Design study of a photon beamline for a soft X-ray FEL driven by high gradient acceleration at EuPRAXIA@SPARC_LAB

We are proposing a facility based on high gradient acceleration via x-band RF structures and plasma acceleration. We plan to reach an electron energy of the order of 1 GeV, suitable to drive a Free Electron Laser for applications in the so called "water window" (2 - 4 nm). A conceptual design of the beamline, from the photon beam from the undulators to the user experimental chamber, mainly focusing on diagnostic, manipulation and transport of the radiation is presented and discussed. We also briefly outline a user end station for coherent imaging, laser ablation and pump-probe experiments

A Tandem Time--of--Flight Spectrometer for Negative--Ion/Positive--Ion Coincidence Measurements with Soft X-ray Excitation

We present a newly constructed spectrometer for negative--ion/positive--ion coincidence spectroscopy of gaseous samples. The instrument consists of two time--of--flight ion spectrometers and a magnetic momentum filter for deflection of electrons. The instrument can measure double and triple coincidences between mass--resolved negative and positive ions with high detection efficiency. First results include identification of several negative--ion/positive--ion coincidence channels following inner-shell photoexcitation of sulfur hexaflouride (SF$_6$)

UPS, XPS, NEXAFS and Computational Investigation of Acrylamide Monomer

Acrylamide is a small conjugated organic compound widely used in industrial processes and agriculture, generally in the form of a polymer. It can also be formed from food and tobacco as a result of Maillard reaction from reducing sugars and asparagine during heat treatment. Due to its toxicity and possible carcinogenicity, there is a risk in its release into the environment or human intake. In order to provide molecular and energetic information, we use synchrotron radiation to record the UV and X-ray photoelectron and photoabsorption spectra of acrylamide. The data are rationalized with the support of density functional theory and ab initio calculations, providing precise assignment of the observed features

The ground and ionized states of azulene : a combined study of the vibrational energy levels by photoionization, configuration interaction, and density functional calculations

A synchrotron-based photoionization spectrum of azulene shows significant additional vibrational fine structure when compared to previous studies. This spectrum was successfully analysed by Franck-Condon (FC) methods. Previously reported zero-kinetic-energy electron spectra (ZEKE) for azulene, have been reinterpreted in FC terms, leading to some alternative assignments to the earlier work. The sequence of ionic states has been determined by ab initio configuration interaction (CI) methods, leading to reliable theoretical values for both the calculated adiabatic and vertical ionization energies (AIE and VIE respectively). VIE were calculated by both symmetry-adapted cluster (SAC-CI), together with Green’s function (GF) and Tamm Dancoff approximation (TDA), single excitation CI methods; AIE for highest states of each symmetry, were determined by open-shell self-consistent field (SCF) methods at the restricted Hartree-Fock level. Complete active space SCF(CASSCF) was used for the pairs of states 12A2 + 22A2 and 12B1 + 22B1 each of which occurs as antisymmetric and symmetric (higher energy) combinations. The combined ionic state sequences (AIE and VIE) from these methods are 12A2 < 12B1 < 22A2 < 22B1. The PES shows a series of broad bands above 11 eV, each of which are attributed to more than one ionisation. The calculated PES sequence of states up to 19 eV shows the SAC-CI and GF results are in almost exact agreement. The internal spacing of the bands is best reproduced by the simpler GF and TDA methods. States involving simultaneous ionization and electronic excitation are considered by both SAC-CI and TDA methods.PostprintPeer reviewe

The ground and ionic states of cyclohepta-1,3,5-triene and their relationship to norcaradiene states. New 1H and 13C NMR spectra, and analysis of a new experimental photoelectron spectrum by ab initio methods

