Photoelectron spectroscopy of atomic oxygen using the Elettra synchrotron source

Atomic oxygen has been studied using angle resolved photoelectron spectroscopy (PES) and constant-ionic-state (CIS) measurements using radiation from the Elettra synchrotron as the photon source. Relative partial photoionization cross-sections and angular distributions for the O⁺(⁴S) ← O(³P) and O⁺(²D) ← O(³P) ionizations have been measured as a function of photon energy from threshold (13.6 eV) to 19.0 eV. Comparison of the results obtained with recent experimental work performed at lower resolution reveals a number of differences and comparison with results of recent calculations shows the need for the inclusion of coupling intermediate between the j-j and L-S limits in future calculations of photoionization cross-sections and angular distributions. This work has demonstrated the feasibility of and results to be expected from angle resolved PES and CIS measurements on reactive intermediates at Elettra, a third-generation synchrotron source, and further studies on small molecular radicals are proposed.     

Chemisorption of atomic oxygen on silicon surface

We investigated the electronic structure of oxygen adsorbed on silicon surface in a head-on position, and showed that only the corresponding bonding structure is able to explain the features in UPS and ELS spectra that were not interpreted satisfactorily before. We also found that charge is transferred from backbonds to SiO bond and induces empty states localized between the first and second layers of the silicon surface.     

Photoelectron and photoion spectroscopy on atomic Lu in the region of the 5p excitation

The resonances of atomic Lu have been investigated by photoelectron and photoion spectroscopy using monochromatized synchrotron radiation in the vacuum ultraviolet energy region. The total photoion yield has been compared to calculations in which the extended Fano theory (Mies formalism) and the Hartree-Fock method were applied. The resonance structure is dominated by the spin-orbit splitting of the 5p core hole. In the photoion yield spectra of singly and doubly charged ions a high fraction of Lu²⁺ ions has been found in the region of the 5p ^-1 (² P _1/2 )nd resonances. Photoelectron spectra, recorded in this resonance region, have been investigated with respect to deexcitation channels connected with Lu²⁺ ions. The 5p ^-1 (² P _1/2 )nd resonances predominantly autoionize by spin-flip into states, which decay in the second step into Lu²⁺ final ionic states. Received: 2 September 1998 / Accepted: 17 September 1998     

Photoelectron spectroscopy of E′ centers in crystalline and glassy silicon dioxide

Radiation-induced E′ centers in SiO₂ were studied to test the possibility of applying optically stimulated electron emission (OSEE) to the spectroscopy of excited states of point defects in dielectrics. The spectral responses of the OSEE of crystalline α quartz and silica glass irradiated by 10-MeV electrons were measured and studied. It was established that volume E′ centers in the crystalline and glassy SiO₂ modifications are dominant emission-active defects. Surface E’_s (1) centers were also detected in glassy SiO₂. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two nonradiative (intracenter and ionization) relaxation channels is proposed. This model was used to explain the mechanism of photothermal decay of the E′ centers and to determine the ionization activation barriers and quantum yields of these centers. The emission, spectral, and kinetic parameters of the volume and surface E′ centers in glassy SiO₂ were obtained, showing the excited states of these defects to have identical atomic configurations.     

Photoelectron spectroscopy of localized levels near surfaces: Scattering effects and relaxation shifts

We calculate the spectrum of photoelectrons excited from localized levels in solids far into the continuum (XPS situation), in a model that takes into account exactly their interaction with a boson-like spatially non-uniform field. The general results are applied to discuss the spectra of photoelectrons in metals, where the dominant interaction is the coupling to bulk and surface plasmons. In particular, we study the modification of spectral sum rules connecting the strength of inelastic processes with the energy shifts in the spectrum, showing that the electron (“extrinsic”) scattering changes the average position of the spectrum with respect to the pure “intrinsic” result in the “sudden approximation”, which leads also to its dependence on the excitation energy and position of localized level. We estimate these deviations in a simple model which nevertheless reproduces correctly the dynamical aspects of the problem.     

