Core Photoelectron Spectra Research Articles

Theoretical study of many‐body effects in the photoelectron spectra of unsaturated hydrocarbons

AbstractA theoretical investigation on the core and valence photoelectron spectra of unsaturated hydrocarbons of increasing complexity, ethylene, trans‐butadiene, and trans‐hexatriene, has been performed in order to analyze the role of the most important many‐body effects on the spectral features and the evolution of these effects along the series. The capability of various configuration interaction (CI) schemes to interpret both the core and the valence ionization satellite structures has been pointed out. The 3h‐2v(π)‐1p and 2h–1p CI schemes employed, together with an accurate choice of the basis set and the explicit inclusion of the relaxation, has proven to be very useful tools for this kind of study, being sufficiently accurate to explain the most important correlation features and extendible, at the same time, to longer chain systems. © 1994 John Wiley & Sons, Inc.

Calculation of excitation and photoionization spectra by quasi‐degenerate perturbation theory

AbstractThe ability of the QDPT CI method, in which the quasi‐degenerate perturbation theory is applied within the configuration interaction (CI) approach, in dealing with the calculation of excitation and photoionization spectra is shown through an overview of comparisons between the QDPT CI values and the full CI ones in the same basis set. A direct comparison with the experimental data is given for the core photoelectron spectrum of Ne and the valence photoelectron spectra of Ar and HCl. The quality of the results obtained is very satisfactory, both as regards the energies and the wave functions, indicating the validity and the flexibility of the method that can confidently be applied also in cases where strong correlation effects are present. © 1994 John Wiley & Sons, Inc.

Quasidegenerate perturbation theory configuration interaction calculations of the core and valence photoelectron spectra of Ne and Ar

Quasidegenerate perturbation theory within the configuration interaction approach (QDPT CI) is applied to the calculation of photoelectron spectra of Ne and Ar, employing the sudden approximation for the calculation of the intensities. This approach proves to be able to take into account properly the dynamical and nondynamical correlation effects for both the valence and core ionizations, provided that static relaxation effects are explicitly taken into account through the use of SCF relaxed orbitals for the core processes. The importance of the choice of the zero-order space, which must comprise the strongly interacting configurations, is discussed and good agreement with the experimental data is obtained also employing zero-order spaces of limited dimension, the possible remaining discrepancies being due to limitations of the basis set employed or of the approach used for the calculation of the intensities. The method proves therefore to be adequate for a quantitative description of the entire photoionization spectra of Ne and Ar, reproducing both the energies and the intensities of the rich satellite structures.

Do shape resonances exist in core electron shake spectra of free molecules?

The high-resolution F1s core photoelectron spectrum of SF 6 is presented and analyzed using a comparison with inner-shell electron energy loss data. It is demonstrated that there is a close relationship between the two different experiments below and above the first and second ionization limits. The first evidence of shape resonances in a molecular shake spectrum is presented. The possibilities of obtaining structural information from the extended shakeoff continuum are discussed.

A high resolution X-ray photoelectron spectroscopy and INDO/S-CI study of the core electron shake-up states in poly(methyl methacrylate) and related model molecules

The O1s and C1s core photoelectron spectra excited by monochromatized X-rays ( hv = 1487 eV) for formic acid, acetic acid, acrylic acid, methyl acrylate, methyl methacrylate and methyl iso-butyrate are presented together with results obtained for solid poly(methyl methacrylate) (PMMA). This series of molecules represents model systems, of varying complexity, for a monomer subunit of PMMA. A rich shake-up structure is found which has been interpreted by means of INDO/S-CI calculations and by studying the systematics in the experimental spectra. The most prominent features are shown to be related to the π system in the molecules. A knowledge of charge transfer processes as well as of hyperconjugation is shown to be essential for the understanding of the experimental results.

The C1s core shake-up spectra of alkene molecules: An experimental and theoretical study

The C1s core photoelectron spectra of a series of alkene molecules, ethene, propene, 1-butene, cis and trans 2-butene, 2-methyl-propene, and 1-pentene are discussed. The experimental spectra are assigned using intermediate neglect of differential overlap-configuration interaction (INDO-CI) calculations and comparative discussions. It is shown that hyperconjugation is a useful concept in the assignment of the transitions. INDO-CI is shown to give a reasonable description of the low energy part of the spectra. The results are used in the discussion of molecular models for the interpretation of the electronic structure of polyacetylene.

