Search

Two synchrotron-based studies on 4H-pyran-4-thione, photoelectron spectroscopy and vacuum ultraviolet (VUV) absorption spectra were performed. A highly resolved structure was observed in the photoelectron spectrum (PES), in contrast to an earlier PES study, where little structure was observed. The sequence of ionic states was determined using configuration interaction and coupled cluster methods. The vibrational structure of the lowest three PES bands was analyzed by configuration interaction and density functional calculations, providing a detailed explanation of the observed profiles. Several vibrational bands in the VUV absorption spectrum showed a similar structure to the bands in the PES and were identified as Rydberg states. [ABSTRACT FROM AUTHOR]

A synchrotron based vacuum ultraviolet absorption spectrum for γ-pyrone has been interpreted in terms of singlet excited electronic states using a variety of coupled cluster, configuration interaction, and density functional calculations. The extremely weak spectral onset at 3.557 eV shows eight vibrational peaks, which following previous analyses, are attributed to a forbidden 1A2 state. A contrasting broad peak with a maximum at 5.381 eV has a relatively high cross-section of 30 Mb; this arises from three overlapping states, where a 1A1 state dominates over progressively weaker 1B2 and 1B1 states. After fitting the second band to a polynomial Gaussian function and plotting the regular residuals over 20 vibrational peaks, we have had limited success in analyzing this fine structure. However, the small separation between these three states clearly shows that their vibrational satellites must overlap. Singlet valence and Rydberg state vibrational profiles were determined by configuration interaction using the CAM-B3LYP density functional. Vibrational analysis using both the Franck–Condon and Herzberg–Teller procedures showed that both procedures contributed to the profiles. Theoretical Rydberg states were evaluated by a highly focused CI procedure. The superposition of the lowest photoelectron spectral band on the vacuum ultraviolet spectrum near 6.4 eV shows that the 3s and 3p Rydberg states based on the 2B2 ionic state are present; those based on the other low-lying ionic state (X2B1) are destroyed by broadening; this is a dramatic extension of the broadening previously witnessed in our studies of halogenobenzenes. [ABSTRACT FROM AUTHOR]

The electronic structure of 2(5H)-thiophenone in the gas phase was investigated by ultraviolet photoelectron spectroscopy and x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C 1s, O 1s, and S 2p edges. All assignments of the experimental results are supported by both ab-initio electron propagator outer-valence Green's function (OVGF) calculations for the valence photoemission bands and density functional theory (DFT) and relativistic time dependent DFT calculations for the core levels XPS and NEXAFS spectra. Overall good agreement between experiment and theory is observed; this is especially true for core electron excitations which has permitted an unambiguous assignment of the observed spectral features in terms of single-particle excitations to virtual molecular orbitals. The assignment of the valence band spectra based on OVGF calculations, although satisfactory, points to the importance of electron correlations effects that partially break the single particle picture of ionization. [ABSTRACT FROM AUTHOR]

The valence and core electronic structure of three non-steroidal anti-inflammatory drugs (methyl salicylate, fenoprofen and ketoprofen) have been studied by photoelectron and soft X-ray absorption spectroscopy, supported by theoretical calculations of the molecular and electronic structure. The conformational landscape has been explored for sixteen low-energy conformers of fenoprofen and ketoprofen, and the energies of both compounds fall into two groups with steric similarities, separated by about 3 kJ mol−1. Valence band photoelectron spectra agree with previous results, and the spectra have been calculated using two approaches. We find the outer valence Green's function method gives good results, but the P3+ method is a little better, particularly for outer valence ionic states. Carbon and oxygen 1s photoemission spectra are reported and are in acceptable agreement with the theory. The C and O K near-edge X-ray absorption fine structure spectra are reported and interpreted by comparison with reference compounds. We analyse the data to provide rough estimates of the energies of the unoccupied orbitals in methyl salicylate. [ABSTRACT FROM AUTHOR]

