Molecular Orbital Spectra Research Articles

Core level photoemission studies of the interaction of pentacene with the Si(1 1 1) (7 × 7) surface

The chemical bonding interactions of molecular pentacene with the atomically clean Si(1 1 1) (7 × 7) surface were investigated by high resolution core level and valence photoemission spectroscopy. Thin films of pentacene were deposited from a thermal evaporator onto the atomically clean Si(1 1 1) surface in ultra-high vacuum at room temperature. Analysis of the Si 2p core level spectra reveal evidence of a strong chemical interaction between the first molecular layer and the surface with a chemically shifted component appearing on the lower binding energy side of the Si 2p core level. A 0.30 eV shift of the Si 2p core level towards the valence band maximum indicates that the bonding interaction between the molecules and the surface removes the bandgap states which pin the Fermi level midgap on the clean surface. Valence band spectra reveal that these interactions result in significant changes to the electronic structure of the chemically bonded molecular layer. Subsequent layers display the characteristic molecular orbital spectrum of the pentacene molecule. A reduction in the workfunction can be attribute to the formation of an interface dipole which reaches a maximum of 0.55 eV for a monolayer coverage.

Geometrical and optical properties of fullerene polymers

The polymerization of solid C 60 is studied theoretically using the semiempirical AM1 Hamiltonian. Model systems consisting of two interacting C 60 molecules are used in order to model polymerization of neutral C 60, as well as of alkali metal doped C 60. The fact that C 60 polymerizes in the charged and in the photoexcited state, but not in the neutral ground state is explained by the Woodward-Hoffmann rules, i.e., the necessary symmetry breaking is introduced by changing the occupancy of the molecular orbital spectrum either by means of electron excitations or by charging of the system. We have also calculated the optical absorption spectrum of both the neutral and the charged system. The charged spectrum is found to give rise to a low lying absorption polarized along the chain direction, a feature which is unique for the polymerized phase.

Structure and bonding in face- and edgebridged octahedral transition metal clusters

A simple and unified picture based on the orbital interaction idea is developed to promote understanding of the complex molecular orbital levels in edge-and face-bridged octahedral clusters. Based on these molecular orbital spectra, the closed-shell electronic structures are discussed. For face-bridged clusters, electron counts of 20 and 24 are derived for those clusters without an interstitial atom or with an interstitial main group. Eighteen or 24 electrons are required to satisfy closed-shell electronic structures for clusters with an interstitial transition-metal atom. For edge-bridged clusters, 14 or 16 electrons meet the closed-shell electronic requirements for those clusters without an interstitial main group atom. For clusters with an interstitial transition-metal atom, 14, 18 and 20 electron counts are possible. Based on the electron counts derived, several possible new clusters are predicted. Copyright © 1996 Elsevier Science Ltd

Molecular-orbital X-rays and their anisotropies in heavy-ion collisions in solids and gases

Molecular-orbital spectra and anisotropies were measured in Cl+NaCl, Cl+KCl, Cl +Ti, and Cl+Ar collisions at bombarding energies between 0.5 and 2.6MeV/a.m.u. The results suggest that at low bombarding energies MO x-rays in the KCl and NaCl targets are partly produced by a two-collision process. Qualitative differences between the anisotropies of gas- and solid-target induced MO x-ray spectra underline this result. Also, new information on the bombarding-energy dependency of anisotropies at higher projectile velocities has been obtained, confirming previous observations in low-Z systems.

Production of x rays beyond theK-series limit in gaseous and solid targets

X rays of energy beyond the K-series limit have been detected during the bombardment of various low-Z, solid and gaseous targets by C/sup +/, N/sup +/, and O/sup +/ ions of energy 0.75 to 1.9 MeV. K..cap alpha cap alpha.. x rays, most likely from the transition (1s/sup -2/2p/sup -1/)..-->..(2s/sup -1/2p/sup -2/), are emitted by both collision partners in gaseous targets. The low-resolution energy measurements are in good agreement with published calculations. The relative yield from the heavier collision partner is larger than simple vacancy-sharing models would indicate, but electron screening may account for the excess. Molecular-orbital (MO) continuum radiation completely obscures the Kaa lines from thin solid carbon targets. The exponential shape of the MO spectrum beyond the united-atom limit is in reasonable agreement with collision-broadened spontaneous emission.

Multilayer X‐ray spectrometry in the 20–80 Å region: A molecular orbital analysis of CO and CO2 in the gas and solid states

AbstractOptimized multilayer X‐ray spectrometry, using lead myristate and lead stearate analyzers (2d values of 80 and 100 Å) has been applied to the measurement of the O‐‐Kα and the C‐‐Kα spectral bands from CO and CO2 in the gas and solid phases. The LII, III spectra were also measured for argon in the gas and solid states under similar conditions in order to identify and to minimize any non‐molecular components in the CO and CO2 spectra. These molecular orbital data have been related to those obtained with X‐ray and ultraviolet photoelectron spectroscopy and grating X‐ray spectroscopy. The consistency of the results of these complementary measurements is excellent. The ionization energies for the C and O 1s levels have been determined to be 295.4 and 542.0 eV for CO and 296.8 and 540.3 eV for CO2. The MO data have also been compared with that predicted from the symmetry, strength and binding energy of the molecular orbital spectral components as calculated using the currently available computational models, CNDO/2, MINDO/3, MNDO, extended Hückel and ab initio. The molecular orbital calculated results for these organic compounds, CO and CO2, are not in as good agreement with the experimental data as previously demonstrated for molecular orbital spectra measured for C‐‐Kα and the LII, III bands of Cl, S and P for both organic and inorganic compounds.