Theoretical study of the satellite structure in the photoelectron spectra of neon and argon

CI calculations are performed for the lowest satellite states in the valence and core photoelectron spectra of Ne and Ar, at various excitation levels, up to 4h-3p/4h-4p. Use of relaxed orbitals relative to the doubly ionized system gives significant improvement at the lowest excitation level, and is subsequently employed in all calculations. Good agreement is obtained both for the energies and intensities, although the latter appear generally underestimated. A better agreement is obtained by the use of the sudden approximation including a plane-wave description of the photoelectron. The spectroscopic factor of the 3p43d satellite in Ar is discussed, and available evidence is found to support the value measured by X-ray photoelectron spectroscopy.

Chemical effects in the X-ray emission (F K ? ) and X-ray photoelectron spectra of alkali-metal and alkaline-earth-metal fluorides

X-Ray emission (XE) and both core and valence-band X-ray photoelectron (XP) spectra have been measured for LiF, NaF, KF, BeF2, MgF2, CaF2, SrF2 and BaF2. Both the F 1s ionisation energies and the F Kα X-ray energies decrease as the cation atomic number increases within a group of the Periodic Table. Knowledge of the F 1s binding energy enabled the valence-band XP spectra to be aligned on a common energy scale for direct comparison with the F Kα peak profile. Both the XE and XP spectra showed broader bands for the lighter cations, which for Li, Be, Mg and Ca could be resolved into two distinct components. Symmetric broadening of the valence-band peaks was rationalized as due to anion–anion interaction whilst additional structure on the low (kinetic) energy side and low (X-ray) energy side of the main XP and XE peaks was related to covalent character in the cation–fluoride bond. Differences in the relative intensities of these components for XE (reflecting F 2p character only) and the XP spectra were ascribed to the presence of F 2s and possibly cation s character in the more tightly bound orbitals of the valence band. Measurement of the BeKα and Mg Kβ XE spectra gave a direct indication of the covalent nature of the bonding in BeF2 and MgF2.

An analysis of configuration-interaction model spaces for the study of the core photoelectron spectra of small molecules

Two-hole, one-particle (2h-1p), 3h-2p and 4h-3p configuration-interaction (CI) calculations of the shake-up in the core photoelectron spectra of H2O, HOF, F2, F2O, N2, CO and O3 have been compared with full configuration interaction (FCI) in a minimal basis set. Explicit inclusion of relaxation is found to be important even at the 3h-2p level, while no definite advantage of the delocalised description is observed beyond the 2h-1p level. The relaxed 3h-2p scheme is found to be adequate for the low-energy part of the spectra; higher excitations become important at higher energy. Both the important correlations and the molecular relaxation appear to be well described within the minimal basis. However, extension of the basis is essential for a quantitative description of the spectrum. The effect of an extended basis has been analysed for the ionisations of CO, using a projection scheme which allows a better definition of the empty orbitals, while keeping the conceptual simplicity of the valence space.

A Stieltjes imaging method for molecular shake-off continua. Application to the core photoelectron spectrum of water

A Stieltjes imaging method for molecular shake-off continua is devised and numerically applied to the core photoelectron spectrum of water. Assuming the independent particle approximation, the full core photoelectron spectrum corresponding to an energy range of 0–100 eV, including main peak, shake-up peaks, and shake-off continua, is constructed in one ansatz. The shake-off cross sections are obtained in both length and velocity gauges and the effect of discrete and continuum interaction between channels with different spin coupling is investigated. The variation of the dipole transition moment vs excitation energy for the primary photoelectron is explicitly calculated, which gives the corrections to the sudden approximation also in the high-energy limit of x-ray excitation. By comparison, the plane wave approximation for the primary photoelectron is found appropriate only for relative cross sections in the shake spectrum, while for the absolute photoionization cross section it significantly deviates from results obtained from the variationally optimized photoelectron functions. The form of the variationally optimized continuum waves close to the oxygen nucleus is compared with the corresponding plane waves and Coulomb waves of different effective charges.