Photoelectron spectroscopy from CO adsorbed on a Cu(111) surface

He II-ultraviolet photoelectron spectra (hv = 40.8 eV) from a carbon monoxide layer adsorbed on a Cu(111) surface exhibit two peaks at 8.5 and 11.6 eV below the Fermi level and a weaker maximum centered at about 13.5 eV. The emission from the Cu d-band is markedly suppressed after adsorption. The results are discussed in terms of the recent models for assigning the UPS peaks observed after adsorption of CO on transtion metals     

Photoelectron spectroscopy of azulenes

The photoelectron spectra of azulene and ten of its derivatives (halo-, methyl-, phenyl-, acetyl- and benzoylazulenes) are presented and assigned. Assignments based on a composite-molecule view are validated by computations (CNDO/S). Conjugative interactions in the phenylazulenes lead to an inversion of the usual order of the two phenyl group e_1g component orbitals. Many of the azulenes have very low ionization energies, and it is suggested that they constitute a class of excellent π-electron donor molecules.     

Photoelectron spin-polarization spectroscopy: A new method in adsorbate physics

Energy-, angle-, and spin-resolved photoemission results from adsorbates are discussed. The measurements employed circularly polarized synchrotron radiation from BESSY, a special photoelectron spectrometer, and a Mott detector for electron spin-polarization (ESP) analysis. It is demonstrated by the example of physisorbed Xe, Kr, and Ar atoms how this method provides independent information complementary to intensity spectroscopy. Almost complete ESP parallel or antiparallel to the photon spin measured at certain conditions allowed a direct assignment of symmetries of electronic states. Sharp photon-inducednp n's resonances (E<80 meV) have been observed, which show only small relaxation shifts compared to the gas phase. The ESP provided an unambiguous quantum-number labelling of the transitions and revealed an additional line splitting due to symmetry lowering on the surface. Photo-ESP spectroscopy is further demonstrated as to increase spectral resolution, to gain an insight into dynamical relaxation mechanisms of electronically excited adsorbates, and to be a sensitive probe of the local symmetry. A detailed discussion is given of line positions and ESP when going from a commensurate overlayer via an incommensurate monolayer and multilayers of different thickness onto a 3D rare-gas crystal.     

Oxidation of silicon surface with atomic oxygen radical anions

The surface oxidation of silicon （Si） wafers by atomic oxygen radical anions （O- anions） and the preparation of metal-oxide-semiconductor （MOS） capacitors on the O-oxidized Si substrates have been examined for the first time. The O- anions are generated from a recently developed O- storage-emission material of [Ca24Al2sO64]^4＋·4O^- （Cl2A7-O^- for short）. After it has been irradiated by an O- anion bean： （0.5 μA/cm^2） at 300℃ for 1-10 hours, the Si wafer achieves an oxide layer with a thickness ranging from 8 to 32 nm. X-ray photoelectron spectroscopy （XPS） results reveal that the oxide layer is of a mixture of SiO2, Si2 O3, and Si2O distributed in different oxidation depths. The features of the MOS capacitor of 〈Al electrode/SiOx/Si〉 are investigated by measuring capacitance-voltage （C - V） and current-voltage （I - V） curves. The oxide charge density is about 6.0 × 10^1 cm^-2 derived from the （C - V curves. The leakage current density is in the order of 10^-6 A/cm^2 below 4 MV/cm, obtained from the I - V curves. The O- anions formed by present method would have potential applications to the oxidation and the surface-modification of materials together with the preparation of semiconductor devices.     

Correlation corrections to the energies and wavefunctions of atomic core levels

We present the results of a calculation of correlation corrections for inner one-electron atomic states, including the dependence of the correlation corrections on the number of outer electrons. Our method also yields the correlation corrections to the wavefunctions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 95–99, November, 1976.     

