Photoionization from the Xe 4d orbitals of XeF2.

We present a comparison of the photoionization dynamics of the 4d shell of XeF2 from threshold to 250 eV to those of the prototypical 4d shell of atomic Xe. The new experimental data include spin-orbit and ligand-field-resolved partial cross sections, photoelectron angular distributions, branching fractions, and lifetime widths for the 4d-hole states. The spin-orbit branching fractions and angular distributions are remarkably similar to the corresponding distributions from atomic Xe across a broad energy interval that includes both the intense shape resonance in the f continuum and a Cooper minimum in the same channel. The angular distributions and branching fractions are also in reasonably good agreement with our first-principles theoretical calculations on XeF2. Data are also presented on the lifetime widths of the substate-resolved 4d-hole states of XeF2. While the trends in the widths are similar to those in the earlier experimental and theoretical work, the linewidths are considerably smaller than in the previous measurements, which may require some reinterpretation of the decay mechanism. Finally, we present new data and an analysis of the Auger electron spectra for ionization above the 4d thresholds and resonant Auger spectra for several pre-edge features.

Valence shell photoelectron angular distributions and vibrationally resolved spectra of imidazole: A combined experimental-theoretical study.

Linearly polarized synchrotron radiation has been used to record polarization dependent valence shell photoelectron spectra of imidazole in the photon energy range 21-100 eV. These have allowed the photoelectron angular distributions, as characterized by the anisotropy parameter ß, and the electronic state intensity branching ratios to be determined. Complementing these experimental data, theoretical photoionization partial cross sections and ß-parameters have been calculated for the outer valence shell orbitals. The assignment of the structure appearing in the experimental photoelectron spectra has been guided by vertical ionization energies and spectral intensities calculated by various theoretical methods that incorporate electron correlation and orbital relaxation. Strong orbital relaxation effects have been found for the 15a', nitrogen lone-pair orbital. The calculations also predict that configuration mixing leads to the formation of several low-lying satellite states. The vibrational structure associated with ionization out of a particular orbital has been simulated within the Franck-Condon model using harmonic vibrational modes. The adiabatic approximation appears to be valid for the X 2A″ state, with the ß-parameter for this state being independent of the level of vibrational excitation. However, for all the other outer valence ionic states, a disparity occurs between the observed and the simulated vibrational structure, and the measured ß-parameters are at variance with the behavior expected at the level of the Franck-Condon approximation. These inconsistencies suggest that the excited electronic states may be interacting vibronically such that the nuclear dynamics occur over coupled potential energy surfaces.

Photoionization dynamics of cis-dichloroethene from investigation of vibrationally resolved photoelectron spectra and angular distributions.

The influence of vibronic coupling on the outer valence ionic states of cis-dichloroethene has been investigated by recording photoelectron spectra over the excitation range 19-90 eV using plane polarized synchrotron radiation, for two polarization orientations. The photoelectron anisotropy parameters and electronic state branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that the photoionization dynamics of the Ã2B2, B̃2A1, C̃2A2, and D̃2B1 states, all of which are formed through the ejection of an electron from a nominally chlorine lone-pair orbital, exhibit distinct evidence of the Cooper minimum associated with the halogen atom. While retaining a high degree of atomic character, these orbital ionizations nevertheless display clear distinctions. Simulations, assuming the validity of the Born-Oppenheimer and the Franck-Condon approximations, of the X̃2B1, Ã2B2, and D̃2B1 state photoelectron bands have allowed some of the vibrational structure observed in the experimental spectra to be assigned. The simulations provide a very satisfactory interpretation for the X̃2B1 state band but appear less successful for the Ã2B2 and D̃2B1 states, with irregularities appearing in both. The B̃2A1 and C̃2A2 state photoelectron bands exhibit very diffuse and erratic profiles that cannot be reproduced at this level. Photoelectron anisotropy parameters, ß, have been evaluated as a function of binding energy across the studied photon energy range. There is a clear step change in the ß values of the Ã2B2 band at the onset of the perturbed peak intensities, with ß evidently adopting the value of the B̃2A1 band ß. The D̃2B1 band ß values also display an unexpected vibrational level dependence, contradicting Franck-Condon expectations. These various behaviors are inferred to be a consequence of vibronic coupling in this system.

An experimental and theoretical study of the photoelectron spectra of cis-dichloroethene: Valence shell vertical ionization and vibronic coupling in the low-lying cationic states.

