Showing papers on "Photoemission spectroscopy published in 1978"

Photoemission study of Au Schottky-barrier formation on GaSb, GaAs, and InP using synchrotron radiation

Abstract: Photoemission spectroscopy, constant-final-state spectroscopy, and ion-depth profiling techniques were applied to the study of the formation of Au Schottky barrier on cleaved GaSb, GaAs, and InP. It is found that the deposited Au interacts strongly with the semiconductors, causing decomposition of their surfaces. Further, the Fermi-level pinning is nearly complete at < 0.2-monolayer Au coverage, when the Au is still ''atomiclike.'' It is suggested that defect states at the interface are responsible for the Schottky-barrier pinning, and a mechanism for their creation is proposed. It appears that many of the known phenomena on Schottky barriers can be explained using a ''defect'' model proposed here.

X-ray photoemission study of Ce-pnictides

Abstract: All the Ce pnictides crys allize in the NaCl structure with lattice constant increasing systematically from CeN to CeBi. Since Ce has the largest $4f$-orbital radius in the entire lanthanide series, it is quite interesting to follow the evolution of his level as the distance between the Ce atoms is varied. The core levels and the valence-band region of CeN, CeP, CeAs, and CeSb have been studied by high-resolution x-ray photoemission spectroscopy. The $4d$ and $3d$ core-level spectra of Ce show clearly that the $4f$ state remains strictly localized in all compounds. In CeN a superposition of lines corresponding to two valences is found which reveals the intermediate valence character of this compound. In the valence-band spectrum, the $4f$ level is found to be pinned at the Fermi level and superimposed on the $5d$ states. The extended valence states originating from the anion $p$ states are located at higher binding energies. In the compounds of heavier rare-earth elements the $4f$ state moves away from the Fermi level and gradually overlaps in energy with $p$ states, so that the probability for interatomic Auger processes in the photoemission final state increases rapidly. The natural width of the $4f$ peak is already 0.8 eV in CeP, and becomes so large in CeAs and CeSb that this line can no longer be unambiguously identified in the other valence-band structure. Finally, it can be concluded from the similarity of the $4d$ spectrum of metallic Ce and CeN that the $4f$ electron remains localized in the $\ensuremath{\alpha}$ phase of this metal.

Photoelectron spectrum of Xe2 and potential energy curves for Xe+2

Abstract: The photoelectron spectrum of the Xe2 van der Waals molecule was determined at 584 A with a resolution of 20 meV, using an apparatus which combines a supersonic molecular beamsource with a hemispherical photoelectron spectrometer. This experiment gives new information on the potential energy curve for the bound Xe+ 2ground state at large internuclear separation. An optimum potential was determined for this state by fitting the observed photoelectron peak to a calculated Franck–Condon distribution. The calculations show specifically that the Morse function is not an adequate approximation to the potential at large internuclear distances. This experiment also gives new information on the potential energy curves for the weakly bound Xe+ 2excited states in the region of their potential minima, thus complementing Xe–Xe+ scattering experiments which probe these potentials in the region of their repulsive walls. Potential energy curves for all of the Xe+ 2 states dissociating to Xe(1 S 0)+Xe+(2 P 3/2 or 2 P 1/2) are constructed primarily from experimental results and are compared with recent theoretical calculations by Wadt. Photoelectron spectra of heavier xenon clusters were also determined and are discussed briefly.

Photoelectron spectra of acetylene with HeI, HeII, Zr Mζ, and Mg Kα radiation sources

Abstract: Valence level photoelectron spectra of gaseous acetylene excited by HeI (21.217 eV), HeII (40.8 eV), Zr Mζ (151.4 eV) with Mg Kα (1253.6 eV) radiations have been obtained. With one exception, the 3σg band under Mg Kα excitation, all of the expected valence level excitations are observed in each case. The best measured orbital ionization potentials are 1πu (11.41 eV), 3σg (16.76 eV), 2σu (18.71 eV), and 2σg (23.65 eV). All values except that for 1πu are adiabatic values. Relative photoionization probabilities (cross sections) have been determined for each of the radiations and compared to those estimated theoretically. The low calculated cross sections for 3σg excitation with Mg Kα radiation provides an explanation for the lack of an observable band due to this ionization in the photoelectron spectrum. A prominent ’’shake‐up’’ peak is observed at 27.6 eV in both the Mg Kα and Zr Mζ excited valence shell photoelectron spectra although it is more pronounced in the former. Similarly the C1s core spectrum show...

