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Shape resonances in physics and chemistry arise from the spatial confinement of a particle by a potential barrier. In molecular photoionization, these barriers prevent the electron from escaping instantaneously, so that nuclei may move and modify the potential, thereby affecting the ionization process. By using an attosecond two-color interferometric approach in combination with high spectral resolution, we have captured the changes induced by the nuclear motion on the centrifugal barrier that sustains the well-known shape resonance in valence-ionized N$_2$. We show that despite the nuclear motion altering the bond length by only $2\%$, which leads to tiny changes in the potential barrier, the corresponding change in the ionization time can be as large as $200$ attoseconds. This result poses limits to the concept of instantaneous electronic transitions in molecules, which is at the basis of the Franck-Condon principle of molecular spectroscopy., 24 pages, 5 figures

Binding of a single water molecule has a dramatic effect on the excited state lifetime of adenine. Here we report a joint nonadiabatic dynamics and reaction paths study aimed at understanding the sub-100 fs lifetime of adenine in the monohydrates. Our nonadiabatic dynamics simulations, performed using the ADC(2) electronic structure method, show a shortening of the excited state lifetime in the monohydrates with respect to bare adenine. However, the computed lifetimes were found to be significantly longer that the observed one. By comparing the reaction pathways of several excited state deactivation processes in adenine and adenine monohydrates, we show that electron-driven proton transfer from water to nitrogen atom N3 of the adenine ring may be the process responsible for the observed ultrafast decay. The inaccessibility of the electron-driven proton transfer pathway to trajectory-based nonadiabatic dynamics simulation is discussed.

The first comprehensive investigation of the effect of conformational flexibility of gaseous D-cycloserine on the valence and core electronic structures is reported here. The seven most stable conformers among the twelve structures calculated at the MP2/6-311++G** level of theory were assumed to properly describe the properties of the investigated compound. Taking into account the contribution of these isomers, the valence photoelectron spectrum (UPS) was simulated by the Outer Valence Green' s Function (OVGF) method. A different sensitivity towards the conformational flexibility of the outermost photoelectron bands was exhibited in the simulated spectrum. The comparison of the theoretical UPS with the experimental one allowed a detailed assignment of the outermost valence spectral region. The composition and bonding properties of the relevant MOs of the most stable conformers were analyzed in terms of leading Natural Bond Orbital (NBO) contributions to the HF/6-311++G** canonical MOs. The C1s, N1s, and O1s photoelectron spectra (XPS) were theoretically simulated by calculating the vertical Ionization Energies (IEs) of the relevant conformers using the ΔSCF approach. The different IE chemical shift spread of the XPS components associated with various conformers, which is expected to affect the experimental spectra, could be evaluated by simulated XPS, thus providing a new insight into the core electronic structure. The comparison of the theoretical results with the experimental ones unraveled that the atomic XPS components are not mixed by conformational flexibility of D-cycloserine, and that the specific vibronic structure of different spectral components should play a crucial role in determining different relative intensities and band shapes observed in the experiment.

Carbon $1s$ photoelectron spectra of CO molecules in gas phase were recorded in the tender x-ray energy range, from 2.3 to 6.9 keV. The intensity ratios of individual peaks from $\ensuremath{\nu}=0$ to 3 within the vibrational progression of the C $1s$ photoelectron spectrum were determined at the various photon energies and are shown to be strongly affected by the photoelectron recoil effect. The experimental vibrational intensity ratios are compared with theoretical predictions at different levels of accuracy. Developments of the recoil model, using generalized Franck-Condon factors, rovibrational coupling, Morse potential energy curves, and accurate angular averaging are presented and applied to the analysis of the experimental results.

Recently we reported a series of numerical simulations proving that it is possible in principle to create an electronic wave packet and subsequent electronic motion in a neutral molecule photoexcited by a UV pump pulse within a few femtoseconds. We considered the ozone molecule: for this system the electronic wave packet leads to a dissociation process. In the present work, we investigate more specifically the time-resolved photoelectron angular distribution of the ozone molecule that provides a much more detailed description of the evolution of the electronic wave packet. We thus show that this experimental technique should be able to give access to observing in real time the creation of an electronic wave packet in a neutral molecule and its impact on a chemical process., Accepted in Scientific Report

