Showing papers on "Photoemission spectroscopy published in 1984"

Ground‐State Vibrational Energy Levels of Polyatomic Transient Molecules

Abstract: The experimentally determined ground‐state vibrational energy levels of approximately 480 covalently bonded transient molecules possessing from 3 to 16 atoms are tabulated, together with references to the pertinent literature. The types of measurement surveyed include laser‐based high resolution gas phase infrared absorption and visible‐ultraviolet emission techniques, ultraviolet photoelectron spectroscopy, and matrix isolation spectroscopy. An assessment of the magnitude of the uncertainty of observations in neon, argon, and nitrogen matrices is given.

Chemisorption of hydrogen on the Si(100) surface: Monohydride and dihydride phases

Abstract: We present a systematic study of the clean and chemisorbed-hydrogen-covered Si(100) surfaces. Experimentally, we used He i and He ii ultraviolet photoemission spectroscopy and were able to resolve important chemisorption features which had not been observed previously. We carried out electronic energy calculations using both empirical tight-binding and extended H\"uckel methods, from which we deduce a consistent interpretation of the spectra. It is found that the low-binding-energy surface states resulting from the asymmetric dimer-bond model are in good agreement with the experimental data, whereas one high-binding-energy surface state associated with the displacements of the subsurface atoms does not appear in the spectra. Hydrogen chemisorbing on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface forms a monohydride phase, and removes most of the surface states, except for the dimer bond and some of the backbonding states. Based on the theoretical and experimental results it is suggested that at high coverage the Si---H bonds of the dihydride phase rotate and depart from the tetrahedral directions. The occurrence of this phase on the other surfaces of silicon, and its possible connections with the observed spectral features, are extensively discussed.

An investigation of the consistency of the ionospheric measurements of the photoelectron flux and solar EUV flux

Abstract: This paper demonstrates that there is an inconsistency between ionospheric measurements of the photoelectron flux and the solar EUV flux. The problem which occurs for EUV wavelengths near 250 A or photoelectron energies near 35 eV has serious implications for theoretical photoelectron flux calculations and could help to explain up to 40 percent of the difference between calculated and measured photoelectron fluxes for electron energies below 35 eV. It is also shown that: (1) only photons with wavelengths shorter than 310 A are of importance in producing photoelectrons; (2) above 20 eV, the photoelectron flux is controlled by the production of primary photoelectrons, while below 20 eV the flux is controlled by degraded photoelectrons; (3) the calculated cascading production rate is insensitive to the cross sections used for the calculation; and (4) the photoelectron flux is inversely proportional to the total inelastic cross sections.

Electronic structure of strontium titanate

Abstract: We have employed the techniques of photoelectron spectroscopy (with tunable synchrotron radiation $30\ensuremath{\le}h\ensuremath{ u}\ensuremath{\le}100$ eV) and inverse photoemission spectroscopy at $h\ensuremath{ u}=9.7$ eV to study respectively the occupied and unoccupied electronic states of $n$-type ${\mathrm{SrTiO}}_{3}$ doped with 1 at.% Nb. Doped samples have the advantage that charging effects are avoided and hence a rather accurate determination of peak positions is possible. The total (occupied and empty) experimental density of states agrees with the calculated density of states of Pertosa and Michel-Calendini when their band-gap energy is readjusted. We have no evidence for intrinsic surface states either in the band gap or in the conduction band, as was calculated by Wolfram et al. The admixture of Ti $3d$ states in the O $2p$ valence states is small, as can be concluded from the Ti $3p\ensuremath{\rightarrow}3d$ resonance behavior.

Electron transfer from anthracenes. Comparison of photoionization, charge-transfer excitation and electrochemical oxidation

Abstract: Quantitative measures of electron detachment from anthracene and various meso-substituted derivatives (D) are examined in the gas phase and in solution. The photoelectron spectra show well-resolved...

Thermal nitridation of silicon: An XPS and LEED investigation

Abstract: Two thermal, low pressure nitridation processes are achieved on silicon(111), using two different nitridant gases, and studied in situ by x‐ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The results show two different growth rates but the same evolution of the electronic and surface crystallographic stucture. A correlation is established between XPS and LEED measurements, associating the characteristic ‘‘quadruplet’’ patterns with the presence of intermediate nitrides. On the contrary, such compounds are absent when the LEED displays ‘‘8×8’’ patterns. Complementary results by reflection high energy electron diffraction (RHEED) and scanning electron microscopy (SEM), permit us to conclude that our thickest films, up to 40 A, are stoichiometric Si3N4, poorly crystallized in the β phase, presenting no surface rugosity.