Innovative, low-loss, and compact optical systems are essential to meet the experimental requirements of emerging novel radiation sources. The microchannel plate (MCP), a metamaterial-based optical device, shows promising potential for shaping, condensing, and focusing soft x-ray radiation at synchrotron radiation (SR) facilities. This study highlights the impact of MCP optical devices on SR beam condensing capability and their sensitivity to the degree of coherence by investigating the profile of transmitted beams through single and double MCP optical devices. Transmitted diffraction patterns of soft x-ray SR radiation change with energy and radiation modes. At 92 eV, the double MCP-based device affects the beam divergence and degree of coherence more than the single MCP. Moreover, the double MCP device shows potential as a condensing optics at shorter wavelengths, i.e., 480 eV. Experiments were performed at the available end-station of the Circular Polarization beamline at the Elettra synchrotron facility in Trieste, using a high-vacuum chamber with a hexapod system, providing the precise movement necessary to align these diffractive optics. The findings contribute to the development of innovative optical systems for SR and free-electron laser beamlines, paving the way for advanced experiments in spectroscopy, microscopy, and imaging in a wide energy range. [ABSTRACT FROM AUTHOR]

A synchrotron-based vacuum ultraviolet absorption spectrum (VUV) of quadricyclane (QC) is reported with energies up to 10.8 eV. Extensive vibrational structure has been extracted from the broad maxima by fitting short energy ranges of the VUV spectrum to high level polynomial functions and processing the regular residuals. Comparison of these data with our recent high-resolution photoelectron spectral of QC showed that this structure must be attributed to Rydberg states (RS). Several of these appear before the valence states at higher energies. Both types of states have been calculated by configuration interaction, including symmetry-adapted cluster studies (SAC-CI) and time dependent density functional theoretical methods (TDDFT). There is a close correlation between the SAC-CI vertical excitation energies (VEE) and both Becke 3-parameter hybrid functional (B3LYP), especially Coulomb-attenuating method-B3LYP determined ones. The VEE for several low-lying s-, p, d-, and f-RS have been determined by SAC-CI and adiabatic excitation energies by TDDFT methods. Searches for equilibrium structures for 11,3A2 and 11B1 states for QC led to rearrangement to a norbornadiene structure. Determination of the experimental 00 band positions, which show extremely low cross-sections, has been assisted by matching features in the spectra with Franck–Condon (FC) fits. Herzberg–Teller (HT) vibrational profiles for the RS are more intense than the FC ones, but only at high energy, and are attributed to up to ten quanta. The vibrational fine structure of the RS calculated by both FC and HT procedures gives an easy route to generating HT profiles for ionic states, which usually require non-standard procedures. [ABSTRACT FROM AUTHOR]

A permanently available molecular-beam injection setup for controlled molecules (COMO) was installed and commissioned at the small quantum systems (SQS) instrument at the European x-ray free-electron laser (EuXFEL). A b-type electrostatic deflector allows for pure state-, size-, and isomer-selected samples of polar molecules and clusters. The source provides a rotationally cold (T ≈ 1 K) and dense (ρ ≈ 108 cm−3) molecular beam with pulse durations up to 100 µs generated by a new version of the Even-Lavie valve. Here, a performance overview of the COMO setup is presented along with characterization experiments performed both with an optical laser at the Center for Free-Electron-Laser Science and with x rays at EuXFEL under burst-mode operation. COMO was designed to be attached to different instruments at the EuXFEL, in particular, the SQS and single particles, clusters, and biomolecules (SPB) instruments. This advanced controlled-molecules injection setup enables x-ray free-electron laser studies using highly defined samples with soft and hard x-ray FEL radiation for applications ranging from atomic, molecular, and cluster physics to elementary processes in chemistry and biology. [ABSTRACT FROM AUTHOR]

