Unoccupied Valence Research Articles

Gas sensing measurements and analysis of the optical properties of poly[3-(butylthio)thiophene] Langmuir–Blodgett films

Films of poly[(3-butylthio)thiophene] were prepared via Langmuir–Blodgett (LB) deposition and casting techniques for applications in gas sensor devices. A description of the preparation of the sensing layer is given for both methods: the LB deposition of the polymer in mixture with arachidic acid and the solvent casting by using directly a solution of the polymer in chloroform. In both cases, alumina substrates equipped with gold interdigitated electrodes have been used. In particular, the samples so prepared show variation in the electrical conductivity when exposed to NO 2-oxidising or NH 3-reducing agents at a working temperature of about 100°C. The refractive index n and the extinction coefficient k at normal incidence have been determined in the 400–800 nm spectral range and an energy gap (highest occupied-molecular-orbital or conduction band (HOMO)–lowest unoccupied molecular-orbital or valence band (LUMO) separation) of about 1.9 eV has been estimated.

Excitation energies and oscillator strengths of Ca I using multireference many-body perturbation theory

Atomic transition lines of neutral calcium were first found in solar spectrum as early as the middle of the 19th century, when the 4227-A line was identified along with the famous singly ionized H and K lines @1#. Neutral calcium lines have also been observed in all types of stellar and interstellar spectra, such as late-type dwarf stars. The crude abundance of Ca I (l56717.7 A ) has been observed in Am binaries@2#. To confirm those identifications and to find new lines, one has to depend on atomic experimental data or computational data based on theoretical models of the calcium atom. The abundances of neutral calcium in these astronomical bodies depend on the oscillator strengths of those lines, which come from the excitation energies of various levels and transition moments among those levels. The difficulty in accurately estimating transition energies and oscillator strengths of neutral calcium atom arises mainly due to the following reasons: ~a! The precise computation of transition energies and oscillator strengths requires a balanced description of the ground and excited states. ~b! The use of an inadequate basis leads to difficulties in describing the excited states and an unbalanced treatment of dynamical correlation and polarization effects. The problems due to basis set inadequacy can be removed partially by enlarging the basis for small and moderate-sized atomic and molecular systems. Here, the size extensivity @3# of the theory plays an important role in handling the proper treatment of electron correlation, and in that way it properly treats the differential correlation energies of the interacting ~initial and final states! zeroth-order states. It ensures that the state energies scale linearly with the number of electrons in a rigorous way. The accuracy of the computed excitation energy depends mostly upon the quality of the unoccupied valence orbitals in which excitation occurs. The traditional choice of some unoccupied valence orbitals from a ground-state selfconsistent-field ~SCF! computation introduces V N orbitals that are best suited for describing negative ions, and not lowlying excited states. Thus the valence orbitals, those that are not occupied in the ground-state SCF, should be taken as more representative orbitals suitable for excited states. One possible choice emerges from the restricted single excitation configuration interaction procedure@4#, where excitations are only permitted from the highest occupied orbitals. A simpler and often equivalent approaches involves using improved virtual orbitals ~IVO’s !@ 5 #in the H v valence space or reference space. Here the IVO’s are generated by single orbital SCF optimization in which the Fock operator is defined by promoting an electron from the highest occupied orbital to the orbital being optimized, while all the previously determined orbitals are kept frozen. Alternatively this can be accomplished by a unitary transformation @6#. The IVO orbital energies obtained in this way are lower than those evaluated from the traditional SCF procedure due to the absence of an extra Coulomb operator in the former procedure. For example, in a neutral calcium atom H v calculation, the twoorbital (4 s,4p) minimal reference space is produced by the sequence

In SituMagnetic-Circular-X-Ray-Dichroism Measurements: An Epitaxial Fe Wedge on Cu(100)

