Showing papers on "Photoemission spectroscopy published in 1976"

Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 7.—The methyl radical

Abstract: The HeI vacuum ultraviolet photoelectron spectrum of the methyl radical produced by pyrolysis of azomethane has been investigated Three ionization potentials, corresponding to the formation of the 1A′1, 3E′ and 1E′ ionic states, have been observed with vertical ionization potentials of 984, 1476 and 1610 eV respectively The observed band positions have been interpreted with the aid of both ab initio and semiempirical molecular orbital calculationsThe band associated with the first ionization potential is the only one for which vibrational structure could be resolved This has been investigated in both CH3 and CD3 using Hel and NeI radiation From the Franck–Condon envelope of this band and the observed shifts on deuteration, it is concluded that the methyl radical is essentially planar in its ground electronic state The factors which control the values of the out-of-plane deformation frequencies in the ground state of CH3 and CH+3 are discussed in detail Jahn–Teller splitting has been detected in the band associated with the second ionization potential of CH3

Surface spectroscopy of Schottky-barrier formation on Si(111) 7 × 7: Photoemission studies of filled surface states and band bending

Abstract: The formation of Schottky barriers between Si(111) 7 \ifmmode\times\else\texttimes\fi{} 7 and group-III metals has been studied by ultraviolet photoemission spectroscopy Modifications of the band bending and of the work function occur for low values of metal coverage (one to four monolayers) The "intrinsic," clean surface states are simultaneously replaced with "extrinsic" metal-related interface states A two-step empirical model is proposed for the formation of the barrier The first step is saturation of interface bonds, and the second step, critical in determining the junction properties, corresponds to the formation of a thin region with properties intermediate between that of a metal and of a semiconductor Our experiments emphasize the need for a detailed theoretical treatment of the interface chemical bonds and underline the inadequacy of "macroscopic" models for metal-semiconductor junctions

Electronic band structures and x-ray photoelectron spectra of ZrC, HfC, and TaC

Abstract: The band structures and densities of states (DOSs) of ZrC, HfC, and TaC were calculated by the augmented-plane-wave method, and the x-ray photoelectron spectra of valence bands of these compounds were observed. The theoretical energy distribution curves (EDCs) were in good agreement with the experimental EDCs. These band structures resemble each other and also those of TiC obtained by our previous work. This fact suggests that the rigid-band model is applicable to the transition-metal carbides with the rock-salt structure. Their DOSs are divided into three parts. Peak I derived from the $\mathrm{C} 2s$ state is isolated from the higher valence-band peak II arising from the $\mathrm{C} 2p$ and the valence electrons of the metal atom. Peak III derived from the $d$ and $s$ states of the metal atom is separated by the Fermi level from peak II. The Fermi level lies at the minimum point of the DOS for the group IV carbides, but for TaC it lies at a relatively large DOS point. The DOS at the Fermi level of ZrC, HfC, and TaC are 0.18, 0.16, and 0.65 electrons/(eV primitive cell), respectively. The characteristic mutual differences among these compounds are a stronger localization of $d$ electrons in ZrC and HfC compared with TiC and an enhancement of the photoelectron spectrum intensity of TaC around the Fermi level.

Chemisorption-Site Geometry from Polarized Photoemission - Si(111)Cl and Ge(111)Cl

Abstract: The chemisorption-site geometries for chlorine on cleaved Si(111) 2\ifmmode\times\else\texttimes\fi{}1 and Ge(111) 2\ifmmode\times\else\texttimes\fi{}1 surfaces have been determined by comparing detailed pseudopotential calculations with energy distribution spectra and polarization selection rules of photoemission spectroscopy. Strong $p$-symmetry polarization effects observed in photoemission allow us to determine that on Si(111) the correct geometry is a onefold covalent site while on Ge(111) the Cl is in a threefold coordinated ionic site.