The EuPRAXIA EU project is at the forefront of advancing particle accelerator research and the development of photon sources through innovative plasma acceleration approaches. Within this framework, the EuAPS project aims to exploit laser wakefield acceleration to build and operate a betatron radiation source at the INFN Frascati National Laboratory. The EuAPS source will provide femtosecond X-ray pulses in the spectral region between about 1 and 10 keV, unlocking a realm of experimental ultrafast methodologies encompassing diverse imaging and X-ray spectroscopy techniques. This paper presents a description of the EuAPS betatron source, including simulations of the photon beam parameters, outlines the preliminary design of the dedicated photon beamline, and provides an insightful overview of its photon science applications. [ABSTRACT FROM AUTHOR]

A synchrotron-based photoionization spectrum up to 27 eV represents a considerable improvement in resolution over early He(I) and He(II) spectra. Symmetry-adapted coupled cluster calculations of the ionic state sequence give the sequence of state vertical ionization energies (VIE) as 12B2 < 12B1 < 12A2 < 22B1 < 12A1. Generally, these symmetry-adapted cluster configuration interactions VIE match reasonably well with the experimental spectrum over this wide energy range. Density functional calculations of the corresponding adiabatic terms (AIE) were also performed. Higher energy ionic states were determined by complete active space self-consistent field methods; these include all π-ionizations and some σ-ionic states. These were analyzed by Franck–Condon (FC) procedures and compared with an experiment. The spectral onset is complex, where two states, later shown to be the 12B2 and 12B1 states, are strongly overlapping. The superposition of the FC vibrational structure in the 12B2 and 12B1 states accounts for most of the peaks arising at the onset of the photoelectron spectra. However, the small separation between these two ionic states makes vibronic interaction fairly inevitable. In the absence of Herzberg–Teller analyses for ionic states, we have sought and determined a transition state between the 12B2 and 12B1 states, showing that vibronic coupling does occur. The lack of degradation in the vibrational envelope of the higher of the two states contrasts with our previous work on the halogenobenzenes, where overlapping state envelopes led to considerable widening of the line width at half-height of the higher energy states. [ABSTRACT FROM AUTHOR]

We have measured, analyzed, and simulated the ground state valence photoelectron spectrum, x-ray absorption (XA) spectrum, x-ray photoelectron (XP) spectrum as well as normal and resonant Auger–Meitner electron (AE) spectrum of oxazole at the carbon, oxygen, and nitrogen K-edge in order to understand its electronic structure. Experimental data are compared to theoretical calculations performed at the coupled cluster, restricted active space perturbation theory to second-order and time-dependent density functional levels of theory. We demonstrate (1) that both N and O K-edge XA spectra are sensitive to the amount of dynamical electron correlation included in the theoretical description and (2) that for a complete description of XP spectra, additional orbital correlation and orbital relaxation effects need to be considered. The normal AE spectra are dominated by a singlet excitation channel and well described by theory. The resonant AE spectra, however, are more complicated. While the participator decay channels, dominating at higher kinetic energies, are well described by coupled cluster theory, spectator channels can only be described satisfactorily using a method that combines restricted active space perturbation theory to second order for the bound part and a one-center approximation for the continuum. [ABSTRACT FROM AUTHOR]

High-resolution carbon K-edge X-ray photoelectron, X-ray absorption, non-resonant and resonant Auger spectra are presented of gas phase trans-1,3-butadiene alongside a detailed theoretical analysis utilising nuclear ensemble approaches and vibronic models to simulate the spectroscopic observables. The resonant Auger spectra recorded across the first pre-edge band reveal a complex evolution of different electronic states which remain relatively well-localised on the edge or central carbon sites. The results demonstrate the sensitivity of the resonant Auger observables to the weighted contributions from multiple electronic states. The gradually evolving spectral features can be accurately and feasibly simulated within nuclear ensemble methods and interpreted with the population analysis. [ABSTRACT FROM AUTHOR]