After sudden ionization of a large molecule, the positive charge can migrate throughout the system on a sub-femtosecond time scale, purely guided by electronic coherences. The possibility to actively explore the role of the electron dynamics in the photo-chemistry of bio-relevant molecules is of fundamental interest for understanding, and perhaps ultimately controlling, the processes leading to damage, mutation and, more generally, to the alteration of the biological functions of the macromolecule. Attosecond laser sources can provide the extreme time resolution required to follow this ultrafast charge flow. In this review we will present recent advances in attosecond molecular science: after a brief description of the results obtained for small molecules, recent experimental and theoretical findings on charge migration in bio-relevant molecules will be discussed, We acknowledge the support from the European Research Council under the ERC grants no. 637756 STARLIGHT, no. 227355 ELYCHE and no. 290853 XCHEM, LASERLABEUROPE (grant agreement no. 284464, European Commissions Seventh Framework Programme), European COST Action CM1204 XLIC, the Ministerio de Ciencia e Innovación project FIS2013-42002-R, European grants MC-ITN CORINF and MC-RG ATTOTREND 268284, UKs Science and Technology Facilities Council Laser Loan Scheme, the Engineering and Physical Sciences Research Council (grant EP/J007048/ 1), the Leverhulme Trust (grant RPG-2012-735), and the Northern Ireland Department of Employment and Learning

The sudden ionization of a molecule by an attosecond pulse is followed by charge redistribution on a time scale from a few femtoseconds down to hundreds of attoseconds. This ultrafast redistribution is the result of the coherent superposition of electronic continua associated with the ionization thresholds that are reached by the broadband attosecond pulse. Thus, a correct theoretical description of the time evolution of the ensuing wave packet requires the knowledge of the actual ionization amplitudes associated with all open ionization channels, a real challenge for large and medium-size molecules. Recently, the first calculation of this kind has come to light, allowing for interpretation of ultrafast electron dynamics observed in attosecond pump–probe experiments performed on the amino acid phenylalanine [Calegari et al., Science 2014, 346, 336]. However, as in most previous theoretical works, the interpretation was based on various simplifying assumptions, namely, the ionized electron was not included in the description of the cation dynamics, the nuclei were fixed at their initial position during the hole migration process, and the effect of the IR probe pulse was ignored. Here we go a step further and discuss the consequences of including these effects in the photoionization of the glycine molecule. We show that (i) the ionized electron does not affect hole dynamics beyond the first femtosecond, and (ii) nuclear dynamics has only a significant effect after approximately 8 fs, but does not destroy the coherent motion of the electronic wave packet during at least few additional tens of fs. As a first step towards understanding the role of the probe pulse, we have considered an XUV probe pulse, instead of a strong IR one, and show that such an XUV probe does not introduce significant distortions in the pump-induced dynamics, suggesting that pump–probe strategies are suitable for imaging and manipulating charge migration in complex molecules. Furthermore, we show that hole dynamics can be changed by shaping the attosecond pump pulse, thus opening the door to the control of charge dynamics in biomolecules.

We describe, in detail, a basis set approach to the multichannel scattering problem. The full set of linearly independent scattering states at each prefixed energy of the continuum spectrum can be obtained via a least-squares approach. To test the algorithm in a concrete setup, we report a parallel implementation of the close-coupling method in which the final states are treated within the configuration interaction singles (CIS) approximation. The method requires, as input, a set of orthonormal orbitals, obtained from any quantum chemistry package. A one-center expansion (OCE) basis set consisting of products of radial B-splines and symmetry adapted angular functions is then used to expand the continuum electron wave function. To assess the quality of the CIS approximation, we compute total and partial cross sections and angular asymmetry parameters for the photoionization of a selection of closed-shell atoms (He, Ne, and Ar), H-2, H2O, and ethylene. Results are compared with the experimental data and with theoretical predictions obtained with time-dependent density functional theory (TDDFT). It is seen that, generally, the photoionization observables obtained at the CIS level compare well with TDDFT predictions. The same basis can be employed to describe molecular multiphoton or strong field ionization.

Photoelectron asymmetry parameters (β) of the gerade and ungerade C 2s−1 derived states of ethyne, ethene and ethane as a function of photon energy have been calculated and experimentally measured, to extend the search of interference effects on angular distributions to polyatomic molecules. The calculations cover the electron energy range from 0 to 1100 eV while the experimental measurements cover the electron energy range from 30 to 220 eV. Clear oscillations are interpreted in terms of interference of the photoelectron wave emitted from the two possible C 2s centres, or equivalently from the gerade and ungerade states associated with them. This is a microscopic analog of Young's double-slit experiment. The effect is however quite small and requires very high experimental accuracy to be detected. It is best evidenced in the behaviour of β difference between the two channels. The connection between β trends and structural parameters shows the expected inverse correlation between oscillation period and distance between the carbon atoms, but do not simply parallel the analogous behaviour found in cross sections.

