies and dissociative nature agree with calculated potential energy curves of the doubly excited states.

Site-specific phenomena in Si:2p core-level photoionization of X 3Si(CH 2) nSi(CH 3) 3 (X=F or Cl, n=0–2) condensed on a Si(111) surface

We used photoelectron spectroscopy and the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific phenomena in the Si:2p photoionization of X 3Si(CH 2) n Si(CH 3) 3 (X=F or Cl, n=0–2) condensed on a Si(111) surface. The site-specific excitation and the occurrence of different chemical shifts at two Si sites were revealed in the total electron-yield spectra and the photoelectron spectra of F 3Si(CH 2) n Si(CH 3) 3 ( n=1, 2), although they were not clearly revealed in those of Cl 3SiSi(CH 3) 3. We conclude that these site-specific phenomena are easily observed in molecules in which the two Si sites are located far apart and in which electron migration between the two Si-containing groups does not occur. This was supported by our ab initio calculation. Site-specific fragmentation was revealed in the ESPEPICO spectrum of F 3SiCH 2CH 2Si(CH 3) 3, although it was negligible for Cl 3SiSi(CH 3) 3 and was less remarkable in F 3SiCH 2Si(CH 3) 3 than in F 3SiCH 2CH 2Si(CH 3) 3. Site-specific fragmentation also occurred when the two Si sites were located far apart.

Photon Stimulated Ion Desorption of Polymer Thin Films Following Core Excitation

We have studied the photon stimulated ion desorption of poly-isopropenyl-acetate (PiPAc) and poly-vinyl-acetate (PVAc) thin films from the viewpoint of site-specific bond scission. PiPAc is the structural isomer of PMMA, which shows site-specific bond scission. The structures of PiPAc and PVAc are different in the substitutional group of the side-chain, a methyl group or hydrogen. H+, CH+, CH2+, CH3+ and COCH3+ are mainly desorbed from PiPAc and PVAc thin films following the oxygen core excitation. The near edge X-ray absorption fine structure (NEXAFS) spectra of these thin films are assigned through a comparison to the inner shell electron energy loss spectra of the related molecules. The features of the partial ion yield (YPi) spectra of CH3+ and COCH3+ ions are different from that of the total electron yield (Ye) spectrum. This indicates that the efficiencies of desorption of these ions depend on the primary excitation and these ions are consequently desorbed site-specifically. The most remarkable difference in the YPi is seen in the YPi(CH3+) and YPi(COCH3+) spectra. In YPi(CH3+) and YPi(COCH3+) spectra, the peak is assigned to the O 1s(C=O) → σ*(C-C) with the help of this result.

Simulation Study of Si–L Total Electron Yield Spectra of Some LiF/Si/LiF Multilayer Using Trial Anisotropic Optical Constants

Simulation study has been made to explain the spectral feature of total electron yield (TEY) spectra of some LiF/Si/LiF multilayer around Si-L edge measured previously for various angles of incidence. The existing formula for the total electron yield has been extended. Optical constants used for the direction perpendicular to the Si layer are those of crystalline Si and those for the direction parallel to the layer is trial ones simulating those of porous Si. The simulated TEY spectra reproduced well the experimental results.