Circularly polarized x-ray radiation is attracting increasing interest as a tool for the characterization of the electronic, magnetic, and chiral properties of low-dimensional structures. Using circular light (with electric field vector parallel to the orbital plane), a dependence of the measured quantity by changing either the orientation of the light polarization or the magnetization is indicative of the existence of magnetic circular dichroism. It can be observed in x-ray absorption spectroscopy (XAS), in which the photon energy is scanned through an absorption threshold exciting a core electron into an unoccupied valence state using circularly polarized light. Synchrotron radiation sources have made this technique possible. It can also be observed in photo-emission spectroscopy from core and valence levels. Here we focus on magnetic circular x-ray dichroism (MCXD) in XAS as an element-specific tool to investigate magnetic properties of ultrathin filmsin situ. The application of magneto-optical sum rules enables the determination of the orbital and spin magnetic moments per atom from XAS spectra, as well as the easy magnetization direction.MCXD-based magnetometry in XAS is extensively used by measuring the L absorption edges of 3d-transition metals, where large intensity changes (up to 60%) of the L-edge white lines are observed upon reversal of either the sample magnetization or the light helicity. The high magnetic contrast obtained, combined with the elemental specificity of the technique, allows for the study of very dilute samples such as ultrathin films. We first concentrate on the selection rules governing MCXD in XAS.

Local aspects of the adsorbate-substrate chemical bond in N/Cu(100) and O/Cu(100)

Both the occupied and unoccupied adsorbate valence states have been probed by x-ray emission and x-ray absorption spectroscopies for N/Cu(100) and the low coverage phase of O/Cu(100). By carrying out experiments using two different measurement geometries, the adsorbate out-of-plane states ${(2p}_{z})$ have been separated from the in-plane states ${(2p}_{\mathrm{xy}}).$ The experimental results are compared to the partial density of states (DOS) obtained from full-potential linear muffin-tin orbital calculations, with good agreement in the case of N/Cu(100). This agreement confirms that it is possible to interpret the experimental data in terms of the adsorbate partial DOS. Our combined experimental and theoretical study also provides a detailed understanding of the local chemical bond for N and O adsorbed on Cu(100).

X-Ray Radiation Damage of Polymers in a Scanning Transmission X-Ray Microscope

Abstract We use the Scanning Transmission X-ray Microscope (STXM) at the National Synchrotron Light Source (NSLS) (1) to acquire images and spectra of polymers. To interpret data correctly, the effects X-ray radiation has on polymers must be understood. We have therefore started to characterize radiation damage in a variety of carbonyl containing polymers in two ways. First, we want to ascertain the critical dose for mass loss and the critical dose for the carbonyl in a variety of polymers and relate the critical dose to the polymer structure. (The critical dose is the radiation dose at which the optical density of the material is decreased by 1/e of its original value.) We also want to understand the damage mechanism. STXM acquires images and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra by using X-ray photons to excite inner shell electrons to unoccupied valence orbitals or to the continuum.

Spin-resolved resonant Auger spectroscopy of condensed iodine on Pt

In molecular iodine condensed onto Pt the N 4,5 VV Auger electron intensity and the valence band photoemission intensity are resonantly enhanced if the photon energy is just the energy necessary for exciting 4d 5/2 and 4d 3/2 core electrons to unoccupied valence states derived from the molecular 5p σ u states. These resonant emission processes are studied by spin resolved electron spectroscopy using circularly polarized radiation for the primary excitation. In the spectra measured on-resonance, spectator and participator emission processes can clearly be distinguished by the energy and by the preferential spin direction of the emitted electrons. In the 4d 3/2 →5p σ u resonance spectrum an emission process following a 4d 3/2 →4d 5/2 Coster–Kronig decay of the primary excited hole-state can be identified by its spin polarization. For the spectator emission processes the two-hole states excited in the decay of the primary holes are predominantly coupled to a singlet.

