Core Photoelectron Spectra Research Articles

Core and valence photoelectron spectroscopy of a series of substituted disulfides.

The valence and core photoelectron spectra of three substituted disulfide systems, α-lipoic acid, trans-4,5-dihydroxy-1,2-dithiane, and di-Boc-cystamine, are presented alongside detailed theoretical analysis based on equation-of-motion coupled-cluster singles doubles for ionization potentials and the nuclear ensemble approach. A comparison of the linear and five- and six-membered ring cyclic structures reveals that the energetic separation of the non-bonding sulfur orbitals can be used to calculate a reliable estimate of the C-S-S-C dihedral angle, even for substituted disulfides, and that the sulfur 2p, oxygen 1s, and valence band photoelectron spectra are a useful site-specific probe of hydrogen bonding.

A photoelectron spectroscopic investigation of aspirin, paracetamol and ibuprofen in the gas phase.

We have investigated the electronic structure of isolated molecules of paracetamol, aspirin and ibuprofen using computational methods and benchmarked the results against valence and core photoelectron spectra. Paracetamol, aspirin and ibuprofen exist as multiple conformers, and we have calculated the free energies and populations of the lowest energy conformers. We find generally good agreement with previous experimental and theoretical structural results. The valence band spectrum of gas phase aspirin has not been reported previously, and we report it and assign the features based on calculations. The effect of acetylation on the frontier orbitals of the parent molecule, salicylic acid, is to increase the ionization potential of the highest occupied molecular orbital (HOMO), and to exchange the energetic ordering of the following two orbitals. The acetyl π bond contributes to the next orbital, which is hybridised with ring π orbitals. The core level spectra of all three molecules are reported and compared with calculations and with the spectra of parent molecules (salicylic acid for aspirin, 4-aminophenol for paracetamol). Observed core ionization energies are in agreement with theory. Although all compounds share a benzene ring, and they also have a number of other chromophores in common, the spectroscopic data indicate chemical diversity, suggesting that their modes of bonding under physiological conditions are likely to be diverse.

Photoemission and photofragmentation of butanoic, hexanoic and octanoic acids in the gas phase

The valence and C 1 s core photoelectron spectra of three carboxylic acids (butanoic, hexanoic and octanoic) as well as the near-edge absorption fine structure spectra and photo-electron photo-ion coincidence spectra have been measured. Changes in the valence spectra are related to the lengthening of the aliphatic chains, and the core spectra have been assigned. Small shifts of the carboxylic carbon 1 s binding energy are related to screening of the core hole by the aliphatic group. The C K edge near-edge absorption fine structure spectra probe the unoccupied density of states and the main changes in the spectra are due to increasing length of the aliphatic chain and related Rydberg states. The coincidence spectra show that soft (near threshold) ionization of all compounds leads to parent ion formation for butanoic and octanoic acids. For hexanoic acid, both parent ions and fragments are produced for near threshold ionization. Fragmentation occurs at the C2C3 bond for all carboxylic acids at the lowest energies, and other channels open for higher internal energy.

X-ray Photoemission Spectra and Electronic Structure of Coumarin and its Derivatives.

The electronic structures of coumarin and three of its derivatives (7-amino-4-methylcoumarin, 7-amino-4-(trifluoro)methylcoumarin, and 4-hydroxycoumarin) have been studied by theoretical calculations, and compared with experimental valence and core photoelectron spectra to benchmark the predicted spectra. The outer valence band spectra of the first three compounds showed good agreement with theoretical calculations for a single isomer, whereas the spectrum of 4-hydroxycoumarin indicated the presence of more than one tautomer, consistent with published results. Calculations of core level spectra of carbon, nitrogen, oxygen, and fluorine of the first three compounds are also in satisfactory agreement with our measurements. The carbon and oxygen 1s spectra of 4-hydroxycoumarin allow us to identify and quantify the populations of the principle tautomers present. The 4-hydroxy enol form is the most stable isomer at 348 K, followed by the diketo form, with 1.3 kJ·mol(-1) lower energy.

