G. J. Lapeyre

Multiple vibrational excitations of H2O and D2O on Si(100)(2 × 1): A HREELS study

Abstract The HREELS (high-resolution electron energy-loss spectroscopy) spectra from H2O and D2O chemisorbed on the clean Si(100)(2 × 1) surface have been investigated in the energy region between about 500 and 1400 meV. For the first time, second overtones have been observed for a chemisorbed species. This observation is believed to be due to the enhancement of the O-H stretching fundamental and overtone modes because of the temporary formation of a negative ion that occurs when the incident electron is briefly trapped in a shape resonance. Additional multiple and combination modes are observed involving the resonance enhanced overtones as well as the fundamental modes. Furthermore, the dissociation energy of the hydroxyl group has been determined and found to be much larger than the actual energy needed to dissociate hydroxyl groups on the surface, indicating that the driving force for the observed atomic rearrangements is probably Si-O bond formation rather than O-H dissociation.

New experimental band width for Ni

Abstract An occupied d -band width of about 4.3 eV is observed in angle-resolved photoemission which agrees well with band calculations and is much larger than earlier reported measurements. The emission azimuthal dependence suggests why the width is not observed in angle-integrated data.

Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth doped optical materials

Abstract Relative energies of 4f n electronic states and crystal band states are important for a fundamental understanding of rare-earth-doped optical materials and a practical understanding of each materials potential performance in specific applications. With this motivation, the 4f n ground state binding energies of rare earth ions have been studied in the gallium garnets using resonant photoemission spectroscopy and compared with the aluminum and iron garnets. The 4d–4f photoemission resonance was used to separate and identify the 4f n and valence band components of the spectra, and theoretical 4f photoemission spectra were fit to experimental results to accurately determine electron binding energies. A two-parameter empirical model was used to successfully describe the relative energies of the 4f n ground states in these materials. The success of this empirical model indicates that measurements on as few as two different rare earth ions in a host are sufficient to predict the energies of all rare earth ions in that host. This analysis shows that systematic shifts in the relative energies of 4f n states and crystal band states between different garnets arise entirely from shifts of the band states, while each rare earth ion maintains the same absolute binding energy for all garnets studied. These results suggest that further studies of additional host compounds using both photoemission and optical spectroscopy will rapidly lead to a broader picture of the host crystals effect on 4f electron binding energies.

Photoemission studies of core exciton decay in KI

Abstract The decay of K + 3 p -core excitons in KI was studied by using new methods for obtaining photoemission spectroscopy data with synchrotron radiation. The Auger and non-radiative direct-recombination decay processes are separately identified in the data and a measure for the Auger energy distribution is obtained.

Initial state symmetries from polarization effects in angular resolved photoemission

Abstract Polarization dependences are present in all angle-resolved photo-emission spectra. When the spectra are obtained with a polarized source such as synchrotron radiation, the effects are manifest. The effects make it possible to use polarization-dependent angle-resolved photoemission UV spectroscopy (PARUPS) to infer initial state symmetry of the electronic states being excited. The ability to measure the initial state symmetry is a newly developed feature of photoemission whose importance parallels that of measuring the surface component of the momentum. The symmetry rules are obtained by examining the symmetry properties of the dipole matrix element in combination with the general result that J. Hermanson has shown for the symmetry of the final states observed in photoemission, i.e. the photoemission experiment only detects even final states when one observes normal emission or emission in a mirror plane. An overview of several applications of these measurement procedures is given for surface state systems and chemisorbed systems. First one wants to measure the initial state symmetry to obtain the properties of the specific system to be measured. For example, CO on nickel, oxygen on nickel or aluminum. Secondly, the polarization dependence can be used to sort out photemission features which are energetically overlapped in a spectrum obtained with mixed polarization.

Measurement of the absolute spectral response of an inverse photoemission detector

The absolute quantum yield of an inverse photoemission detector is reported. The detector consists of a 650‐A KBr photoemission film on the mouth of a channeltron with either a CaF2 or a SrF2 window for a low‐pass cutoff filter. The spectral response for the CaF2 window peaks at 9.8 eV with a FWHM of 1.6 eV and a maximum yield of 0.18 pulse/photon, while for the SrF2 window the peak is at 9.3 eV with a FWHM of 1.4 eV and a maximal yield of 0.06 pulse/photon. Performance of the detector was tested by measuring the inverse photoemission spectra from a W(001)‐(1×1) surface.

Optical Properties of Molybdenum and Ruthenium

In the spectral range 0.5 14 eV (23 eV for Mo) the reflectance was represented by an inverse power function where the exponent was determined with the aid of the reflectance measured at 50° incidence for hv<11.9 eV. The imaginary part of the dielectric constant, ∊2, has shoulders at about 2.2 and 4.0 eV for Mo and 1.6 eV for Ru. The loss functions Im[1/∊] and Im [1/(1+∊)] have peaks at 10.8 and 9.9 eV, respectively, for Mo, and 10.2 and 8.7 eV, respectively, for Ru. Interband transitions dominate for 0.5≲hv≲10.0 eV.

Photoemission measurement of the d-band energies in calcium ☆

For hν<8 eV the energy distribution curves from deposited films of calcium are dominated by a single peak. The peak results from initial states near the Fermi energy. An overlap of 0.3 eV of the s- and d-like bands is estimated from the width of the peak. The spectral dependence of the width of the peak is used to locate final d-band structure at approximately 4.7 and 5.7 eV above the Fermi energy. These results are consistent with recent APW calculations.

Study of dissociation of water on Ge(100) by high-resolution electron-energy-loss spectroscopy

Abstract High-resolution electron energy loss spectroscopy has been used to study the interaction of the clean Ge(100) surface with water under ultrahigh vacuum conditions. We observe no detectable molecular adsorption nor dissociation at room temperature up to 100 L water exposure. At 100 K both molecular and dissociated water are present on the surface even for 0.1 L exposure. Vibrational modes of the hydroxyl group have been identified by observing the isotopic shift between the H 2 O and D 2 O energy loss spectra. Dissociation and desorption of water have been studied in the 100–620 K range. The data show that at room temperature and above water is completely dissociated into H and OH. At higher temperatures (> 450 K) the OH radical also starts to dissociate, leaving oxygen and hydrogen bonded to Ge. Upon annealing to 620 K only atomic oxygen is detectable on the surface.

Surface structure effects via photoexcited core electron diffraction for Na on Ni

Abstract Photoemission from the Na 2p core level has been measured in the range ℏ ω = 40–120 eV for a c(2×2) Na overlayer on a Ni(001). surface. Weak oscillations in the Na 2p emission intensity vs. wavelength were observed and are interpreted as due to photoelectron scattering. The observed peaks closely match, in energy, those derived in recent calculations which assume the Na occupies a hollow site 2.23A above the Ni surface layer. The data therefore indicate that observation and calculation of such effects may allow surface structure determinations.