The excitation wavelength dependent photodynamics of pyrrole are investigated by nonadiabatic trajectory-surface-hopping dynamics simulations based on time dependent density functional theory (TDDFT) and the algebraic diagrammatic construction method to the second order (ADC(2)). The ADC(2) results confirm that the N-H bond dissociation occurring upon excitation at the origin of the first excited state, S-1(pi sigma*), is driven by tunnelling [Roberts et al., Faraday Discuss., 2013, 163, 95] as a barrier of DE = 1780 cm(-1) traps the population in a quasi-bound minimum. Upon excitation to S-1(pi sigma*) in the wavelength range of 236-240 nm, direct dissociation of the N-H bond takes place with a time constant of 28 fs. The computed time constant is in very good agreement with recently reported measurements. Excitation to the lowest B-2 state in the 198-202 nm range returns a time constant for N-H fission of 48 fs at the B3LYP/def2-TZVPD level, in perfect agreement with the experiment [Roberts et al. Faraday Discuss., 2013, 163, 95]. For the same wavelength range the ADC(2)/aug-cc-pVDZ decay constant is more than three times longer than the experimentally reported one. The accuracy of the B3LYP/def2-TZVPD dynamics is checked against reference complete-active-space second-order perturbation theory (CASPT2) calculations and explained in terms of correct topography of the pi pi* surface and the lack of mixing between the pi pi* and the 3p(x) Rydberg states which occurs in the ADC(2) method.

A joint experimental and theoretical approach has been used to study the valence shell electronic structure and photoionisation dynamics of s-triazine (1,3,5-triazine). Synchrotron radiation has been employed to record angle resolved photoelectron spectra of the complete valence shell for photon energies between 17.5 and 100 eV, thereby allowing photoelectron anisotropy parameters and branching ratios to be determined. Absolute photoionisation partial cross sections have been estimated as the product of these branching ratios and the absolute photoabsorption cross section. The Kohn–Sham and the time-dependent version of density functional theory methods have been used to calculate photoelectron anisotropy parameters and photoionisation partial cross sections, and these have been compared with the corresponding experimental data. The calculations predict that shape resonances affect the photoionisation dynamics of several of the molecular orbitals. The angle resolved photoelectron spectra suggest that the 1 a 2 ″ (π) orbital is more tightly bound than the 5e′(σ) orbital, and that the 4 a 1 ′ (σ) orbital is more tightly bound than the 1 a 2 ′ (σ) orbital, in agreement with the predicted molecular orbital sequence. For the outer valence orbitals the single-particle picture of ionisation holds but electron correlation becomes increasingly important for the inner valence 4e′, 3e′ and 3 a 1 ′ orbitals and leads to a redistribution of intensity. Instead of a readily distinguishable main-line, associated with each of these orbitals, the photoelectron intensity is spread over numerous satellite states and the resulting band is broad and featureless.

The most recent advances in the theoretical description of molecular photoionization by means of the Density Functional Theory with B-splines basis functions is reviewed. From the methodological point of view, the formalism of the Time Dependent Density Functional Theory applied to molecular continuum is considered, with a new implementation based on a non-iterative algorithm. Also, applications of the Kohn–Sham method for the calculation of dichroism in photoionization of chiral molecules and of dynamical parameters beyond the dipole approximation are considered. The methods have proved successful to accurately reproduce experimental data and to suggest reliable assignments to observed spectral features.

The time-dependent density functional theory (TD-DFT) has been applied to study the cross-section and asymmetry parameter profiles of valence and core photoionization dynamic processes of acetylene. The effect of the interaction between different ionisation channels has been analysed by the comparison of the TD-DFT results and the results from the simpler Kohn–Sham (KS) scheme. The significant structures of the valence region arising from indirect photoionization processes such as autoionization have been reproduced by the TD-DFT approach and the major calculated features have been compared with the experimental data. The photoionization from the two symmetry resolved initial states (1σg and 1σu) of the core C1s have been considered; the calculated intensity ratio is found in good agreement with the recent experimental results. The account of the multichannel interaction given by TD-DFT proves substantially accurate in reproducing the main features observed in the photoionization continuum of the acetylene molecule.

