Localized Magnetic States of Fe, Co, and Ni Impurities on Alkali Metal Films

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) have been used to study transition metal impurities on K and Na films. The multiplet structure of the XAS spectra indicates that Fe, Co, and Ni have localized atomic ground states with predominantly d7, d8, and d9 character, respectively. XMCD shows that the localized impurity states possess large, atomiclike, magnetic orbital moments that are progressively quenched as clusters are formed. Ni impurities on Na films are found to be nonmagnetic, with a strongly increased d10 character of the impurity state. The results show that the high magnetic moments of transition metals in alkali hosts originate from electron localization

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

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

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

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 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

High-level studies of the ionic states of norbornadiene and quadricyclane, including analysis of new experimental photoelectron spectra by configuration interaction and coupled cluster calculations

Synchrotron-based photoelectron spectra (PES) of norbornadiene (NBD) and quadricyclane (QC) differ significantly from those in previous studies. The adiabatic ionization energy (AIE1) for NBD, assigned to the 2B1 state at 8.279 eV, shows a progression of 18 members with decreasing vibration frequency from 390 cm−1 to 340 cm−1; our calculated frequency is 381 cm−1. Similarly, the AIE1 for QC at 7.671 eV, assigned to the 2B2 state, discloses a vibrational progression of nine or more members with vibration frequency decreasing from 703 cm−1 to 660 cm−1; our calculated vibration frequency is 663 cm−1. These AIEs, determined by coupled cluster and fourth order Møller–Plesset perturbation theory, were very similar to the corresponding second order perturbation theory results. The calculated AIE symmetry sequences are 2B1 < 2A1 < 2A2 < 2B2 for NBD and 2B2 < 2A2 < 2B1 < 2A1 for QC. The overall PES vertical ionization energy profiles for both compounds were closely reproduced by Tamm–Dancoff approximation energies and intensities. The vibrational structure of the ionic states, determined using Franck–Condon methods, gave a good account of the observed spectra, but the observed envelopes for both IE1 are complex sets of vibrations, rather than single progressions. The NMR spectra for QC showed residual second order properties at 300 MHz; both QC and NBD have been theoretically analyzed in greater detail using AA/BB/CC/XX/ spectra, where all H are coupled; the magnetic shielding and spin–spin coupling constants obtained are similar to experimental values.PostprintPostprintPeer reviewe