On the electronic properties of β-rhombohedral boron interstitially doped with 3d transition metal atoms

The controlled modification of the p-type character of pure boron-rich solids to n-type by suitable doping is an essential prerequisite for technical applications. Interstitial doping of β-rhombohedral boron with V, Cr, Fe and Ni fulfils this requirement, while Cu and Co do not. Comparative investigations of the Seebeck coefficient, the DC electrical conductivity, the optical absorption and the dynamical conductivity of BV, BCo and BFe lead to the conclusion that donor levels positioned between the conduction band and the uppermost intrinsic electron trapping level and of sufficient density to overcompensate unoccupied valence and gap states are necessary for n-type conductivity. In contrast to Fe, the interstitial accommodation of V atoms seems to reduce the concentration of possible gap states.

The electronic structure of the layered cuprate Ba 2Cu 3O 4Cl 2

We report spectroscopic studies of the occupied and unoccupied valence band states of Ba 2Cu 3O 4Cl 2 single crystals, which adopt a layered body-centered tetragonal structure related to that of the cuprates, but with an extra copper site resulting in a plane with Cu 3O 4 stoichiometry. Using angle-resolved photoemission we observe for the Γ-(π,π)-direction of the Brillouin zone a dispersive feature with a bandwidth of 400 ± 80 meV, approaching to within ∼ 0.9 eV the Fermi level at ( 1 2 π, 1 2 π) .

Electronic and Solvent Relaxation Dynamics of a Photoexcited Aqueous Halide

The details of the electronic and solvent relaxation dynamics following two-photon excitation of an aqueous halide ion are studied via nonadiabatic quantum molecular dynamics simulation. It is found that the branching ratio at very early times (<50 fs) between two channels, a minor channel involving direct electron detachment to a spatially separated solvent void and a dominant channel characterized by delayed adiabatic detachment following a cascade through excited electronic states of the ion, is determined by the effect of solvent dynamics on the values of the ionic and void electronic energies, as well as the relative small matrix elements for tunneling into void states. Solvent dynamics is also found to be important in controlling the rate of electron transfer which leads to geminate recombination of proximal electron−halogen atom pairs. The sensitivity of this recombination rate to the relative energy levels of the unoccupied valence electron hole on the solvated halogen and that of the hydrated ele...

Electronic structure study of K xC 60 compounds with x > 6

We present C 1s X-ray absorption and photoemission results for K x C 60 with x > 6, the so-called ‘superfullerides’. These are the first measurements of the unoccupied valence band for these systems and confirm that more than six electrons can be transferred to each fullerene. We observe saturation of the K-to-C 60 charge transfer, with part of the t 1g LUMO + 1 orbital remaining unoccupied, so that fewer than 12 electrons are accepted per C 60 molecule. This fullerene state exhibits metallic behaviour, while a transition to insulating behaviour is observed as lower charge states are formed by adding more C 60. The superfullerides appear to follow the pattern of strongly correlated solids.

Initial and final state rules in X-ray spectroscopies of adsorbates

The so-called “initial and final state rules” for X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are applied to CO on Ni(1 0 0). In XAS the final state core hole strongly modifies the distribution of unoccupied valence states from that of the ground, whereas in XES the final state reflects the ground state (without core hole) occupied density of states. The intensities of the X-ray transitions are related to the electronic state occupancy in the initial state. This is used to derive electronic state populations from XAS since the initial state corresponds to the ground state.

Inner-shell electron excitation in the methylmethoxysilanes

The effect of electron-donating substituents on the low-lying unoccupied valence orbitals in the series of silanes Si(CH 3) x (OCH 3) 4− x is investigated in an examination of electron-impact energy-loss spectra near the Si 2p ionization edge, and by means of ab initio calculations of the core-hole states populated by the inner-shell excitation. The substituent effect on the LUMO energies, as well as upon the 29Si NMR shieldings, is considered along with the consequences of valence—Rydberg mixing.