Charge density waves in layered metals observed by X-ray photoemission

Abstract: Temperature dependent X-ray photoemission spectroscopy (ESCA or XPS) studies on the Ta 4f lines in 1T-TaS2, 1T-TaSe2, 1T-Ta0.97Hf0.03S2 and 2H-TaSe2 are reported. Striking effects arising from charge density waves (CDWs) are observed at low temperature. While electron diffraction evaluates the periodicity and direction of the CDWs, these XPS observations allow the different configurations of the CDWs with respect to the crystal lattice to be deduced. The results indicate the existence of CDW fluctuations in commensurate phases, and that in some incommensurate and ‘quasi-commensurate’ phases the CDWs cannot maintain long-range coherence. In 1T-Ta0.97Hf0.03S2 the Hf impurities pin the incommensurate CDWs into domains.

Atomic Photoelectron Spectroscopy, Part I

Abstract: Publisher Summary This chapter explains the theoretical photoionization process and experimental techniques involved in the atomic photoelectron spectroscopy. Recently, photoionization was investigated experimentally almost entirely via photoabsorption measurements, but in the last 15 years direct observations of the ejected photoelectrons (photoelectron spectroscopy) have been made in ever-increasing numbers. The angular distribution of the photoelectrons can also be measured using photoelectron spectroscopy. The intensity of a photoelectron line is proportional to the photoionization cross section of that energy. The simplicity of wave functions consisting of single slater determinants—that is, antisymmetric products of one-electron functions can be maintained while still treating exchange correctly. Recent conference proceedings have also contained a wealth of information on the experimental methods of photoelectron spectroscopy. The proceedings of the International Conference on Electron Spectroscopy held in 1971 and the one held in 1974 contain many papers on all the phases of the experimental situation, particularly on photoelectron analysis and detection.

Study of the N–Cu(100) system

Abstract: The reaction of a clean Cu(100) surface with atomic N has been studied with low energy electron diffraction (LEED), Auger electron spectroscopy, ultraviolet photoemission spectroscopy (UPS), and x‐ray photoemission spectroscopy (XPS). Atomic nitrogen, formed by electron dissociation of N2 forms a c(2×2) overlayer on the Cu(100) surface. An analysis of LEED intensity profiles averaged over constant momentum transfer indicates that the N binds in a fourfold symmetric site, 0.145 nm above the first layer of Cu atoms. UPS data reveal small adsorbate‐induced electronic levels at 1.3 and 5.8 eV below the Fermi level, in addition to rather complex changes in the Cu d band. XPS measurements yield 397.3 eV for the binding energy of the N 1s electrons with respect to the Fermi level. Our UPS and Auger data on Cu(100)/N in terms of N‐induced states is shown to be plausible.

The photoelectron spectra of diazene, diazene‐d2, and trans‐methyldiazene

Abstract: The photoelectron spectra of the transient species diazene, N2H2; diazene‐d2, N2D2; and trans‐methyldiazene, MeNNH have been recorded and interpreted. Four ionization potentials in the HeI energy range (21.2 eV) have been detected in the case of N2H2, where the orbital energies are found to follow the sequence n+, π, n−, and σ. N2H2 exhibits a highly structured photoelectron spectrum which has been fully resolved and analyzed in conjunction with its deutero analog. Several features of the spectrum are discussed, in particular the magnitude of the n+/n− separation, which is the largest yet observed, a comparison with N2F2 and the perfluoro effect, and the position of the π ionization. Our results also suggest that the 1700 A band in the absorption spectrum of N2H2 is a n+→3p Rydberg transition. In addition, using the experimental Franck–Condon factors, we have deduced the geometry of the N2H2 and N2D2 ground state ions and compared the photoelectron spectra with the isoelectronic molecules B2H6, C2H4, CH2N...