The B-spline least-squares approach for the determination of the continuum wavefunctions, which proves to be very accurate and numerically stable in the non-relativistic case has been generalized to the relativistic case. The least-squares scheme has been tested in the free-particle and Coulomb problems. Expression of relativistic Coulomb functions in terms of the corresponding non relativistic ones is spelled out in detail and employed for testing the spline solutions. The convergence properties of the algorithm with respect to the B-spline order and the radial grid are also analyzed.

The B-spline density functional method for the electronic continuum is employed to calculate the cross section and the asymmetry parameter profiles of valence and core photoionization in M@C60 (M = Be, Mg and Ca). For the valence photoionization significant differences are identified through the series and oscillations similar to those observed in the HOMO/HOMO-1 ratio in C60 have been calculated for Ca@C60. Core M 1s and Ca 2p ionizations are studied with the help of the `dipole prepared' continuum orbitals, which allows the identification of the leading mechanisms which give rise to the characteristic sharp and intense shape resonances, typical of these compounds.

The time dependent-density functional theory (TD-DFT) has been applied to study the photoionization from the two symmetry resolved initial states (1σg and 1σu) of the core N 1s in N2. The results are compared with very recent experimental data and with previous ab initio calculations. The calculated intensity ratio is in excellent agreement with the experiment and performs better than previous calculations. Good quantitative results are also obtained for the absolute partial cross-section. The calculated asymmetry parameters are also predicted to be very different in the two contributions, so a new experiment to investigate the angular distribution of the photoelectron is suggested.

Vibrationally resolved molecular frame photoelectron angular distributions (MFPADs) of fixed-in-space molecules have been evaluated for diatomic and polyatomic molecules. Calculations have been performed by using an extension of the static-exchange density functional theory formerly developed by P. Decleva [1] and coworkers and extended in order to include the nuclear motion in the Born-Oppenheimer approximation. The method proved to be very accurate for diatomic molecules [2, 3, 5], particularly at high energy of the photoelectron. In this work, we present the results obtained for the inner shell photoionization of C2H2, NH3, CH4, CF4, BF3 and SF6.

This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on Volume 346, 17 october 2014, DOI: 10.1126/science.1254061., In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub-4.5-fs temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems, We acknowledge the support from the European Research Council under the ERC grants no. 227355 ELYCHE and 290853 XCHEM, from LASERLAB-EUROPE (grant agreement no. 284464, EC's Seventh Framework Programme), from European COST Action CM1204 XLIC, the MICINN Project FIS2010-15127, the ERA-Chemistry Project PIM2010EEC-00751, the European Grants MC-ITN CORINF and MC-RG ATTOTREND 268284, the UK’s STFC Laser Loan Scheme, the Engineering and Physical Sciences Research Council (grant EP/J007048/1), the Leverhulme Trust (grant RPG-2012-735), and the Northern Ireland Department of Employment and Learning

We report evidence of intramolecular scattering occurring in inner shell photoionization of small molecules. Pronounced oscillations of the ratios between vibrationally resolved cross sections (v-ratios) as a function of photon energy have been observed theoretically and experimentally. Qualitative agreement with a 1st Born model confirms that they are due to intramolecular scattering: when an electron is ejected from a very localized region in the center of a polyatomic molecule, such as the C(1s) orbital in a CF4 molecule, it is diffracted by the surrounding atomic centers, encoding the geometry of the molecule [1, 2].

The differential photoionization cross section ratio (? = 1)/(? = 0) for the symmetric stretching mode in the C 1s photoionization of CF4 was studied both theoretically and experimentally. We observed this ratio to differ from the Franck?Condon ratio and to be strongly dependent on the photon energy, even far from the photoionization threshold. The density-functional theory computations show that the ratio is significantly modulated by the diffraction of the photoelectrons by the neighbouring atoms at high photon energies. At lower energies, the interpretation of the first very strong maximum observed about 60?eV above the photoionization threshold required detailed calculations of the absolute partial cross sections, which revealed that the absolute cross section has two maxima at lower energies, which turn into one maximum in the cross section ratio because the maxima appear at slightly different energies in ? = 1 and ? = 0 cross sections. These two strong, low-energy continuum resonances originate from the trapping of the continuum wavefunction in the molecular potential of the surrounding fluorine atoms and from the outgoing electron scattering by them.