The strong inter-relationship between cyclohepta-1,3,5-triene (CHT) and norcaradiene (NCD) observed in some reactions, has been extended to include the energy surfaces for some low-lying ionic states. Whilst equilibrium structures of 2A/ symmetry containing the CHT skeleton were routinely found, the structures emerging with 2A// symmetry were found to be NCD ionic states. Surface studies, by variation of the C1 to C6 distance, showed minima for both state symmetries. Curve crossing which occurs in CS symmetry, is avoided by distortion to C1 symmetry. The CHT → NCD structural change is attributed to initial conrotatory closure of the singly occupied molecular orbital. A new synchrotron-based study of the photoelectron spectrum (PES) for CHT up to 25 eV shows little vibrational structure. We have assigned the overall PES up to 17 eV in considerable detail, using a variety of theoretical methods. The vertical ionization energy (VIE) sequence in the PES for CHT ions, is predicted to be: 12A/ < 12A// < 22A/ < 22A//. The calculated lowest ionic state, with lowest vibrational frequency 87 cm-1, leads to a high density of vibrational states. The Franck-Condon (FC) envelopes of the two lowest PES bands have been analysed. The identity of the PES spectrum, as derived from CHT rather than NCD, was demonstrated by 1H and 13C nuclear magnetic resonance (NMR) spectra of the sample; this agrees with the predicted PES spectra of CHT and NCD. The NCD predicted spectrum shows significant differences from CHT.PostprintPostprintPeer reviewe

Study of the electronic structure of short chain oligothiophenes

The electronic structure of short-chain thiophenes (thiophene, 2,2'-bithiophene and 2,2':5',2'-terthiophene) in the gas phase has been investigated by combining the outcomes of Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) at the C K-edge with those of density functional theory (DFT) calculations. The calculated NEXAFS spectra provide a comprehensive description of the main experimental features and allow their attribution. The evolution of the C1s NEXAFS spectral features is analyzed as a function of the number of thiophene rings; a tendency to a stabilization for increasing chain length is found. The computation of the binding energy allows to assign the experimental XPS peaks to the different carbon sites on the basis of both the inductive effects generated by the presence of the S atom as well as of the differential aromaticity effects

The vacuum ultraviolet absorption spectra of norbornadiene : vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations

A synchrotron-based vacuum ultraviolet absorption spectrum (VUV) of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analysed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high resolution photoelectron spectral (PES) analysis of the X2B1 ionic state. Additional vibrational detail in the region of this Rydberg state is observed in its VUV spectrum, when compared with the PES 2B1 ionic state; this is attributed to underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction (CI) and time dependent density functional theoretical methods (TDDFT). Several low-lying singlet valence states, especially those which arise from ππ* excitations, conventionally termed NV1 to NV4, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 00 band positions have been made from these fits. The anomaly of observed UV absorption by the 1A2 state of NBD is attributed to HT effects. Generally the HT components are less than 10% of the FC terms. The calculated 5b13s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analysed in detail in terms of triplet states.PostprintPeer reviewe

Vibrationally resolved NEXAFS at C and N K-edges of pyridine, 2-fluoropyridine and 2,6-difluoropyridine: A combined experimental and theoretical assessment

In the present work, the near edge X-ray absorption spectroscopy (NEXAFS) spectra at both C and N K-edges of pyridine, 2-fluoropyridine, and 2,6-difluoropyridine have been studied both experimentally and theoretically. From an electronic point of view, both transition potential density functional theory and time-dependent density functional theory approaches lead to reliable results provided that suitable basis sets and density functionals are employed. In this connection, the global hybrid B3LYP functional in conjunction with the EPR-III basis set appears particularly suitable after constant scaling of the band positions. For the N K-edge, vertical energies obtained at these levels and broadened by symmetric Gaussian distributions provide spectra in reasonable agreement with the experiment. Vibronic contributions further modulate the band-shapes leading to a better agreement with the experimental results, but are not strictly necessary for semi-quantitative investigations. On the other hand, vibronic contributions are responsible for strong intensity redistribution in the NEXAFS C K-edge spectra, and their inclusion is thus mandatory for a proper description of experiments. In this connection, the simple vertical gradient model is particularly appealing in view of its sufficient reliability and low computational cost. For more quantitative results, the more refined vertical Hessian approach can be employed, and its effectiveness has been improved thanks to a new least-squares fitting approach