Photoelectron spectroscopy of 9,10-dihaloanthracenes

The photoelectron spectra of anthracene and its 9,10-dichloro- and dibromo-derivatives have been obtained. Analysis of the spectra is based mainly on the shifts in the anthracene molecular orbital ionization energies observed upon substitution. These shifts are interpreted on the basis of a perturbation model comprising short-range and long-range inductive terms and a mesomeric term. The electronic perturbation caused by the halogen atom is observed to depend largely on the unperturbed energy of the halogen np electrons and the charge distribution in the anthracene molecular orbitals. It has been possible to separate out Coulombic interactions and mesomeric interactions which depend on the energy gap between hydrocarbon and substituent orbitals. Although the observations are not conclusive, owing to the fact that only two orbitals have measurable mesomeric shifts, results indicate that α_Cl′, the Coulombic perturbation parameter, may well be negative, in contrast to previous work.     

Photoelectron spectroscopy of liquids: External ionization energies

A method is reported here for the determination of ionization energies of liquids or solutions by means of photoelectron spectroscopy (PES). It is, to our knowledge, the first determination of its kind in liquid-phase ultraviolet PES. A number of applications is foreseen.     

Photoelectron spectroscopy of sulfur heterocycles adsorbed onto a Cu(110) surface

Photoelectron spectra of Cu(110) after exposure to thiacyclobutane, thiacyclopentane, and thiacyclohexane, as well as of the clean Cu(110) surface, have been obtained using He(I) radiation. Comparison of the spectra of the absorbates with gas-phase spectra suggests a strong sulfur-to-copper dative bond. Sulfur lone-pair orbital stabilization energies were obtained for thiacyclobutane (1.4 eV), thiacyclopentane (1.1 eV), and thiacyclohexane (0.6 eV), and these stabilization energies are correlated with the dative sulfur-to-surface bond energies. Ring-strain changes are suggested as the reason for additional small shifts of absorbate-bond energies in the thiacyclobutane spectrum. Relaxation energies were found to decrease in the order 4- > 5- > 6-membered ring. An empirical linear relation was found between relaxation shifts and lone-pair orbital stabilization energies for monoheteroatom heterocycles bonded to Cu(110) by a lone-pair dative bond. All adsorbates studied showed marked angular dependence, suggesting nonrandom orientation on the surface. Al Kα X-ray data were used to calculate percentage surface coverages. Other sulfur-containing adsorbates are discussed, including thiophene, allyl mercaptan, and thiacyclopropane. Thiacyclopropane was found to decompose upon exposure to copper.     

Photoelectron spectroscopy of phosphites and phosphates

The ultraviolet photoelectron spectra (UPS) of trimethyl and triethyl phosphite, trimethyl and triethyl phosphate and four substituted phosphates are presented. Assignments are based on analogies to the UPS of phosphorus trichloride and phosphoryl trichloride and are substantiated by CNDO/2 computations. The mechanism of PO (axial) bond formation is discussed.     

Photoelectron spectroscopy of some biological molecules

The ultraviolet photoelectron spectra (UPS) of the following biologically-active compounds are reported and/or assigned: 2,4-dinitrophenol, nicotinic acid, nicotinamide, barbituric acid, xanthine, hypoxanthine, uric acid, uracil, thymine, cytosine, adenine, guanine, β-carotene, menadione, purine and pyrimidine. The importance of UPS data is exemplified in two ways: First, by investigating the validity of the Pullman k-index approach ¹⁴ to ionization energies; and, second, by generating an experimental scale of electron-donating ability.     

Photoelectron spectroscopy of doped helium nanodroplets

The photoionization dynamics of aniline doped helium droplets has been investigated by photoelectron spectroscopy. The photoelectron spectra resemble closely that of gas phase aniline, except for a droplet-size-dependent shift. This shift is caused by lowering of the ionization threshold upon solvation and can be readily estimated. The individual peaks in the photoelectron spectrum are broadened towards lower kinetic energy which is attributed to the relaxation of the photoelectrons as they pass through the helium droplet.