The valence shell photoelectron spectrum of cis-dichloroethene has been studied both experimentally and theoretically. Photoelectron spectra have been recorded with horizontally and vertically plane polarized synchrotron radiation, thereby allowing the anisotropy parameters, characterizing the angular distributions, to be determined. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function has been employed to compute the complete valence shell ionization spectrum. In addition, the vertical ionization energies have been calculated using the outer valence Green's function (OVGF) method and the equation-of-motion coupled-cluster, with single and double substitutions for calculating ionization potentials (EOM-IP-CCSD) model. The theoretical results have enabled assignments to be proposed for most of the structure observed in the experimental spectra, including the inner-valence regions dominated by satellite states. The linear vibronic coupling model has been employed to study the vibrational structure of the lowest photoelectron bands, using parameters obtained from ab initio calculations. The ground state optimized geometries and vibrational frequencies have been computed at the level of the second-order Møller-Plesset perturbation theory, and the dependence of the ionization energies on the nuclear configuration has been evaluated using the OVGF method. While the adiabatic approximation holds for the X̃2B1 state photoelectron band, the Ã2B2, B̃2A1, and C̃2A2 states interact vibronically and form a complex photoelectron band system with four distinct maxima. The D̃2B1 and Ẽ2B2 states also interact vibronically with each other. The potential energy surface of the D̃2B1 state is predicted to have a double-minimum shape with respect to the out-of-plane a2 deformations of the molecular structure. The single photoelectron band resulting from this interaction is characterized by a highly irregular structure, reflecting the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces forming a conical intersection close to the minimum of the Ẽ2B2 state.

An experimental and theoretical study of the valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine.

The valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine have been studied both experimentally and theoretically. Synchrotron radiation has been employed to record angle resolved photoelectron spectra in the photon energy range 20-100 eV, and these have enabled anisotropy parameters and branching ratios to be derived. The experimental results have been compared with theoretical predictions obtained using the continuum multiple scattering Xα approach. This comparison shows that the anisotropy parameter associated with the nominally chlorine lone-pair orbital lying in the molecular plane is strongly affected by the atomic Cooper minimum. In contrast, the photoionization dynamics of the second lone-pair orbital, orientated perpendicular to the molecular plane, seem relatively unaffected by this atomic phenomenon. The outer valence ionization has been studied theoretically using the third-order algebraic-diagrammatic construction (ADC(3)) approximation scheme for the one-particle Green's function, the outer valence Green's function method, and the equation-of-motion (EOM) coupled cluster (CC) theory at the level of the EOM-IP-CCSD and EOM-EE-CC3 models. The convergence of the results to the complete basis set limit has been investigated. The ADC(3) method has been employed to compute the complete valence shell ionization spectra of 2-chloropyridine and 3-chloropyridine. The relaxation mechanism for ionization of the nitrogen σ-type lone-pair orbital (σN LP) has been found to be different to that for the corresponding chlorine lone-pair (σCl LP). For the σN LP orbital, π-π* excitations play the main role in the screening of the lone-pair hole. In contrast, excitations localized at the chlorine site involving the chlorine πCl LP lone-pair and the Cl 4p Rydberg orbital are the most important for the σCl LP orbital. The calculated photoelectron spectra have allowed assignments to be proposed for most of the structure observed in the experimental spectra. The theoretical work also highlights the formation of satellite states, due to the breakdown of the single particle model of ionization, in the inner valence region.

Ionization of pyridine: Interplay of orbital relaxation and electron correlation.

The valence shell ionization spectrum of pyridine was studied using the third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function and the outer-valence Green's function method. The results were used to interpret angle resolved photoelectron spectra recorded with synchrotron radiation in the photon energy range of 17-120 eV. The lowest four states of the pyridine radical cation, namely, 2A2(1a2-1), 2A1(7a1-1), 2B1(2b1-1), and 2B2(5b2-1), were studied in detail using various high-level electronic structure calculation methods. The vertical ionization energies were established using the equation-of-motion coupled-cluster approach with single, double, and triple excitations (EOM-IP-CCSDT) and the complete basis set extrapolation technique. Further interpretation of the electronic structure results was accomplished using Dyson orbitals, electron density difference plots, and a second-order perturbation theory treatment for the relaxation energy. Strong orbital relaxation and electron correlation effects were shown to accompany ionization of the 7a1 orbital, which formally represents the nonbonding σ-type nitrogen lone-pair (nσ) orbital. The theoretical work establishes the important roles of the π-system (π-π* excitations) in the screening of the nσ-hole and of the relaxation of the molecular orbitals in the formation of the 7a1(nσ)-1 state. Equilibrium geometric parameters were computed using the MP2 (second-order Møller-Plesset perturbation theory) and CCSD methods, and the harmonic vibrational frequencies were obtained at the MP2 level of theory for the lowest three cation states. The results were used to estimate the adiabatic 0-0 ionization energies, which were then compared to the available experimental and theoretical data. Photoelectron anisotropy parameters and photoionization partial cross sections, derived from the experimental spectra, were compared to predictions obtained with the continuum multiple scattering approach.

A fresh look at the photoelectron spectrum of bromobenzene: A third-order non-Dyson electron propagator study.