Reliability of Density Functional and Perturbation Theories for Calculating Core-Ionization Spectra of Free Radicals

The C 1s, N 1s, and O 1s ionization energies were calculated for the three stable nitroxide free radicals, viz., tempo and its two analogues, and compared to their most recent high-resolution core photoelectron spectra. We compare the performance of unrestricted and restricted open shell based ΔHF, ΔMP2, and ΔB3LYP methods. A mixed basis set is employed in all cases, which consists of the core-valence correlation-consistent triple-ζ basis set (cc-pCVTZ or cc-pwCVTZ) on the atom whose core-electron binding energy is calculated, model core potentials on the remaining first row atoms, and the cc-pVDZ basis set on the hydrogen atoms. The best overall performance for the three free radicals is offered by the restricted open shell based ΔB3LYP method. Surprisingly, both the restricted open and unrestricted second-order perturbation theories perform relatively poorly and typically do not warrant additional computational effort over the reference ΔHF methods. This is particularly true of the ΔZAPT method, which exhibits a number of grave failures that render it unsuitable for calculating the core-ionization spectra.

Interference and diffraction in photoelectron spectra

Theoretical calculations are employed to disentangle the effect due to coherent emission from equivalent centres and diffraction from neighbouring inequivalent atoms in core and valence photoelectron spectra. The molecules investigated are mono and disubstituted fluoro and iodo acetylenes, compared to the simple acetylene system. The two effects appear well separated and additive in the core region, with diffraction giving oscillations of smaller amplitude, shorter period, and more strongly damped. Their interplay is more complex in the valence region giving rise to irregular patterns which, although rich in information, are more difficult to analyze. It is shown that the use of an external standard molecule can be profitably used to reveal diffraction patterns in the case where no internal ratio is available.

Charge models for electron spectroscopy of disordered alloys

Disorder broadening can be observed in the core photoelectron spectra of metallic alloys. This effect can be simulated using a model in which site charges are assumed to be proportional to number of ``unlike'' atoms in the nearest-neighbor shell. This linear charge model (LCM) gives a sensible description of the variation in Madelung potential in disordered alloys and is reasonably self-consistent but significantly overestimates core disorder broadening as compared with ab initio core eigenvalue calculations and experimental core-level binding-energy measurements. Two generalizations of the LCM are investigated: an electronegativity model, in which charge is exchanged between unlike nearest neighbors in a nonlinear fashion, and a linear multishell model. Analytical and computational results are presented in each case and the implications for the analysis of core-level photoelectron spectra are discussed.

Giant Chiral Asymmetry in the C 1s Core Level Photoemission from Randomly Oriented Fenchone Enantiomers

Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer using circularly polarized synchrotron radiation demonstrate huge chiral asymmetries, approaching 20%, in the angular distribution of photoelectrons ejected from carbonyl C 1s core orbitals. This asymmetry in the forward-backward scattering of electrons along the direction of the incident soft X-ray radiation reverses when either the enantiomer or the left-right handedness of the light polarization is exchanged. Calculations are provided that model and explain the resulting photoelectron circular dichroism with quantitative accuracy up to approximately 7 eV above threshold. A discrepancy at higher energies is discussed in the light of a comparison with the closely related terpene, camphor. The photoelectron dichroism spectrum can be used to identify the absolute chiral configuration, and it is more effective at distinguishing the similar camphor and fenchone molecules than the corresponding core photoelectron spectrum.

Intrinsic and extrinsic excitations in deep core photoelectron spectra of solid Ge

AbstractIn the quantitative analysis of buried interfaces or the exploration of local electronic structures around component atoms in bulk solids, spectroscopy of transitions in the deep atomic core has an increasingly significant role to play. A key issue in understanding the deep core photoelectron spectra is the identification and separation of the contributions that are attributable to different excitations leading to loss of energy of the emitted photoelectrons. In this work, Ge 1s and 2s photoelectron spectra are analyzed using various models to interpret the measured spectral shapes. A detailed comparison of the results derived from the different approaches is presented. Copyright © 2006 John Wiley & Sons, Ltd.

Shakedown in core photoelectron spectra from aligned laser-excited Na atoms

In an experiment combining a state-of-the-art visible laser system with the vacuum ultraviolet beamline I411 at the storage ring MAX II, shakedown satellite lines in the 2p photoelectron emission of excited Na atoms have been observed. A close investigation of these lines shows a strong dependency of the line intensities from the linear polarization angle of the laser light. The shakedown electrons are preferably emitted into the direction of the laser electric field vector.