A new vacuum ultraviolet absorption (VUV) spectrum of azulene vapor has been obtained by using a synchrotron radiation source. The onset of the ultraviolet spectrum, previously reported by Sidman et al., has been analyzed in detail by Franck–Condon (FC) and Herzberg–Teller (HT) methods. The photoelectron spectral profile identifies the 3px-Rydberg state 00 band to be 131 cm−1 from the VUV maximum. Excited state energy levels were calculated by three independent methods: the wide scan VUV spectrum was correlated with symmetry adapted cluster configuration interaction calculations. The low energy portion of the spectrum was studied by both time dependent density functional theoretical methods (TDDFT) and multi-reference multi-root CI (MRD-CI). Equilibrium structures were determined for valence states at the TDDFT level. Rydberg states were determined by both TDDFT and MRD-CI. The FC + HT analyses were performed on the TDDFT wave-functions. The HT intensity profiles are generally low in intensity, relative to the FC ones; however, HT is dominant in the second singlet state (S2, 11A1). As a result, numerous non-symmetric modes, their overtones, and combination bands show considerable intensity in that band. Energies obtained from use of extremely diffuse s-, p-, d-, or f-character functions enabled realistic extrapolation to the IE1 for many Rydberg states (RS). The lowest RS (3b13s) based on IE2 lies at 4.804 eV with a quantum defect of 0.714. Differentiation between valence and RS is readily made using the second moments of the charge distribution. [ABSTRACT FROM AUTHOR]

Studies of persistent organic radical films on conductive metal surfaces can pave the way for diverse applications such as improved spin probes and labels, data control and storage, spintronics, and quantum computing. We grew monolayer films of three nitroxyl radicals (NRs), viz. TEMPO and two carbamoyl-proxyl radicals (nit8 and nit9) under ultra-high vacuum conditions on Au(111) and Cu(111) surfaces. The electronic properties of the films and NR adsorption mechanisms were analyzed by means of X-ray photoelectron (XPS) and absorption (NEXAFS) spectroscopies, with the aid of density functional theory (DFT) and time-dependent DFT computations performed on large unit cells (rev-PBE) and clusters (CAM-B3LYP). We found that all three NRs physisorb weakly on Au. In the case of nit8 and nit9, H-bonded monolayers are formed that recline parallel to the Au surface. Stronger interactions with Cu resulted in chemisorption and robust films, with nit8 and nit9 exhibiting upright orientation due to the amide group acting as an efficient binding anchor. Conversely, TEMPO binds to Cu necessarily via NO which is observed to lead to the destruction of the spin-carrying NO functionality. Computational evidence highlighted the decisive role of Cu surface defects in the partial fragmentation of the CONH2 anchor upon chemisorption of nit8 and nit9. [ABSTRACT FROM AUTHOR]

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown. [ABSTRACT FROM AUTHOR]

A synchrotron-based photoionization spectrum of azulene shows a significant additional vibrational fine structure when compared to previous studies. This spectrum was successfully analyzed by using Franck–Condon (FC) methods. Previously reported zero-kinetic-energy electron spectra 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 using ab initio configuration interaction (CI) methods, leading to reliable theoretical values for both the calculated adiabatic ionization energy (AIE) and vertical ionization energy (VIE). VIEs were calculated by both symmetry-adapted cluster (SAC-CI), together with Green's function (GF) and Tamm–Dancoff approximation (TDA), and single excitation CI methods; AIEs 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 was used for the pairs of 12A2 + 22A2 and 12B1 + 22B1 states, 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 photoelectron spectrum (PES) shows a series of broadbands above 11 eV, each of which is attributed to more than one ionization. The calculated PES sequence of states of up to 19 eV shows that 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. [ABSTRACT FROM AUTHOR]