Laser photoelectron spectroscopy of vibrationally relaxed CH−2: A reinvestigation of the singlet–triplet splitting in methylene

Abstract: The photoelectron spectrum of CH−2 has been reinvestigated using a new apparatus that incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Earlier photoelectron studies [J. Chem. Phys. 74, 5460 (1981)] employing gas discharge and sputter ion sources were plagued by hot CH−2 ions whose vibrational population distribution, for reasons not yet explicable, could not be detectably altered by modification of source conditions. In contrast, the spectrum of cooled CH−2 displays a markedly changed vibronic band intensity profile for the CH−2 (2B1) →−e− CH2(3B1) transition. These results enable several previously observed spectral features to be positively identified as vibrational hot bands. The new CH−2 photoelectron spectrum is consistent only with the methylene singlet–triplet splitting of approximately 9 kcal/mol determined by numerous recent experimental and theoretical studies.

Angle‐resolved photoelectron cross section of CF4

Abstract: Partial photoelectron cross sections σ and angular distribution parameters β were obtained for the first five valence orbitals in CF4: 1t1, 4t2, 1e, 3t2, and 4a1, as a function of photon energy from 17 to 70 eV. These data were taken with the aid of angle‐resolved photoelectron spectroscopy and synchrotron radiation. The results were compared with earlier data on CCl4. Substantial differences were found. These are explained partly in terms of the absence of a Cooper minimum with a fluorine compound as opposed to the presence of a Cooper minimum with chlorine compounds and partly in terms of the position of shape resonances. Data on CF4 were also compared with recent calculations of Stephens et al., who used the multiple‐scattering Xα method. Structure in the photoelectron spectrum of CF4 lying on the low energy side of the third band was identified as due to autoionization and evidence is given as to its specific nature.

Photoelectron studies of resonant multiphoton ionization of molecular nitrogen

Abstract: Photoelectron studies of three photon resonant, four photon ionization of N2 via the b 1Πu, v′=0–5, c 1Πu, v′=0, 1, and c′ 1Σ+u, v′=0, 1 levels are presented. A qualitative analysis of the observed photoelectron spectra in terms of homogeneous perturbations among the b 1Πu, c 1Πu, o 1Πu states and among the b′ 1Σ+u, c′ 1Σ+u, and e′ 1Σ+u states is partially successful. However, several of the photoelectron spectra exhibit marked deviations from the expected behavior. These deviations are discussed in terms of more complex interactions among the intermediate levels, and in terms of the dynamics of excited state photoionization. In addition, the N2 a 1Πg, v′=0–5 levels have been observed in two photon resonant, four photon ionization, and the results are compared with the results of earlier two‐photon laser induced fluorescence studies.

X-Ray photoelectron spectroscopy of nitroso compounds: relative ionicity of the closed and open forms

Abstract: Etude de plusieurs composes nitroso dimeres («ouvert» et «ferme») par spectrometrie photoelectronique de rayons X. Comparaison d'un calcul de charge-potentiel par la methode CNDO/2 et d'un calcul de «chaleur de formation» par la methode MINDO/3 avec les deplacements observes

A photoelectron spectroscopic study of the ground state of CF+ via the ionization process CF+(X 1Σ+)←CF(X 2Π)

Abstract: The CF (X 2Π) radical has been investigated with vacuum ultraviolet photoelectron spectroscopy and ab initio molecular orbital calculations. Only one band was observed and it was assigned to the ionization CF+(X 1Σ+)←CF(X 2Π). For this band, the adiabatic and vertical ionization potentials were measured as (9.11±0.01) and (9.55±0.01) eV, respectively, and analysis of the vibrational structure in this band allowed we and re for CF+(X 1Σ+) to be determined as (1840±30) cm−1 and (1.148±0.005) A. Comparison is made with other first row diatomic fluorides that have also been studied with PES.

Electronic structure of yttrium hydride studied by X-ray photoemission spectroscopy

Abstract: The core levels and valence bands of Y, YH2.1, and YH3 have been studied using X-ray photoemission spectroscopy. For YH2.1 and YH3, hydrogen-induced states appear at binding energies of about 6 eV. YH2.1 shows emission at the Fermi level and the core-level spectra show highly asymmetric lineshape with shoulders at the high-binding-energy side of the main peaks. For YH3, emission at the Fermi level is weak and the core-levels are nearly symmetric, consistent with a semiconductor or a semimetal. The main peaks of the core levels shift to higher binding energies from Y to YH2.1 to YH3. Taking into account different core-hole screening mechanisms in these materials, however, a charge transfer from Y to H is evident in YH2, but no clear further Y to H charge transfer is seen in going from YH2 to YH3. Importance of the Y 5sp-H 1s bond is suggested from the valence-band spectra.