KLL resonant Auger electron emission from silicon compounds in solid phase

The resonant Auger electron emissions from solid silicon compounds with different relaxation properties (Si, SiC, SiO 2) have been studied using synchrotron radiation. We found that the energy of the SiKL 2,3L 2,3 Auger electrons from SiO 2 is split into two peaks, i.e. normal and resonant Auger peaks, when the photon energy is close to the Si 1s → 3p resonance. Such energy splits of the Auger peaks are not observed in Si and SiC, because the core holes are well screened and the charge is delocalized before the core hole decay happens. It was also found that the energy of the resonant Auger peak of SiO2 increases almost in proportion to the photon energy. This gradual energy gain is interpreted as the broad band width of unoccupied valence orbitals of solid Si02 compared to isolated atoms and molecules in gas phase.

Influence of many-electron effects in the C60 carbon K-shell absorption spectrum.

In the first part of this work we develop a model describing the influence of many-electron effects in ${\mathrm{C}}_{60}$ K-shell photoemission (only the \ensuremath{\pi} valence electrons are taken into account). Then we extend this model to the calculation of the ${\mathrm{C}}_{60}$ K-shell absorption spectrum. We first determine a one-electron spectrum obtained by considering only direct transitions from the 1s to the unoccupied valence levels. This spectrum exhibits three main peaks. Then, following a previous Friedel's analysis, we add two kinds of many-electron effects. In the first step, we include the transitions with shake-off excitations. The obtained spectrum is similar to the one-electron spectrum for the positions and the relative intensities of the three peaks, however, a high-energy tail appears. In a second step we also include replacement transitions. Then the relative intensities of the three peaks vary and are in better agreement with experiment than in the one-electron spectrum. The differences with experiment which still remain are discussed.

Shape of core level spectra in adsorbates

The spectrum of core levels in adsorbates is calculated taking into account the transfer of screening charge to the adsorbate. An exact numerical solution for a model including this effect is obtained. The position of the main peak and the overall shape of the spectrum are found to depend strongly on the width and position of the unoccupied adsorbate valence level.

Two-electron excitations and one-electron multiple-scattering resonances in the x-ray absorption of solid neon.

The local and partial density of states of the conduction band of solid neon in a large energy interval has been extracted from the analysis of the K-edge x-ray-absorption near-edge structure (XANES) spectrum by using a full multiple-scattering method. The absorption cross section for one-electron transitions from the Ne 1s level to the conduction-band final states in the solid Ne is predicted by first principles. The absorption cross section is described as due to an atomic factor and to a structure factor determined by the local and partial density of states for a large cluster formed by about 80 atoms. The experimental spectrum is obtained by describing the final state in the fully relaxed potential in the presence of the core hole. The exciton peak at threshold is shown to be due to the lowest unoccupied valence state pushed down in the bound-state region by the core hole. In the 100-eV XANES energy range the single-scattering approximation fails to predict the XANES of solid Ne indicating that the full multiple-scattering approach is required. The present analysis allows us to identify strong two-electron excitations 1${\mathit{s}}^{2}$2${\mathit{s}}^{2}$2${\mathit{p}}^{6}$\ensuremath{\rightarrow}1${\mathit{s}}^{1}$2${\mathit{s}}^{2}$2${\mathit{p}}^{5}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{i}}$${\mathit{p}}^{2}$ in the energy range of 30 eV above the K edge, involving monopole 2p\ensuremath{\rightarrow}\ensuremath{\varepsilon}p transitions to the lowest valence states at energy \ensuremath{\varepsilon}. The intensity of the two-electron excitations is found to be so strong that it should be considered in standard extended x-ray-absorption fine-structure--XANES data analysis for local structure investigation.