A synchrotron-based vacuum ultraviolet (VUV) absorption spectrum of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analyzed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high-resolution photoelectron spectral analysis of the X2B1 ionic state. Additional vibrational details in the region of this Rydberg state are observed in its VUV spectrum when compared with the photoelectron 2B1 ionic state; this is attributed to the underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction and time-dependent density functional theoretical methods. Several low-lying singlet valence states, especially those that 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 the 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 analyzed in detail in terms of triplet states. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet (VUV) spectrum for cyclohepta-1,3,5-triene up to 10.8 eV shows several broad bands, which are compared with electron impact spectra. Local curve fitting exposed groups of sharp vibrational peaks, which are assigned to Rydberg states. The vertical excitation profile of the VUV spectrum, reproduced by time dependent density functional theory (TDDFT), gives a good interpretation of the principal regions of absorption. Fourth order Möller–Plessett perturbation theory, including single, double, and quadruple excitations, showed that the lowest singlet and triplet states retain CS symmetry. This contrasts with TDDFT where several low-lying excited states are planar. Detailed vibrational analysis of the first UV band was performed by Franck–Condon, Herzberg–Teller, and their combined methods. These show the dominance of mid-range frequencies, while the lowest frequency (75 cm−1) has negligible importance. In contrast, the second excited (Rydberg) state shows a major progression with separations of 115 (6) cm−1. This is interpreted by re-analysis of the X2A′ ionic state at the anharmonic level. Extremely low exponent Gaussian functions enabled several low-lying Rydberg state energies to be determined theoretically; extrapolation of the 3s-, 4s-, and 5s-Rydberg state calculated energies gives the adiabatic ionization energy as 7.837 eV (4) with δ 0.964 (2). Similarly, extrapolation of the centroids of the observed Rydberg states gave the vertical ionization energy (VIE) as VIE1 = 8.675 ± 0.077, close to the photoelectron spectroscopy VIE value [8.55 (1) eV]. [ABSTRACT FROM AUTHOR]

The strong inter-relationship between cyclohepta-1,3,5-triene (CHT) and norcaradiene (NCD) systems observed in some reactions has been extended to include the energy surfaces for some low-lying ionic states. Equilibrium structures for ionic states of CHT with 2A′ symmetry were routinely found; the structures emerging with 2A′′ symmetry were NCD ionic states. A detailed analysis of these surfaces as a function of the C1 to C6 distance showed that while minima occurred for both state symmetries, curve crossing occurs in CS symmetry, which 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 photoelectron spectrum (PES) for CHT up to 25 eV shows little vibrational structure. We have assigned the PES up to 17 eV using a variety of theoretical methods. The calculated lowest ionic state, X2A′, is predicted to have a very low vibrational frequency of 87 cm−1, leading to a high density of vibrational states. The Franck–Condon envelopes calculated for the two lowest states are almost completely contained within the envelope of the lowest PES band. A comparison of the predicted PES of CHT and NCD showed much closer agreement of the PES with that of CHT. An analysis of the 1H and 13C nuclear magnetic resonance (NMR) spectra of CHT showed no evidence of NCD. The increased chemical shifts arising from the higher frequencies used here lead to significant changes in appearance when compared with earlier NMR spectra. [ABSTRACT FROM AUTHOR]

We performed a theoretical investigation on the influence of electronic correlation effects on the B1s NEXAFS spectrum of boronic acid derivatives, namely, boric acid [B(OH)3], phenyl boronic acid (PBA), and 1,4-phenyl diboronic acid (PDBA), employing different computational schemes of increasing complexity, ranging from the purely one-electron scheme based on the transition potential method of density functional theory (DFT-TP), time-dependent DFT (TDDFT), and multiconfigurational self-consistent field (MCSCF). We also report experimental measurements of the B1s NEXAFS spectra of the aforementioned molecules together with the high-resolution C1s NEXAFS spectrum of PBA. We demonstrate that due to the shallow B1s core energy levels compared to C, O, and N, the inclusion of static correlation effects, which can be incorporated by using multireference approaches to excited states, assumes a decisive role in reconciling experiment and theory on B1s core-electron excitation energies and oscillator strengths to valence states. This claim is corroborated by the good agreement that we find between the DFT-TP calculated C1s NEXAFS spectrum and that experimentally measured for PBA and by the failure of both DFT-TP and TDDFT approaches with a selection of xc functionals kernels to properly describe the B1s NEXAFS spectrum of PBA and PDBA, at variance with the good agreement with the experiment that is found by employing the MCSCF wave function approach. [ABSTRACT FROM AUTHOR]