Observation of Selected Molecular-Orbital X Rays in Coincidence with Separated-AtomKX Rays

A cascade relationship between molecular-orbital (MO) and separated-atom x rays has been utilized to study selected MO transitions. Depending on the mechanism of vacancy production, the observed MO spectra are interpreted as reflecting $2p\ensuremath{\sigma}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ transitions or a mixture of $2p\ensuremath{\sigma}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ and $2p\ensuremath{\pi}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ transitions. The coincidence selection also directly corroborates a previous assignment of MO transitions to the $2p\ensuremath{\sigma}$ state.

Observation of Selected Molecular-Orbital X Rays in Coincidence with Separated-AtomKX Rays

A cascade relationship between molecular-orbital (MO) and separated-atom x rays has been utilized to study selected MO transitions. Depending on the mechanism of vacancy production, the observed MO spectra are interpreted as reflecting $2p\ensuremath{\sigma}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ transitions or a mixture of $2p\ensuremath{\sigma}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ and $2p\ensuremath{\pi}\ensuremath{\rightarrow}1s\ensuremath{\sigma}$ transitions. The coincidence selection also directly corroborates a previous assignment of MO transitions to the $2p\ensuremath{\sigma}$ state.

Recoil effects on molecular-orbital X-ray spectra in solid targets

Using the work of Lindhard et al. (1963, 1968) on radiation effects of ions penetrating into solids the authors compute recoil effects on molecular-orbital (MO) spectra in solid targets. In symmetric collisions the dominant effect is due to scattered projectile and recoiling target atoms which carry an inner-shell vacancy into subsequent collisions where MO X-rays are emitted. In asymmetric collisions in which the target is the higher Z partner, the dominant effect is due to recoiling target atoms which emit MO X-rays in subsequent multiple-collision processes. The authors apply their expressions to MO X-rays emitted in Ni+Ni and Ne+Al collisions. In the presently available 12-92 MeV Ni+Ni MO spectra recoil effects are too small to be detectable. In 0.3-1.6 MeV Ne+Al MO spectra measured by Knaf and Presser (1974) recoil effects appear at the lower bombarding energies, which agree well with the authors' calculations.

The valence shell spectra of acetylene and hydrogen excited by Zr M ζ x-rays. An example of the advantages provided by an ultra-soft x-ray source

Excitation of the valence shell molecular orbital spectrum of acetylene with Zr M ζ (151.4 eV) radiation shows clearly all four expected bands 1π u (11.4 eV), 3σ g (16.8 eV), 2σ u (18.8 eV) and 2σ g (23.5 eV) with approximately equal intensity in contrast to He or Mg K α excited spectra. A spectrum of molecular hydrogen can also be obtained with this radiation.

X-ray photoemission molecular orbitals of hydrogen fluoride and the fluorinated methanes

The x-ray photoemission molecular orbital spectra of gaseous hydrogen fluoride and the fluorinated methanes (CHn F4−n, 0≥ n ≥4) are reported. Ab initio, Koopman's theorem energies and a three-parameter relaxation model were used to fit the experimental binding energies. Excellent agreement was obtained. Relative intensities of molecular orbitals were computed using the model of Gelius with both CNDO/2 and ab initio atomic populations. In the case of ab initio results, net populations were found to be superior to gross populations in reproducing the experimental intensities. In most cases, the theory predicted intensities quite well. Some trends exhibited by the fluoromethane series are noted.

X-Ray Photoemission Cross-Section Modulation in Diamond, Silicon, Germanium, Methane, Silane, and Germane

The high-resolution x-ray-photoemission-spectroscopy (XPS) valence-band spectrum from a cleaved natural-diamond single crystal is reported. The absence of extrinsic structure allows a reliable comparison with band theory. The XPS molecular-orbital spectra of methane, silane, and germane are also given. The modulation of photoelectric cross sections in the valence bands of diamond, Si, and Ge are discussed and compared with atomic XPS cross sections derived from the spectra of C${\mathrm{H}}_{4}$, Si${\mathrm{H}}_{4}$, and Ge${\mathrm{H}}_{4}$.

Ion-Neutralization Spectroscopy.

The ion-neutralization spectroscopy (INS) is discussed in comparison with other spectroscopies of solids. It is shown that INS probes the local density of states of the solid at or just outside the solid surface. It is believed that this accounts for the clear-cut differences between INS results and those of other spectroscopies. Because of its unique specificity to the surface region INS is particularly useful in studying the surface electronic structures of atomically clean surfaces and of surfaces having ordered arrays of known atoms adsorbed upon them. In the latter case INS determines a portion of the molecular orbital spectrum of surface molecules formed from the adsorbed foreign atom and surface atoms of the bulk crystal. Such spectra provide information on local bonding symmetry and structure and electrical charging within the surface molecule which is as yet unavailable by any other method. INS is the first attempt to base a spectroscopy of electronic states on a two-electron process. More recent work on experimental and mathematical problems which such a spectroscopy entails are also briefly mentioned in this paper.

Ion-Neutralization Spectroscopy

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