Photoelectron spectroscopic investigation of the adsorption and catalytic decomposition of formic acid by copper, nickel and gold

Abstract: The interaction of formic acid with polycrystalline copper, nickel and gold surfaces (and with a Cu(100) single crystal) in the temperature range 80‒500K has been studied by photoelectron spectroscopy. Between 80 and 150K formic acid forms multilayers and the ultraviolet photoelectron spectrum (u. p. s.) is rather similar to that of formic acid vapour. There are, however, small but significant differences in the u. p. s. and also in the X-ray induced photoelectron spectrum (X. p. s.). These differences are shown to be a consequence of hydrogen bonding in the condensed phase. Changes in the spectra with increasing temperature are interpreted as reflecting molecular transformation to the formate ion. This is shown to involve the following sequence of molecular events:( a ) hydrogen bond cleavage in the condensed phase, ( b ) reorientation of the molecule from being parallel to being perpendicular to the plane of the surface and ( c ) removal of a proton. These conclusions are based on the observed surface carbon to oxygen ratio, estimated from the O(1s) and C(1s) spectra, O(1s) f. w. h. m. values at different temperatures, the energies of the peaks in the He I and He II spectra and comparison of these with theoretical cal­culations of the energies of the orbitals of the formate ion and the formic acid molecule. Spectroscopic monitoring of the surface species showed that with gold the formate ion decomposes into gaseous products between 180 and 200K; in the case of nickel decomposition occurs slowly in vacuo at 295K but with copper, the surface form ate ion does not break down until above 400K. The following activation energies have been estimated for decom­position of the surface anion: Au (50kJ mol ‒1 ), Ni (90kJ mol ‒1 ) and Cu ( ca . 120kJ mol ‒1 ). The density of states near the Fermi level has been studied and there is no evidence for the participation of an electronic factor in the catalytic decomposition process. In a limited l. e. e. d. study with a Cu(100) surface no evidence for ordered structures of either the adsorbed formic acid molecule or the anion was obtained. We believe this is the first investigation of a heterogeneously catalysed reaction by both u. v. and X-ray electron spectroscopy, and the results are compared with previous infrared and catalytic studies.

PES of atomic and molecular bismuth

Abstract: HeI photoelectron spectra of Bi and Bi2 were recorded by studying bismuth vapor at 750°C. The spectrum of Bi was found consistent with a j–j coupling description of the ground state. However, relativistic effects seemed to have little influence upon the relative photoionization cross sections of the 3P0,1,2 lines. Satellite peaks 1D2 at 11.49 eV, and possibly also 3D1,2 at 17.12 eV and 3F2,3 at 17.47 eV, were observed. They were assigned to admixtures of the configurations [(6s1/2)26p1/2(6p3/2)2]3/2 and [6s26p6d2]3/2 into the main ground‐state configuration [(6s1/2)2(6p1/2)26p3/2]3/2. An autoionization process at the energy of HeIγ radiation (23.74 eV) was suggested by the large intensity of the HeIγ spectrum. The photoelectron spectrum of Bi2 showed three distinguishable bands at 7.53, 8.94, and 9.30 eV corresponding, respectively, to the ionic states 2Πu, 3/2 , 2Πu, 1/2 , and 2Σ+g due to ionization of a (πu6p) or (σg6p) electron. A probable fourth band was observed at 14.87 eV, and was tentatively assig...

HeI photoelectron spectrum of the P2(X1∑+g) molecule

Abstract: The HeI photoelectron spectrum of the P2(X1∑+g) molecule has been recorded and interpreted with the aid of ab initio and semi-empirical molecular orbital calculations. Three ionic states of the diphosphorus molecule are found to be accessible with HeI radiation. Comparison is made between the properties of these states and the corresponding well established ionic states of nitrogen.

Directional uv photoemission from (100) and (110) molybdenum surfaces

Abstract: A study of the (100) and (110) molybdenum surfaces by directional photoemission spectroscopy is presented. Energy distribution spectra formed by photoelectrons emitted normal to the surfaces have been measured for photon energies between 10.2 and 21.2 eV. The results are discussed in terms of calculated band structure within the framework of the ${K}_{\ensuremath{\parallel}}$-conservation assumption. A good agreement is found between the main features of the experimental spectra and the emission expected from the band structure along the corresponding symmetry line in the Brillouin zone, assuming essentially direct-transition and surface-emission processes. Two extra structures are interpreted in terms of surface states or resonances: on the (100) surface, 0.5 eV below ${E}_{F}$; on the (110) surface, 4.5 eV below ${E}_{F}$ in the $s$-$d$ hybridization gap.