Vapor phase x-ray core-photoelectron spectra of N,N-dimethyl-p-nitrosoaniline. An experimental and theoretical study

Vapor phase x-ray core-photoelectron spectra of N,N-dimethyl-p-nitrosoaniline. An experimental and theoretical study

Fourier transform-based deconvolution techniques for resolution of overlapping bands in x-ray photoelectron and fourier-transform infrared spectra of bisphenol-a-polycarbonate/dimethyl siloxane block copolymers

A series of bisphenol-A-polycarbonate/poly(dimethylsiloxane) (BPAC/DMS) block copolymers is studied by x-ray photoelectron (x.p.s.) and Fourier-transform infrared (F.t.i.r.) spectroscopies. Previous quantitative results, obtained without resolution enhancement, provided calibration of the signal-processing techniques used. A Van Cittert technique for x.p.s. is evaluated for the deconvolution of the carbon 1 s core photoelectron spectrum, and self-deconvolution by Fourier methods is evaluated for the overlapping Si-CH 3 and aromatic C—O stretch bands at 1261 and 1228 cm -1, respectively, in the i.r. spectrum. These features can be utilized in quantitative analysis of the surface and bulk of the BPAC/DMS copolymers. The ability of deconvolution techniques to narrow the bands artificially so as to resolve complex spectral envelopes, while retaining the quantitative data is discussed. The studies illustrate the ability of modern signal processing techniques to extract exact complex data from spectroscopic results, and allow quantitative evaluation without standards for complex polymer systems.

Theoretical calculations of shake-up transitions and intensities using the relativistic local-density R Xi method

The Slater transition-state concept within the relativistic local-density R Xi method is applied to the study of shake-up energies in the core photoelectron spectra of alkali atoms and rare gases. The intensities of the shake-up transitions are calculated using the sudden approximation. It is found that the shake-up energies are well described by the present method and the intensities are in agreement with the experimental values. In these shake-up processes, the relativistic effects are found to be very small but the relaxation effects are important. The present simple one-electron picture seems to be adequate for the interpretation of the core photoelectron spectra.

Interstitial atoms in metal-metal bonded arrays: The synthesis and characterization of heptascandium decachlorodicarbide, Sc 7Cl 10C 2, and comparison with the interstitial-free Sc 7Cl 10

Reaction of Sc strips, ScCl/sub 3/, and graphite at 860-1000/sup 0/C gives Sc/sub 7/Cl/sub 10/C/sub 2/ in quantitative yields, with transport occurring at the higher temperatures; adventitious carbon will also produce the phase. The compound has been shown to be isostructural with Er/sub 7/I/sub 10/ by single-crystal X-ray diffraction (a = 18.620 (4) A, b = 3.4975 (6) A, c = 11.810 (2) A, ..beta.. = 99.81 (2)/sup 0/; space group C2/m, Z = 2, R = 0.029, R/sub w/ = 0.046 for 676 reflections, MoK..cap alpha.., 2 theta < 50/sup 0/). The phase contains double chains of condensed scandium octahedra sharing edges with a carbon approximately centered in each (anti d(Sc-C) = 2.31 A) together with isolated scandium atoms in a parallel chain of chloride octahedra. The arrangement is very similar to that in the previously known Sc/sub 7/Cl/sub 10/, from which the heavy atom arrangement can be derived by displacement of all metal atoms by b/2 so as to convert chlorine functions on the metal chain from face-capping to edge-bridging. The driving force for this is thought to be the reduction of carbon-chlorine repulsive interactions. Core and valence x-ray photoelectron spectra (XPS) for Sc/sub 7/Cl/sub 10/, Sc/sub 7/Cl/submore » 10/Cl/sub 2/, Sc/sub 2/Cl/sub 2/C, ScCl/sub 3/, and Sc are presented to demonstrate the appearance of a carbide-like state for the interstitial, the presence of two different types of scandium in the first two compounds, the oxidation of the chain that accompanies the carbon insertion, and a substantial Sc-C covalency. The latter arises through mixing of the interstitial's 2s and 2p valence orbitals with metal-metal bonding cluster orbitals of the same symmetry. 37 references, 6 figures, 7 tables.« less

Screening effects in photoemission from weakly bound adsorbates: CO on Ag(110)

Abstract Photoelectron spectra of the core and valence region of CO adsorbed on Ag(110) at low temperature are reported. The valence spectra show that the large bonding shift of the 5σ level observed for CO adsorbed on transition metals, as well as the large relaxation shifts are absent in CO/Ag(110). This proves that the CO molecule is physisorbed on Ag(110). Polarisation-dependent spectra indicate that CO is not bound to the surface in an upright geometry. The molecule is probably bound to the surface with its molecular axis parallel to the surface, or in a random orientation. The C 1s and O 1s core levels exhibit pronounced satellite structure. By comparison with the spectra from the CO/Ni and CO/Cu adsorption systems, it is shown that the satellite structure and the differences in extra-atomic relaxation shifts can be understood from a unified point of view, by invoking a model in which both core and valence photoelectron spectra are shaped by the strength of the screening of the photon-induced hole through metal-adsorbate charge transfer.