A theoretical study of the role of the hydrogen bond on core ionization of the water dimer

Motivated by the interest in using X-ray spectra for probing of hydrogen bonded networks we developed a quantum model for simulations of the electronic-vibrational profile of the X-ray core photoelectron spectrum of the water dimer. It is found that the potential surfaces of the donor and acceptor O1s core-ionized states of this system display a qualitative difference. Large gradients of the potential in the core ionized state along some intermolecular coordinates combined with small vibrational frequencies breaks down completely the harmonic approximation. The band profiles are therefore treated using a quasi-continuum approximation. The weak hydrogen bonding and the drastic change of water dimer potential under core ionization is responsible for the anomalously strong vibrational broadening: 0.4 eV for the acceptor band and 0.6 eV for the donor band. The core ionization of the donor oxygen is accompanied by proton transfer which should be observable in X-ray fluorescence or Auger spectra.

Oxidation state of chromium associated with cell surfaces of Shewanella oneidensis during chromate reduction

Employing electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we demonstrate that in both aerobic and anaerobic culture Shewanella oneidensis cells are capable of chromate reduction. No Cr(VI) or Cr(V) species were identified at the cell surfaces in Cr 2p 3/2 core photoelectron spectra. More chromium was associated with cell surfaces recovered from anaerobic medium than aerobic. Multiplet-splitting models derived for Cr(III) and Cr(IV) were employed to determine contributions from each ion to Cr 2p 3/2 photopeaks collected from the various cell treatments. Whilst in all cases Cr(III) was the major ion associated with cell surfaces, a significant contribution was identified due to Cr(IV) in anaerobically grown cells. The Cr(IV) contribution was far less when cells were grown aerobically. Moreover, when anaerobically grown cells were exposed to oxygen very little re-oxidation of Cr-precipitates occurred, the precipitates were again identified as a mixture of Cr(III) and Cr(IV). A positive relationship was observed between amounts of chromium and phosphorous associated with cell surfaces resulting from the various treatments, suggesting the precipitates included Cr(III)-phosphate. The fact that Cr(IV) remained associated with precipitates following re-oxidation suggests that under anaerobic conditions the intermediate ion is afforded sufficient stability to be incorporated within the precipitate matrix and thus conferred a degree of protection from oxidation.

Observation of disorder broadening of core photoelectron spectra of CuZn alloys

The distribution of site potentials in CuZn alloys is investigated using x-ray-excited photoelectron spectroscopy. A small `disorder broadening' of the core-level photoelectron spectra is observed for the alloys. Experimental results are compared with results of model and first-principles calculations.

Laser ablation deposition of silicon nanostructures

Pulsed laser deposition with an off-axis geometry in a low-pressure argon ambient was used to produce films of nanosized silicon clusters or crystallites. Size selection was achieved by the distance of the deposit from the target. The local film structure and the crystallite size of the films were studied as a function of the distance from the target using micro-Raman spectroscopy. The nanocrystal size on the off-axis placed substrate varied in the range 1…5 nm, increasing with increasing distance from the target; this made it possible to achieve size selection of the nanocrystals. The surface morphology of the deposits was studied using scanning electron microscopy. Room temperature photoluminescence of the samples with 514.5 nm excitation was rather weak. Silicon 2p core photoelectron spectra showed the existence of a silicon dioxide layer on the surface of the nanoparticles.

Photoelectron spectra of metallofullerenes, GdC 82 and La 2C 80: Electron transfer from the metal to the cage

Ultraviolet photoelectron spectra of metallofullerenes, GdC 82 and La 2C 80 were measured with synchrotron orbital radiation. The specimens were dried-up films prepared from their solution instead of the sublimed films commonly adopted for the measurement. GdC 82 gave structure rich spectra reflecting its electronic structure. The band intensity of the GdC 82 spectra depends on the incident photon energy, as observed in other fullerenes. The similarity of the GdC 82 spectra to those of LaC 82 indicates that their electronic structures are analogous. The spectrum of La 2C 80 does not show any characteristic structures that have been observed in those of other fullerenes, and it resembles that of amorphous carbon. The C ls core photoelectron spectra of these metallofullerenes obtained with the Mg Kα radiation are also presented.

Highly correlated QDPT-CI calculations of valence and core photoelectron spectra of Ne

Quasidegenerate perturbation theory within the configuration interaction approach (QDPT-CI) is applied to the calculation of the valence and core photoelectron spectra of the Ne atom. A detailed investigation on the requirements of basis set and correlation level for convergent calculations is performed relative to the main ionizations, showing the importance of the inclusion of high angular momentum functions in the basis set; also the contribution of core valence correlation is pointed out. A basis set limited to f functions enlarged to Rydberg components is used to analyse the convergence of the results in a series of QDPT-CI calculations with a zero-order space R progressively enlarged, allowing to obtain absolute errors on the energies within 0.2 eV in the entire manifold of the ionic states. The well-defined scheme 2h-1p/4h-3p QDPT-CI appears adequate to describe quantitatively both the energies and intensities of the core and valence satellite structure of Ne and can represent a good compromise for applications to larger systems.