A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by configuration interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiconfiguration self-consistent field methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene. [ABSTRACT FROM AUTHOR]

A synchrotron-based study of the photoelectron spectrum (PES) for cyclooctatetraene (COT) is reported, and this has been subjected to theoretical analysis in unprecedented detail. Weak vibrational structure was observed on the lowest ionization energy (IE1), but the peaks generally show very broad features. Multiconfiguration self-consistent field study confirms that the adiabatic IE (AIE) sequence is 12A1 < 12B1 < 22A1 < 12A2 < 22B1 < 32A1. The Tamm-Dancoff approximation gives an acceptable interpretation of the PES below 20 eV. Vibrational analysis of the PES bands by Franck-Condon methods predicts well-defined vibrational structure for these ionic states. The principal contributors to the PES envelopes are the a1 modes, and only a few are responsible for the overall shape of most bands. The high density of vibrational states, together with the known D2d ⇆ D2d interconversion process, where the C=C and C—C bonds interchange, is attributed to the lack of the observed structure. The transition state (TS) structures for the interconversion above of several ionic states of COT have been elucidated. The intrinsic reaction coordinate procedure gave a planar TS for the X1A1 (D2d) ⇆ 1A1g (D4h) ⇆ X1A1 (D2d) process; this shows alternating C—C (1.4791 Å), C=C (1.3261 Å), and C—H (1.0780 Å) bond lengths. The planar TS is a very shallow maximum, with energy varying with the method used, Hartree-Fock 0.575 eV and second order Møller-Plesset 0.653 eV. A polynomial containing quadratic, quartic, and sextic terms gives an effectively exact fit to the surface. The lowest ionic state of COT (X2A1) shows a similar TS for the process X2A1 (D2d) ⇆ 2B2u (D4h). This structure has alternating C—C (1.4366 Å), C=C (1.3572 Å), and C—H (1.0756 Å) bonds. [ABSTRACT FROM AUTHOR]

Carbon K-edge resonant Auger spectra of gas-phase allene following excitation of the pre-edge 1s → π* transitions are presented and analysed with the support of EOM-CCSD/cc-pVTZ calculations. X-Ray absorption (XAS), X-ray photoelectron (XPS), valence band and non-resonant Auger spectra are also reanalysed with a series of computational approaches. The results presented demonstrate the importance of including nuclear ensemble effects for simulating X-ray observables and as an effective strategy for capturing Jahn–Teller effects in spectra. [ABSTRACT FROM AUTHOR]