The vacuum ultra-violet photoelectron spectrum of the SiO(X1Σ+) molecule

Abstract: The vacuum ultra-violet photoelectron spectrum of the SiO(X1Σ+) molecule has been recorded and interpreted with the aid of both ab initio molecular orbital and multiple-scattering Xα calculations. Three bands have been observed with vertical ionization potentials of 11·61, 12·19 and 14·80 eV, corresponding to ionization to the X2Σ+, A2Π and B2Σ+ states of SiO+ respectively. Estimates of r e, and D e for these ionic states have been made.

Photoionization processes at threshold. II. Threshold photoelectron, photoionization, and coincidence ion‐threshold photoelectron spectra of BF3

Abstract: Threshold photoelectron spectra and photoionization efficiency curves have been measured for BF3 from the onset of ionization to 19.7 eV. This range includes four electronic states of BF+3. Vertical ionization energies of 15.96, 17.12, and 19.181 eV are observed for the first, third, and fourth states, respectively. The relative ionization cross sections for production of these states agree with those previously observed in the HeI photoelectron spectrum, but the cross section for ionization in the vicinity of the second state is considerably larger in the threshold photoelectron spectrum. The vibrational structure of the neutral BF3 molecule is not observed in either the first or third electronic states of BF3+, but the symmetric stretching mode progression appears in the band for the fourth state. It appears likely that production of each of the three lowest states of the ion involves a change in molecular geometry with respect to the ground state of the neutral. A breakdown curve was obtained between i...

The electronic structure of magnesium dialuminium tetraoxide (spinel) using X-ray emission and X-ray photoelectron spectroscopies

Abstract: The Mg-Kβ1,3, O-Kα1,2, and Al-L2,3X-ray emission spectra and also the X-ray photoelectron spectrum of spinet, MgAl2O4, are reported. A model for the bonding is derived using simple molecular-orbital theory, which is adequate for a qualitative rationalisation of the observed spectra. The shape of particular X-ray emission peaks is shown to be strongly dependent on the bonding commitments and co-ordination of the emitting atoms.

Atomic and electronic structure of semiconductor surfaces

Abstract: The electronic structure of surfaces, which can be determined by various electron/ion spectroscopies including ultraviolet photoemission spectroscopy (UPS), is dependent on the surface atomic geometry. Thus, it is possible to determine the surface atomic structure via structure‐dependent theoretical analyses of these experimental spectra. Because of the complexity of such systems, a simple but accurate theoretical scheme is required. We show that the valence electronic states and atomic structures of semiconductor surfaces—both clean and with chemisorbed species as well as relaxed and/or reconstructed—can be determined using a tight‐binding model. Such calculations together with UPS data show that the annealed Si(111)7×7 surface contains about 25% vacancies. For GaAs(110), we have found tha the Ga atoms move into, and As atoms move out of the surface with small bond length distortions so that a plane through them makes an angle of about 20° with the surface. We show that H chemisorbed on Si(111) has two d...

Multiple-scattering approach to angle-resolved photoemission

Abstract: A Green's-function formulation of angle-resolved ultraviolet photoemission spectroscopy is presented for surfaces with overlayer adsorbed systems. Multiple scatterings of the initial state are included through the use of cluster wave functions solved from a self-consistent $X\ensuremath{\alpha}$ scattered-wave method. The final-state process is expressed in terms of a scattering $T$ matrix which propagates the final-state electron through surface layers of the crystal lattice, in the presence of inelastic damping. It is found that major angular asymmetries in the photoemission intensity profiles arise from multiple scatterings off neighboring ion cores. Our formulation, in the limit of taking a single wave function from a single ion-core, reduces to the photoemission theory of a core level.