Spin Polarization of Photoelectrons from Si(001)

Si 2p core photoelectron spectra are calculated for Si(001) for different elliptical polarization of the incident radiation and normal incidence. The degree of the elliptical polarization is varied continuously from right to left circularly polarized light including different cases of linear polarization [100], [110], [010] and [110]. Due to the surface the cubic C 4 screw axis and the (100) glide plane disappear. There remain a C 2 axis and the mirror planes (100) and (110). Thus we get different intensities for the polarizations [110] and [110], respectively. Due to the mirror planes (Φ 1 = 45°;135°) the relation P(θ, Φ 1 + Φ, ∈(α,ζ)) = -P(θ, Φ 1 - Φ, ∈ (2Φ 0 - α, -ζ)) connects the intensities of spin up and spin down electrons. The angles α and ζ describe the direction of the polarization. This relation is an essential view of the spin polarization. Results are presented in dependence on the azimuth angle Φ for fixed polar angle θ with respect to the surface normal direction.

The vibrationally resolved C 1s core photoelectron spectra of methane and ethane

Recent progress in the development of high-resolution electron spectrometers in combination with highly monochromatized undulator radiation has allowed observation of the vibrationally resolved gas-phase C 1s photoelectron spectra of methane and ethane. For both molecules, the C–H stretching modes are well resolved and for ethane the active C–C stretching mode has been observed for the first time. The spectra have been measured at low kinetic energies and detailed fittings using post-collision interaction line profiles have been made both, using a free parameter fit and a fit adhering to a linear coupling model. The free parameter fit allows for any anharmonicity in the vibrational energies. The linear coupling model, on the other hand, assumes that the initial and final state potential curves are harmonic and differ only in the normal coordinates. This simple model is used to reduce the number of free parameters in the fit, which greatly simplifies the analysis. An intensity model based on the linear coupling predicts that the intensities of the C–H stretching modes are directly related to the number of C–H bonds around the core ionized atom. The result is verified for ethane and shows a potential for further reduction of free parameters for large molecules and polymers. Ab initio calculations of molecular geometry and vibrational frequencies have also been carried out using the equivalent core (Z+1) approximation. The values predicted for the decrease in bond length have then been compared to those determined empirically by the linear coupling approach. The calculation of ethane indicates that symmetric C–H and C–C stretching modes are important upon core ionization. The corresponding vibrational frequencies have been calculated and agree well with observed values.

Spin-polarized core photoelectron diffraction from magnetic and non-magnetic surface layers

A method is considered to calculate core photoelectron spectra including multiple scattering contributions and all curved wave corrections. Furthermore this method allows to calculate the spectra in dependence on the polarization (linearly or circularly) of the X-ray beam and on the spin-polarization of the photoelectrons. The results show that the spin-polarized spectra give more detailed information on the structure of the surface layer. Results are shown for the Sb 4 d 3 2 and 4 d 5 2 spectra of the Sb InP(110) surface where we found a strong circular dichroism of the spin-polarization. Furthermore the system Cr Ag(100) is investigated where experimental spectra are available for the Cr 2 p 1 2 level. In this case it is shown to what extent the spectra depend on the orientation of the magnetization of the Cr layers besides the dichroism. The results should encourage experimentalists to do spin-polarized measurements with circularly polarized light.

Correlation effects in core and valence photoelectron spectra of alkene molecules

The core, the inner and outer valence ionization spectra of propene, 1-butene, 1-pentene, cis-butene, trans-butene and iso-butene have been calculated at the ab initio configuration interaction (CI) level, in order to analyze the influence of different substitutions on the double bond on the appearance of the spectra. The CI schemes adopted, i.e. 3h-2v(π) + 2h − 1p CI for the core part, 2h−1v CI for the valence part and 2h−1p CI for core and valence, have proven to be adequate for the description of correlation effects which cause the appearance of complex structures in the spectra. Satisfactory reproduction of the experimental features has been achieved and for their simplicity the CI schemes adopted can be put forward for the calculation of the spectra of larger molecules of the same type which allows a clearer insight of the evolution of correlation effects with the increasing complexity of the systems.