W.D. Grobman

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

Abstract We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH 3 OH), dimethyl ether (CH 3 OCH 3 ) formaldehyde (H 2 CO), acetaldehyde [H(CH 3 )CO], and acetone [(CH 3 ) 2 CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH 3 OH and H(CH 3 )CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mullikens theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.

Orbital energy shifts associated with chemical bonding of organic molecules on ZnO nonpolar surfaces

Abstract Ultraviolet photoemission studies show that adsorption of organic molecules on semiconducting ZnO surfaces produces: (i) bonding shifts of molecular energy levels which indicate chemisorption via π and lone-pair orbitals; and (ii) nearly uniform extramolecular relaxation/polarization shifts of the adsorbate valence orbital energies.

Ultraviolet-photoemission studies of formic acid decomposition on ZnO nonpolar surfaces

Abstract Ultraviolet photoemission studies of HCOOH decomposition on ZnO (1 1 00) at 120 and 300 K show that the reaction products remaining on the surface include CO and atomic O. Both the 5 σ (lone-pair) and 1 π orbitals of CO show chemical bonding shifts due to adsorption; this multiple-bonding behavior contrasts that observed on transition metal surfaces and apparently reflects special properties of the ZnO semi-conductor surface.

Intramolecular screening of crystal fields and the X-ray-photoemission determination of charge transfer in TTF-TCNQ

Abstract A recent controversy concerns the XPS data for TTF-TCNQ, which shows two binding energies for the N−1s level. We show that XPS chemical shifts due to charge flow on TCNQ, in response to the crystal potential, can cancel the Madelung splitting of the N−1s levels. Then we decompose the N−1s spectrum into its two components, which have widths characteristic of N−1s in TCNQ0 and TCNQ−, and whose intensities suggest charge transfer of ∼ 0.56–0.67.

Cyclohexane Dehydrogenation of Clean Pd Surfaces Studied by UV Photoemission.

Abstract Ultraviolet photoemission studies show that cyclohexane is adsorbed without decomposition on clean polycrystalline Pd and single-crystal Pd (111) surfaces at low temperature (≤ 120 °K) but that at 300 °K it dehydrogenates to leave chemisorbed benzene on the surface.

Chemisorption of organic molecules on ZnO(11̄00) surfaces: C5H5N, (CH3)2CO, and (CH3)2SO

Abstract Ultraviolet photoemission studies of pyridine, acetone, and dimethyl sulfoxide adsorbed on ZnO(1100) show the involvement of molecular orbitals having π, π ★ , and nitrogen and oxygen lone-pair character in formation of the chemisorption bond. No decomposition products from adsorption of these molecules are observed on the surface.

A comparative study of amorphous and crystalline superconducting molybdenum films by ultraviolet photoemission spectroscopy

Abstract Ultraviolet photoemission measurements at two photon energies are carried out, to understand the strong enhancement of the superconducting transition temperature T c in the nitrogen stabilized amorphous molybdenum films. The rise of T c can be attributed to a 40% increase of the electronic density of states at the Fermi level N(E F ) and a 15% softening of the phonon spectrum.

Ultraviolet photoemission from polymeric sulfur nitride polycrystalline films and single crystals

Abstract The (SN)x valence band structure, for polycrystalline films as well as for single crystal samples, has been studied using He I and He II resonance radiation. In angle-resolved photoemission energy distributions from single crystals, structure in the spectra is selectively enhanced offering a possibility of assigning the photoemission as originating from particular regions of the Brillouin Zone. The observed onset of photoemission 0.2 eV below the Fermi edge is discussed.

Dipolar polarization binding energy of TTFTCNQ

Abstract The dipolar polarization energy that arises from the electric field induced dipoles at the atomic sites been calculated for TTFTCNQ and found to be 0.091 eV per donor-acceptor pair for the fully charge transferred crystal. This is, therefore, a significant contribution to the binding energy, but is not sufficiently large to account for the existence of the charge transferred ionic ground state of this crystal.

Modulated photoemission spectroscopy: Application to Au☆

Abstract We describe modulated photoemission spectroscopy, in which an internal (sample) parameter such as temperature, or an external (apparatus) parameter such as wavelength is varied. A general formalism is developed for modulated photoemission spectroscopy and then illustrated using temperature modulated photoemission spectra and yields for Au obtained in the ∼ 6 to 11.6 eV photon energy range. Modulated s-p band photoemission data are described in terms of photoemission critical points in order to explain the nature of the modulated structures in the s-p band region and relate experiment to energy band thresholds obtained from a recent band calculation for Au. Application of the formalism to modulated d-band emission leads to a method for extracting d-band deformation potentials. For example, we find that the upper d band edge moves upward with respect to E F at a rate of 2 to 6 × 10 −4 eV/K. Modulation of the quantum yield is described and our measurements are compared with modulated optical data.