A valence- and inner-shell electronic and photoelectron spectroscopic study of the frontier orbitals of 2,1,3-benzothiadiazole, C 6H 4SN 2, 1,3,2,4-benzodithiadiazine, C 6H 4S 2N 2, and 1,3,5,2,4-benzotrithiadiazepine, C 6H 4S 3N 2

HeI photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, C1s and N1s edges, and S1s synchrotron radiation photoabsorption spectroscopy have been used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole, 1,3,2,4-benzodithiadiazine, and 1,3,5,2,4-benzotrithiadiazepine. The term values obtained from the S1s, S2s and N1s oscillator strength spectra are sensitive to the aromatic/anti-aromatic character in the thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively constant. The combination of valence photoelectron and core excitation results, aided by semi-empirical (MNDO) molecular orbital calculations, provides a useful probe of the frontier molecular orbitals of these aromatic/anti-aromatic molecules.

Polarized x-ray emission studies of methyl chloride and the chlorofluoromethanes.

A new technique sensitive to molecular orientation and geometry, and based on measuring the polarization of x-ray emission, has been applied to the Cl-containing molecules methyl chloride (${\mathrm{CH}}_{3}$Cl) and the chlorofluoromethanes (${\mathrm{CF}}_{3}$Cl, ${\mathrm{CF}}_{2}$${\mathrm{Cl}}_{2}$, and ${\mathrm{CFCl}}_{3}$) in the gas phase. Upon selective excitation using monochromatic synchrotron radiation in the Cl K-edge (Cl 1s) near-threshold region, polarization-selective x-ray emission studies reveal highly polarized molecular valence x-ray fluorescence for all four molecules. The degree and the orientation of the polarized emission are observed to be sensitive to the incident excitation energy near the Cl K edge. In some cases, the polarization direction for x-ray emission reverses for small changes in incident excitation energy (a few eV). It is shown that the polarized x-ray emission technique can be used to infer, directly from experiment, symmetries of occupied and unoccupied valence molecular orbitals, an- isotropies in absorption and emission, and orientational and geometrical information. It is suggested that the x-ray polarized-fluorescence phenomenon, reported here for simple molecules, can be used as a new approach to study more complicated systems in a variety of environments.

Laser-excited hot-electron induced desorption: A theoretical model applied to NO/Pt(111)

We present a theoretical model for stimulated desorption due to the interaction of energetic substrate carriers with molecular adsorbates. The model is based on the premise that optically excited hot electrons scatter into an unoccupied valence electron resonance of the adsorbate, thus forming a temporary negative molecular ion which then experiences an enhanced attraction towards the substrate. Neutralization of the ion returns the adsorbed molecule to one of the continuum states of the molecule/substrate potential energy surface, possibly in an internally excited state. The consequences of such a model are worked out using semiclassical wave packet dynamics which, in the short time limit relevant to the present situation, can be brought to an analytic realization. The model provides considerable insight into recent experiments on the laser-induced desorption of NO from Pt(111).

ChemInform Abstract: Some Observations Concerning the Structure of Dilithiomethane

As small-volume cationic charge centers and as a source of unoccupied valence orbitals, the early main group metals have a well-documented but remarkable organometallic chemistry. Unusual structural configurations are observed for organolithium, beryllium, and aluminum compounds including semibridging {sigma}-{pi} coordination in organometallic acetylenes such as (C{sub 6}H{sub 5}){sub 2} AlC{triple bond}CC{sub 6}H{sub 5}{sup 2} {pi} back-donation which leads to planar three coordinate metal atom configurations as in (BeN(CH{sub 3}){sub 2}){sub 3}, alkyl C-H coordination as in LiB(CH{sub 3}){sub 4} or (LiC{sub 6}H{sub 11}){sub 2}, and frontier orbital directed coordination away from charge potential energy minima in {pi} carbanions. The authors present here the results of recent structural studies of Li{sub 2}CD{sub 2} as obtained from neutron and x-ray scattering experiments. The nontrivial synthesis of a pure phase of Li{sub 2}CD{sub 2} used in this study was carried out by the Kawa-Lagow modified Ziegler technique.