Synchrotron photoemission investigation: Fluorine on silicon surfaces

Abstract: High resolution core level photoemission spectroscopy has been used to obtain the first direct identification of the chemical species remaining on silicon surfaces after exposure to fluorine. Both Si(111) 2×1 and Si(111) 7×7 were exposed to fluorine via the dissociative chemisorption of XeF2. For fluorine coverages in the monolayer regime, SiF1, SiF2, and SiF3 were all present although their relative abundance varied significantly between the two surfaces. No evidence for the existence of unreacted interestitial fluorine was found. These results suggest the need for modification of current models describing plasma and reactive ion etching of silicon.

X-ray photoemission of polynuclear aromatic carbon

Abstract: The evolution of the C(1s) x‐ray photoelectron spectrum is examined as a function of the size of the polynuclear aromatic molecules. The transition from a symmetric line shape to an asymmetric one occurs in the narrow range of the layer size between 10 and 22 A. It is shown that the asymmetry reflects the area of the layer over which the π electrons can be delocalized and that it is more closely related to the actual physical size of the layer than measurements from x‐ray diffraction. The latter is illustrated in coal tars which have undergone heat treatment. The effect of the evolution of the line‐shape asymmetry on the determination of the fraction of polynuclear aromatic carbon in heterogeneous carbon materials by x‐ray photoelectron spectroscopy is also discussed.

Photoelectron spectroscopy of the valence electronic structure of polymers

Abstract: Photoelectron spectroscopy has been highly developed as an investigative tool of the bulk and surface chemical and electronic structure of condensed matter during the past 2 decades. The use of soft X-ray sources led to the development of electroq spectroscopy for chemical applications (ESCA),1,2 which is also known to the physicst as X-ray photoelectron spectroscopy (XPS). These developments were followed by the use of ultraviolet sources for the study of gases3 and the bulk and surface electronic structure of solids.4,5 Recently, the use of the continuous spectral distribution of synchrotron radiation has had a major impact upon the study of solid surfaces6,7 and has made the largely historical terminology of XPS (or ESCA) and ultraviolet photoelectron spectroscopy (UPS) somewhat less meaningful. Angle-resolved UPS and XPS (ARUPS and ARXPS) can provide additional information about electronic band structure, surface states, and adsorbates, but are typically applied to the study of the surfaces o...

Photoelectron Angular Distribution and Assignment of Photoelectron Spectrum of Ozone

Abstract: For the first three ionized states of ozone the ordering of 2A1, 2B2 and 2A2 states has so far been controversial both experimentally and theoretically. The present work establishes assignment of the 2A2 state by using the characteristic of electron energy dependence of an asymmetry parameter (β) of photoelectron angular distribution measured with NeI, HeI, and NeII resonance radiation. From measurements of photoelectron spectra of ozone with NeI, HeI, and HeII radiation, we newly find that two bands centered at 16.50 and 17.6 eV exhibit vibrational structures with the spacings of about 850 and about 1000 cm−1, respectively. Furthermore abnormally weak bands are also confirmed at 22.7, 24.1, and 26.8 eV in the Hell spectrum.

On the electronic structure of the semiconducting compounds Mg3Bi2 and Mg3Sb2

Abstract: Bulk samples of the intermetallic compounds Mg3Bi2 and Mg3Sb2 have been investigated experimentally using soft X-ray emission and X-ray photoemission spectroscopy. The electronic band structure has been calculated using an extended Huckel technique. The results are discussed and critically compared with existing data on electrical conductivity and soft X-ray absorption.

Photoelectron spectroscopy of isomeric C4H7 radicals. Implications for the thermochemistry and structures of the radicals and their corresponding carbonium ions

Abstract: The photoelectron spectra of the C/sub 4/H/sub 7//sup +/ radicals provide structural and thermochemical information about several of the gas-phase isomers. The first photoelectron bands of 1-methylallyl, 2-methylallyl, allylcarbinyl, and cyclobutyl radicals have been obtained. Analysis of the data indicates that these isomers have structures corresponding most closely to slightly puckered cations. In addition, thermal decomposition and isomerization products of 1- and 2-methylallyl, allylcarbinyl, cyclobutyl, and cyclopropylcarbinyl radicals are identified in the photoelectron spectra. The cyclopropylcarbinyl radical itself is not observed, due to its facile ring-opening isomerization to allylcarbinyl radical.

Photoemission studies of atomic redistribution at gold–silicon and aluminum–silicon interfaces

Abstract: We have used soft x‐ray photoemission spectroscopy (SXPS) to monitor the rearrangement of Si and metal atoms during the initial stages of Au or Al interface formation with UHV‐cleaved (111) or (100) Si surfaces. From Si 2p core level spectra as a function of metal overlayer thickness and as a function of incident photon energy, we obtain evidence for strong Si bond changes at submonolayer Au coverages but only weak Al–Si interactions. Marker experiments reveal that Au diffuses into Si (Si diffuses into Al) with the first few deposited metal overlayers. We examine our new information on the interface evolution at room and elevated temperatures in relation to the corresponding bulk phase diagrams.