F1s and C1s photoelectron angular distributions are considered for CH3F, a molecule which does not support any shape resonance. In spite of the absence of features in the photoionization cross section profile, the recoil frame photoelectron angular distributions (RFPADs) exhibits dramatic changes depending on both the photoelectron energy and polarization geometry. Time-dependent density functional theory calculations are also given to rationalize the photoionization dynamics. The RFPADs have been compared with the theoretical calculations, in order to assess the accuracy of the theoretical method and rationalize the experimental findings. The effect of finite acceptance angles for both ionic fragments and photoelectrons has been included in the calculations, as well as the effect of rotational averaging around the fragmentation axis. Excellent agreement between theory and experiment is obtained, confirming the good quality of the calculated dynamical quantities (dipole moments and phase shifts). (C) 2014 AIP Publishing LLC.

The present work concerns the study of high-energy structures in the photoionization of Mg and Be metallocenes due to photoelectron diffraction. The influence of geometrical structure is studied by varying the metal–ring distance in MgCp2, as well as that in the permethylated compounds MgCp2* and BeCp2*. The cross section ratios relative to the two outermost valence ionizations have been studied and found to be very sensitive to the value of the metal–ring distance and to be able to resolve ambiguities in present experimental values. Further differences are attributed to minor changes in the electronic structure. The results confirm that long-range oscillations in molecular photoemission cross sections constitute a general phenomenon and are an easily measurable observable that can be used to obtain important information on the geometric and electronic structure of the target.

Interference effects in the outer valence ionization cross sections have been theoretically investigated in simple hydrocarbons, and found comparable to those observed in core and inner valence ionizations. Notably short period oscillations give strong evidence of molecular orbital delocalization over the entire molecule. Also in the case of unresolved ionizations, interference patterns still survive in band ratios and convey useful information.

The HOMO/HOMO-1 ratio of the partial photoionization cross-sections in C 60 is computed to high accuracy employing a convergent one-centre expansion and a local density approximation (LDA) hamiltonian over an extended energy range. Excellent agreement is obtained with available experimental data in the low energy range. The high energy behaviour shows no sign of predicted amplitude modulation or of high energy damping. A modulation in the oscillating frequency is instead clearly revealed, consistent with the few available high energy data, which suggest however a more regular increase with increasing energy.

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.

We have measured the angular correlation between the B K-VV Auger electrons of BF(3) molecules and the coincident fragment-ion pairs of BF(2)(+)-F(+). Then, we have found that the measured angular correlation patterns depending on the mutual angle between the light polarization direction and molecular orientation are affected by the anisotropic axis distribution of the molecular ensemble of BF(3)(+) reflecting the anisotropic nature of photon-molecule interaction. In this context, we have pointed out generally that for coincidence experiments, so-called molecular frame Auger electron angular distributions are realized only if the axis distribution of the molecular ion ensemble is isotropic.

A comparison between theoretical and experimental results for the photoelectron intensity and circular dichroism is reported for the valence electron states of the metal complex Delta-Cobalt(III) tris-acetylacetonate. The measured intensities of the valence features and their dichroic behaviour are reproduced in the time dependent density functional theory (TDDFT) framework. The satisfactory agreement between theoretical and experimental results in the photon energy range 15-29 eV proves that the TDDFT procedure is suitable for describing electronic processes and circular dichroism.

Photoionization cross sections are evaluated at the local density level avoiding any potential shape approximation by employing the Stieltjes imaging (ST) technique in conjunction with large (STO ) basis LCAO calculations. The ST technique proves accurate in comparison with full continuum calculation on the noble gases ionization. Several choices for the final-state potentials, including different exchange-correlation potentials, are tested on the noble gases, as well as in H{sub 2}O and CO. Finally, several molecules are investigated employing the transition-state X{alpha} potential and the results compared with available experimental data and previous calculations. The accuracy of the proposed approach appears quite comparable to the ab initio static-exchange level, removing spurious resonances associated to the muffin-tin approximation, and should prove useful in the analysis and interpretation of cross-section data for large systems, where it is shown that basis-set requirements become less stringent. The accuracy obtainable for the cross-section profiles allows close comparison with experimental data, which, although showing good general agreement for the shape of the cross-section profile, indicates the need of a further refinement of the theoretical model. 55 refs., 15 figs., 1 tab.