Configuration interaction calculation of the O2 + ion and study of the photoelectron spectra of O2

Abstract: Ab initio calculations within the minimal basis of Slater-type orbitals are performed on the O2 + ion. Full-valence configuration interaction is carried out at seven internuclear separations between 1·0 A and 2·0 A. For eight observed bound states, the calculated ordering agrees with experiment, and the root mean square of the differences between the eight calculated and observed vertical excitation energies is 0·45 eV. An analysis of the intensity relations in the photoelectron spectrum of O2 for ionization of an electron from the 2σ u or 1π u molecular orbitals is presented. This leads to a better understanding of some characteristics of these bands and to a new assignment of a few observed peaks.

The x-ray photoelectron spectrum of atomic sodium

Abstract: The Na(1s) binding energy in atomic sodium has been determined by x‐ray photoelectron spectroscopy to be 1079.1(±0.1) eV. The spectrum exhibits a satellite 8.4 eV to the high binding energy side of the primary peak with a relative intensity of 19(±1) %. It is assigned to a 3s→4s excitation accompanying ionization. Ab initio ’’ΔSCF’’ calculations are in good agreement with experiment for both the separation and intensity of the satellite. The unusually large intensity of the shakeup peak compared to those found in the inert gases is discussed. Theoretical Na(1s) binding energies in the series Na, Na2, and Na3, suggest that ∼90% of the binding energy shift between the free atom and the metal (5.1 eV) is due to the extraatomic relaxation in the metal.

Interelectronic-state perturbation effects on photoelectron spectra and emission quantum yields

Abstract: Model calculations are carried out on the photoelectron and ion emission spectra of a hypothetical diatomic molecule in the absence and presence of intramolecular coupling between two emitting excited states B + and A + of the ion. We show that interelectronic state coupling has relatively little influence on the frequency distribution of the photoelectron spectrum while modifying considerably the optical emission spectrum. Shifts of intensity from apparent B + to apparent A + emission spectral regions are shown to occur. This result explains the observation of apparent state quantum yields different from unity for emission from [Btilde] 2Σ u + and A 2Π u states of CO2 +, as well as the discrepancy between apparent populations of these two excited states inferred from photoelectron and emission spectral intensities.

Evidence for a new type of metal-semiconductor interaction on GaSb

Abstract: The formation of Au-semiconductor Schottky barriers is studied using soft-x-ray photoemission spectroscopy. The Au-GaSb interface is highly nonstoichiometric due to selective removal of Sb. This nonstoichiometry results in acceptorlike states which pin the Fermi level at the interface and determine the Schottky barrier height.

Correlation satellites in the outer-shell photoelectron spectrum of argon

Abstract: The authors present satellite photoelectron spectra from the outer shell of argon whose binding energies range from 32 to 43 eV, using synchrotron radiation in the 40-80 eV photon energy region. It has been possible to obtain satellite spectra much better resolved than those previously obtained by Spears et al. (1974) as well as to observe new satellite lines appearing only at low photon energy. An assignment is proposed for all the satellites observed by the authors. These experimental results show that the 3s3p6 2S to 3s23p4ns, nd 2S transfer has been fairly well accounted for by the theoreticians, but they also show clearly that the satellite spectra following photoionisation in the outer shell of argon arise from both the 3s and 3p subshells and that the final-ionic-state configuration-interaction mechanism describes only part of the spectrum.

Satellite lines in the 5s-5p photoelectron spectrum of xenon

Abstract: ACO synchrotron radiation has been used to obtain photoelectron spectra of the outer shell of xenon in the 75-100 electron volts energy range. Satellite peaks corresponding to the formation of excited ionic states have been observed, in addition to the main peaks corresponding to the production of the 5s 2 5p5 2P and 5s 5p6 2S states of Xe+. A comparison is presented of the intensities of the satellites relative to the main lines for different energies of the incident radiation. There are several ways to excite satellite lines, via core rearrangement, virtual super Coster-Kronig processes, ground state correlation and final state cattering processes involving the primary photoelectron. In this paper we present theoretical calculations involving one particular process, namely virtual Coster-Kronig process, which gives good agreement with experimental 5s binding energy and gives the correct order of magnitude for the relative intensities of the satellites corresponding to the 5s2 Sp4 5d 2S and 5s2 5p4 6d 2S final states.