The valence-shell ionization spectrum of bromobenzene, as a representative halogen substituted aromatic, was studied using the non-Dyson third-order algebraic-diagrammatic construction [nD-ADC(3)] approximation for the electron propagator. This method, also referred to as IP-ADC(3), was implemented as a part of the Q-Chem program and enables large-scale calculations of the ionization spectra, where the computational effort scales as n(5) with respect to the number of molecular orbitals n. The IP-ADC(3) scheme is ideally suited for investigating low-lying ionization transitions, so fresh insight could be gained into the cationic state manifold of bromobenzene. In particular, the present IP-ADC(3) calculations with the cc-pVTZ basis reveal a whole class of low-lying low-intensity two-hole-one-particle (2h-1p) doublet and quartet states, which are relevant to various photoionization processes. The good qualitative agreement between the theoretical spectral profile for the valence-shell ionization transitions generated with the smaller cc-pVDZ basis set and the experimental photoelectron spectrum measured at a photon energy of 80 eV on the PLÉIADES beamline at the Soleil synchrotron radiation source allowed all the main features to be assigned. Some theoretical aspects of the ionization energy calculations concerning the use of various approximation schemes and basis sets are discussed.

The influence of the bromine atom Cooper minimum on the photoelectron angular distributions and branching ratios of the four outermost bands of bromobenzene.

Angle resolved photoelectron spectra of the X̃(2)B1, Ã(2)A2, B̃(2)B2, and C̃(2)B1 states of bromobenzene have been recorded over the excitation range 20.5-94 eV using linearly polarized synchrotron radiation. The photoelectron anisotropy parameters and electronic branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that ionization from the 8b2 orbital and, to a lesser extent, the 4b1 orbital is influenced by the Cooper minimum associated with the bromine atom. The 8b2 and 4b1 orbitals are nominally bromine lone-pairs, but the latter orbital interacts strongly with the π-orbitals in the benzene ring and this leads to a reduced atomic character. Simulations of the X̃(2)B1, B̃(2)B2, and C̃(2)B1 state photoelectron bands have enabled most of the vibrational structures appearing in the experimental spectra to be assigned. Many of the photoelectron peaks exhibit an asymmetric shape with a tail towards low binding energy. This asymmetry has been examined in the simulations of the vibrationally unexcited peak, due mainly to the adiabatic transition, in the X̃(2)B1 state photoelectron band. The simulations show that the asymmetric profile arises from hot-band transitions. The inclusion of transitions originating from thermally populated levels results in a satisfactory agreement between the experimental and simulated peak shapes.

Does additional exercise improve trunk function recovery in stroke patients? A meta-analysis.

BACKGROUND: The restoration of trunk function following stroke is a key component of rehabilitation, however there is limited evidence of the efficacy of additional trunk training. OBJECTIVES: To evaluate the efficacy of trunk exercises added to conventional rehabilitation on functional outcomes. METHODS: Relevant randomised controlled trials (RCTs), published up to July 2012, evaluating the effect of the addition of trunk exercises to conventional rehabilitation on functional outcomes were identified in Medline, Cinahl, Embase, Pubmed, PEDro, Web of Science and Scopus databases. Findings were summarised across studies as mean or standardised mean differences (MD or SMD) with 95% confidence intervals. RESULTS: Six RCTs with 155 participants and a mean PEDro score of 6.5 (range 6 to 8) were included. Data from two to five studies were pooled in meta-analyses that showed a moderate, non-significant effect of additional trunk exercise on trunk performance, (SMD = 0.50; 95% CI -0.25, 1.25; P = 0.19); large effects on standing balance, SMD = 0.72 (95% CI -0.01, 1.45 P = 0.05); and walking ability, (SMD = 0.81; 95% CI 0.30, 1.33. P = 0.002) and a small, non-significant effect, MD = 10.03 (95% CI -15.70, 35.75. P = 0.44) on functional independence. CONCLUSIONS: There is moderate evidence that the addition of specific trunk exercise to conventional early stroke rehabilitation significantly improve standing balance and mobility after stroke; however the evidence was weak for the effect of additional trunk exercise on trunk performance and in functional independence.

Theoretical study on the circular dichroism in core and valence photoelectron angular distributions of camphor enantiomers.

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xalpha methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.

An angle resolved electron-ion recoil vector correlation study of alternate ion dissociation channels in A CF3I+

Electron-ion recoil vector correlations are examined for the ionization and subsequent dissociation of A state CF3I+. The magnitude of the electron and fragment ion recoil vectors permits the energetics of two alternative decays to I+ and CF3+ to be compared, while differences between the angular correlations are interpreted as molecule-frame photoelectron angular distributions which, in the I+ channel, are smeared by molecular rotation between ionization and dissociation. Quantitative estimates of sub-ps I+ decay lifetimes are extracted, indicating very different decay rates for the alternative dissociation channels. Surprisingly, the ka1 and ke photoelectron continua exchange polarization dependence in the I+ channel correlations and vibronic interactions are postulated in explanation. This can also rationalize the non-adiabatic A CF3I+-->I+ decay mechanism and the branching competition between the CF3+ and I+ channels.