Here we present a comparative computational study of the photoionization of furan from the ground and the two lowest-lying excited electronic states. The study aims to assess the quality of the computational methods currently employed for treating bound and continuum states in photoionization. For the ionization from the ground electronic state, we show that the Dyson orbital approach combined with an accurate solution of the continuum one particle wave functions in a multicenter B-spline basis, at the density functional theory (DFT) level, provides cross sections and asymmetry parameters in excellent agreement with experimental data. On the contrary, when the Dyson orbitals approach is combined with the Coulomb and orthogonalized Coulomb treatments of the continuum, the results are qualitatively different. In excited electronic states, three electronic structure methods, TDDFT, ADC(2), and CASSCF, have been used for the computation of the Dyson orbitals, while the continuum was treated at the B-spline/DFT level. We show that photoionization observables are sensitive probes of the nature of the excited states as well as of the quality of excited state wave functions. This paves the way for applications in more complex situations such as time resolved photoionization spectroscopy.

This is an author-created, un-copyedited version of an article published in Journal of physics B: atomic molecular and optical physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/0953-4075/45/19/194008, Vibrationally resolved photoelectron angular distributions from randomly oriented and fixed-in-space N 2 and CO molecules have been evaluated by using an extension of the static-exchange density functional theory that includes the nuclear motion. Both K-shell and valence-shell photoionization have been considered. Comparison with the experimental data, only available for randomly oriented molecules, is very good. Our predictions for molecular-frame photoelectron angular distributions of N 2 show the signature of electron confinement and coherent two-centre interferences as those previously found in H 2. For CO, they exhibit diffraction patterns associated with the scattering of the ejected electron by the neighbouring atomic centre. The conclusions reported in this work suggest that vibrationally resolved photoelectron angular distributions can be a useful instrument to determine structure parameters in these simple molecules, We thank Mare Nostrum BSC, Cineca and CCC-UAM for allocation of computer time. Work supported by the MICINN project Nos. FIS2010-15127, ACI2008-0777 and CSD 2007-00010 (Spain), the ERA-Chemistry project PIM2010EEC-00751, the European COST Action CM0702, the Marie Curie ITN CORINF, and the XCHEM Advanced Grant 290853 of the European Research Council

Photoelectron distributions of the polarized $2{p}^{5}3d$ Rydberg states of neon have been studied with a newly built velocity map imaging analyzer. The atoms were polarized by absorption of synchrotron radiation and ionized by an infrared laser. The asymmetry parameters ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\beta}}_{4}$ characterizing two-photon resonant ionization have been extracted from the measured images and compared with the results of a quantum defect treatment. To achieve a good theoretical description of the data, it is necessary to take into account the dependence of the dipole transition matrix elements and phases of the partial waves on the angular momentum quantum numbers pertaining to various continuum channels.

To investigate the dependence of ligand (adsorbate) core hole spectra on the electronic structure of the metal substrate, we performed ab initio 2h1p and 2h2p/3h2p CI calculations of the core hole spectra of the linear PdCO molecule using an extended basis set. The main line is the one-hole state and takes a much larger intensity than for NiCO and NiN 2 but still smaller than for free CO. As in the case of NiCO and NiN 2 , also for PdCO the π charge transfer (CT) shakeup satellite of a small intensity is obtained near the main line peak. The most striking spectral feature of PdCO which differs from NiCO and NiN 2 is the absence of the 5 eV giant σ shakeup satellite in the carbon spectrum. In the oxygen spectrum the corresponding satellite of a small satellite intensity, is shifted toward the higher energy (around 8 eV). However, with an increase of the bond length this satellite also disappears. As in the case of NiCO, the π to π * shakeup satellites are obtained around 9 eV for both carbon and oxygen spectra. These dramatic spectral feature changes are explained in terms of the different degree of the dσ-s hybridization and s-dσ promotion in the local metal configuration in the ground and ionized states. We point out the possibility that the DES spectrum becomes similar to the AES spectrum in the spectator Auger decay energy region even when initial core hole state differs.

A variational approach for the determination of continuum orbitals, recently proposed in the one-dimensional case, is generalized to the multicenter molecular problem. A test application is performed on the H2+ molecule in the one-center approximation, with a radial basis or B-spline functions. The resulting eigenvalue equation is solved by inverse subspace iteration giving a full set of K-matrix normalized solutions at each prefixed energy. Excellent numerical stability allows expansion up to l=20, giving very satisfactory agreement with exact results. The potential of a multicenter LCAO-type implementation is pointed out.