Screening effects in photoemission from weakly bound adsorbates: CO on Ag(110)

Photoelectron spectra of the core and valence region of CO adsorbed on Ag(110) at low temperature are reported. The valence spectra show that the large bonding shift of the 5σ level observed for CO adsorbed on transition metals, as well as the large relaxation shifts are absent in CO/Ag(110). This proves that the CO molecule is physisorbed on Ag(110). Polarisation-dependent spectra indicate that CO is not bound to the surface in an upright geometry. The molecule is probably bound to the surface with its molecular axis parallel to the surface, or in a random orientation. The C 1s and O 1s core levels exhibit pronounced satellite structure. By comparison with the spectra from the CO/Ni and CO/Cu adsorption systems, it is shown that the satellite structure and the differences in extra-atomic relaxation shifts can be understood from a unified point of view, by invoking a model in which both core and valence photoelectron spectra are shaped by the strength of the screening of the photon-induced hole through metal-adsorbate charge transfer.

The lifetime of the koopmans-theorem state in core photoelectron spectroscopy. An estimate based upon the sudden approximation

A lifetime for the unrelaxed or Koopmans-theorem state can be estimated from the second moment of the spectral profile of a core photoelectron spectrum assuming validity of the sudden approximation. For Ne 1s the second moment 2850.9 eV 2 leads to a lifetime of 1.0 × 10 −17 s. This time is two orders of magnitude shorter than the lifetime of individual electronic states in the spectrum but is of the same magnitude as times considered to be zero in the sudden approximation. Within the validity of this approximation the decay of the Koopmans-theorem state is therefore instantaneous.

Theoretical study of shake-up in the core photoelectron spectra of the alkali atoms

Relativistic and non-relativistic Hartree-Fock calculations have been performed for the shake-up lines relative to core ionization of the alkali atoms. Good general agreement with the experimental data is achieved both for the energy and the intensity. Relativistic effects are found to be small, amounting to = 0.2 eV in Cs. As concerns the shake-up energies, a correlation effect is detected, the magnitude of which increases along the series.

Core hole induced charge‐transfer reactions in disubstituted benzenes

AbstractThe application of the charge‐transfer concept on core electron ionization in donor–acceptor molecules is analyzed. A single excitation picture, involving the highest occupied and lowest unoccupied molecular orbitals is compared with a full π‐valence orbital active space model. The connections between the notion of charge transfer in donor–acceptor species with the shake‐up picture used for core photoelectron spectra of small molecules and with the dynamical screening concept applied on surface–adsorbate spectra are discussed.

Electronic structure of uranium halides and oxyhalides in the solid state. An x-ray photoelectron spectral study of bonding ionicity

Core and valence X-ray photoelectron spectra of 18 uranium halides (U/sup 3 +/ to U/sup 5 +/) and oxyhalides (U/sup 3 +/ to U/sup 6 +/) (fluorides, chlorides, and bromides) are described. The results are discussed, as a function of halogen nature and compounds stoichiometry, in terms of ionicity of the bonds, of U 5f participation in bonding, and effect of oxygen. Evidence is produced that UF/sub 3/ has an unexpectedly strong covalent character. The experimental study of uranium halides (F, Cl, Br) and oxyhalides has brought to light some important information about the nature of chemical bonding in these compounds: (a) The combined examination of valence photoelectron peaks (particularly U 5f), core levels, and U 4f shake-up satellites is a powerful tool in the determination of the ionicity of the solid. Only core level chemical shifts are not sufficient by themselves. (b) There is evidence for U 5f participation in chemical bonding; this contribution increases when the compound becomes more covalent. (c) Ionicity is largest for the higher halides (higher uranium oxidation state) and with the more electronegative halogens; and exception to this rule is UF/sub 3/, which is more covalent than UCl/sub 3/. (d) The oxygen in oxyhalidesmore » shows a growing tendency to form uranyl groups, as the uranium oxidation state goes up. These uranyl groups have covalent bonding and behave as a single species vs. the halogen.« less