X-ray absorption spectra of gas-phase VOCl(3) and CrO(2)Cl(2) have been measured in the metal L(2,3)-edge and O K-edge regions. The assignment of the spectral features is based on the relativistic two-component ZORA TDDFT approach. The calculations provide results in excellent agreement with the experimental spectra and prove the importance of including both configuration mixing and spin-orbit coupling in the theoretical description to obtain a reliable simulation of the transition metal L(2,3)-edge. The calculations are extended also to the MnO(3)Cl molecule to discuss the spectral variations along the series of the oxychlorides both in the metal L(2,3) and ligand O K spectra.

The electronic structure and ionization energies of the first transition series of isoelectronic carbonyl nitrosyls were studied using ab initio 2h-1p configuration interaction (CI) and local density calculations. A 2h-1p CI scheme is found to be fairly successful except for Fe (CO)2(NO)2, and notably superior to ΔSCF calculations, although definite deficiencies are clearly indicated. Excellent results are afforded by the local density approach based on the Xα potential, which confirms the ability of the method to deal with complex correlation effects. A detailed analysis of higher excitation levels in selected orbital spaces was performed on Fe(CO)2(NO)2, showing the importance of quasi-degenerate correlation effects and of up to 4h-3p excitations. Despite the great improvement over 2h-1p, the energy separation obtained is still unsatisfactory. A dependable assignment of the available spectra is afforded by the combined theoretical results and correlation along the series. The problem of d orbital participation is discussed and the usefulness of further spectroscopic information is indicated. A peculiar behaviour of the still unobserved MnCO(NO)3 spectrum is predicted at the 2h-1p level.

Photoionization dynamics of the RNA base Uracil is studied in the framework of Density Functional Theory (DFT). The photoionization calculations take advantage of a newly developed parallel version of a multicentric approach to the calculation of the electronic continuum spectrum which uses a set of B-spline radial basis functions and a Kohn-Sham density functional hamiltonian. Both valence and core ionizations are considered. Scattering resonances in selected single-particle ionization channels are classified by the symmetry of the resonant state and the peak energy position in the photoelectron kinetic energy scale; the present results highlight once more the site specificity of core ionization processes. We further suggest that the resonant structures previously characterized in low-energy electron collision experiments are partly shifted below threshold by the photoionization processes. A critical evaluation of the theoretical results providing a guide for future experimental work on similar biosystems.

Contributions of the first-order nondipole terms to the photoelectron angular distributions from randomly oriented nitrogen molecules have been calculated in the framework of density functional theory and by employing a single centre expansion for the bound and continuum wavefunctions. Both valence and inner-shell ionizations have been considered in a photon energy range from threshold up to 2000 eV. Our results agree reasonably with calculations based on the random phase approximation and with the experimental data available. Our results for core ionizations confirm the most recent experimental finding of rather small nondipole effects in the near-threshold range. However, nondipole terms turn out to be quite large, even at the threshold, when individual contributions from the 1 sigma(-1)(g) and 1 sigma(-1)(u) ionization channels are considered. Strong interference effects leading to high-energy oscillations in the dipole and nondipole asymmetry parameters are satisfactorily explained within the Cohen-Fano model.

Circular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented 3-hydroxytetrahydrofuran enantiomers (Th-S and Th-R) has been observed in gas-phase experiments using circularly polarized vacuum ultraviolet (VUV) light. The measured dichroism for both Th-S and Th-R, acquired at the single magic angle theta=234.73 degrees and at photon energies of 22, 19, 16, and 14 eV, points to an asymmetric forward-backward scattering of the photoelectrons from their highest occupied molecular orbitals (HOMO) HOMO-1 and HOMO-2, of up to 5%, depending on the photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or the configuration of Th. The photoionization dichroic D parameters of Th-S and Th-R have been measured and their values discussed in the light of LCAO B-spline density functional theory (DFT) predictions. While an acceptable agreement is found between the dichroic parameter measured and calculated at the highest photon energy for the HOMO and HOMO-2 orbitals of Th, a significant discrepancy is observed for the HOMO-1 state which is attributed to the floppiness of Th, in particular to the comparatively large sensitivity of the size and shape of its HOMO-1 on nuclear vibrational motion.

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients Di(É) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Inner-shell excitation spectra of gas phase TiCl4, VOCl3 and CrO2Cl2 have been recorded in the Cl 2p and O 1s edge regions. The assignment of the core excitations is based on ab initio calculations in the relaxed 1h–1p CI scheme. The computational approach has provided excitation energy patterns and photoabsorption oscillator strengths in satisfactory agreement with the experimental spectra, indicating the reliability of the theoretical model. The calculations are extended also to the compound MnO3Cl and to the Cl 1s core excitation spectra of the whole series of molecules. The interpretation of the final excited states in terms of orbital composition is also discussed as well as the spectral variations along the series with the progressive substitution of the chlorine atom with the oxygen atom.

A density functional method based on a large-scale one-centre expansion in B-spline is employed to calculate the valence photoionization cross-section and the asymmetry parameter profiles in C 6 H 6 . The convergence of the method with increasing values of angular momentum is analysed. The partial cross-section and asymmetry parameter profiles are discussed for each orbital ionization. Information regarding the spatial extension of the wavefunction of the initial orbital can be gained by the cross-section contributions. Comparison with respect to a previous experiment and calculations is reported as well.

Calculations of the discrete core excitation spectra, including Rydberg transitions, are reported for CO, H2CO and F2CO employing ab initio and density functional approaches. The highly correlated QDPTCI approach compares well with experimental data and other accurate ab initio results. It appears that reliable values can be currently obtained for small molecules, although considerable uncertainty still affects intensity values, notably those experimentally derived. Comparison with the simpler 1h–1p CI approach indicates some deficiencies of the latter, attributed to the use of inadequate orbitals. The density functional approach proves generally reliable and can be profitably employed for the interpretation of experimental data in large systems, although presently limited to the case of nondegenerate core holes.

Valence partial photoionization cross sections relative to N2, CO2, N2O, C2N2, SF6, TiCl4 and Cr(CO)6 are evaluated at the local density level. The muffin-tin potential approximation is avoided by the use of large basis set LCAO calculations and the Stieltjes imaging approach. Comparison with experimental data and accurate ab initio calculations in the smaller systems proves the reliability of the approach, which furnishes results comparable to the ab initio ones at the Hartree-Fock level. Significant improvement over comparable MS-Xα calculations is obtained for the linear systems, notably the removal of spurious resonances. Notably the computational effort does not increase inordinately in the larger systems, where convergence in the moment expansion is even better, owing to the increased basis set size. The technique appears valuable in extracting information and assisting interpretation of experimental cross sections in complex systems, and as an aid to the resolution of assignment problems.

The purely algebraic variational approach in a spline basis, which has recently given sub-nanohartree accuracy in the LCAO expansion of H+2 and HeH2+ bound states, has been applied also to the continuum wavefunction calculations of these molecules. An efficient algorithm to treat linear dependence problems without loss of accuracy is presented. The accuracy in cross section calculations, with few angular momenta, is further improved beyond the maximum one-center limit (with lmax=50 and lmax=35) previously obtained. The LCAO scheme proposed may yeild accurate continuum orbitals for general polyatomic molecules.

The adequacy of the 2h-1p CI scheme for the interpretation of the valence ionization shake-up was first analysed in neon and argon, paying special attention to the choice of an adequate basis set, the explicit inclusion of relaxation by the use of relaxed orbitals and the correlation of the ground state. The scheme was then applied to the calculation of the shake-up of the isoelectronic series of hydrides: i.e. HF, H20, NH3 and CH4; HCl, H2S, PH3 and SiH4. The results obtained compare well with experiment and previous calculations, showing that in the hydrides the shakeup is dominated by the same effects as in neon and argon, and providing a consistent interpretation of the available experimental data.

Inner-shell excitation spectra of gas phase TiCl4, VOCl3 and CrO2Cl2 have been recorded in the Cl 2p and O 1s edge regions. The assignment of the core excitations is based on ab initio calculations in the relaxed 1h–1p CI scheme. The computational approach has provided excitation energy patterns and photoabsorption oscillator strengths in satisfactory agreement with the experimental spectra, indicating the reliability of the theoretical model. The calculations are extended also to the compound MnO3Cl and to the Cl 1s core excitation spectra of the whole series of molecules. The interpretation of the final excited states in terms of orbital composition is also discussed as well as the spectral variations along the series with the progressive substitution of the chlorine atom with the oxygen atom.

A comparison between the results of different configuration interaction excitation levels, up to SDTQ/4h-3p for the ground and ionised states, has been performed on LiH and Li2. Strong correlation effects are found in the latter molecule. The qualitative relevance of the lowest approximation, 1 det/2h-1p, is pointed out as well as its limitations. Such a calculation on LiF shows the absence of similar effects. Available experimental data are discussed in the context of the present results.

Multiple scattering Xα calculations with overlapping spheres were performed on CH3F and CH2F2 for all-valence region ionization potentials. Comparison with previous Xα calculations shows a substantial improvement.