New synchrotron based studies of the photoelectron ionization spectra (PES) for the isomeric 1- and 2-methyltetrazoles (1- and 2-MeTet) show markedly higher resolution than previous reports. The unusual spectral profiles suggest that a considerable overlay of the ionic states occurs for both molecules. Under these circumstances of near degeneracy of two or more ionic states, mutual annihilation of vibrational fine structure occurs for all except the strongest vibrational states; the PES just reflects the resultants rather than full spectra. Theoretical determination of the adiabatic ionization energies (AIEs) proved a challenge; the most successful method was second order Møller-Plesset perturbation theory (MP2). These calculations suggest that the lowest PES bands for both isomers contain ionization both from lone pair σ-orbitals (2A′) on the N-atoms (LPN) and π-orbitals (2A″). The lowest experimental AIEs are as follows: 1-MeTet is 10.315 eV assigned to 12A′, while 2-MeTet is 10.543 eV assigned to 12A″. Franck-Condon analysis shows that the lowest ionization energy regions of both spectra are dominated by IE from the LPN2A′ manifold, even though the 2A″ states have a higher absolute intensity. In this example, we have utilized a VUV Rydberg state to assist simplification of the PES; more frequently, the PES assignment is simpler and assists the location of Rydberg states in the VUV. The very slow spectral onset for 2-MeTet demonstrates the importance of vertical ionization energy calculations since maxima are more readily measured than slow onsets. These were performed at the equilibrium structure of the X1A′ state, using both multi-reference multi-root configuration interaction and the ionization potential variant of the equations-of-motion coupled cluster method, with single and double excitations (EOMIP-CCSD). This enabled the principal ionization bands to be identified over a wider range of energy. Attempts to study the higher ionic states by EOMIP-CCSD showed that several states of each symmetry are close to degenerate for 1-MeTet, in particular. A multi-configuration self-consistent field study confirmed the small separation of ionic states, but state switching during the optimization process largely disabled this method. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet (VUV) spectrum for CH2F2 from a new synchrotron study has been combined with earlier data and subjected to detailed scrutiny. The onset of absorption, band I and also band IV, is resolved into broad vibrational peaks, which contrast with the continuous absorption previously claimed. A new theoretical analysis, using a combination of time dependent density functional theory (TDDFT) calculations and complete active space self-consistent field, leads to a major new interpretation. Adiabatic excitation energies (AEEs) and vertical excitation energies, evaluated by these methods, are used to interpret the spectra in unprecedented detail using theoretical vibronic analysis. This includes both Franck-Condon (FC) and Herzberg-Teller (HT) effects on cold and hot bands. These results lead to the re-assignment of several known excited states and the identification of new ones. The lowest calculated AEE sequence for singlet states is 11B1 ∼ 11A2 < 21B1 < 11A1 < 21A1 < 11B2 < 31A1 < 31B1. These, together with calculated higher energy states, give a satisfactory account of the principal maxima observed in the VUV spectrum. Basis sets up to quadruple zeta valence with extensive polarization are used. The diffuse functions within this type of basis generate both valence and low-lying Rydberg excited states. The optimum position for the site of further diffuse functions in the calculations of Rydberg states is shown to lie on the H-atoms. The routine choice on the F-atoms is shown to be inadequate for both CHF3 and CH2F2. The lowest excitation energy region has mixed valence and Rydberg character. TDDFT calculations show that the unusual structure of the onset arises from the near degeneracy of 11B1 and 11A2 valence states, which mix in symmetric and antisymmetric combinations. The absence of fluorescence in the 10.8–11 eV region contrasts with strong absorption. This is interpreted by the 21B1 and 11A1 states where no fluorescence is calculated for these two states, which are only active in absorption. The nature of the two states, 11B1 and 21B1, is fundamentally different, but both are complex owing to the presence of FC and HT effects occurring in different ways. The two most intense bands, close to 12.5 and 15.5 eV, contain valence states as expected; the onset of the 15.5 eV band shows a set of vibrational peaks, but the vibration frequency does not correspond to any of the photoelectron spectral (PES) structure and is clearly valence in nature. The routine use of PES footprints to detect Rydberg states in VUV spectra is shown to be inadequate. The combined effects of FC and HT in the VUV spectral bands lead to additional vibrations when compared with the PES. [ABSTRACT FROM AUTHOR]

The development of novel coherent and brilliant sources, such as soft X-ray free electron laser (FEL) and high harmonic generation (HHG), enables new ultrafast analysis of the electronic and structural dynamics of a wide variety of materials. Soft X-ray FEL delivers high-brilliance beams with a short pulse duration, high spatial coherence and photon energy tunability. In comparison with FELs, HHG X-ray sources are characterized by a wide spectral bandwidth and few- to sub-femtosecond pulses. The approach will lead to the time-resolved reconstruction of molecular dynamics, shedding light on different photochemical pathways. The high peak brilliance of soft X-ray FELs facilitates investigations in a nonlinear regime, while the broader spectral bandwidth of the HHG sources may provide the simultaneous probing of multiple components. Significant technical breakthroughs in these novel sources are under way to improve brilliance, pulse duration, and to control spectral bandwidth, spot size, and energy resolution. Therefore, in the next few years, the new generation of soft X-ray sources combined with novel experimental techniques, new detectors, and computing capabilities will allow for the study of several extremely fast dynamics, such as vibronic dynamics. In the present review, we discuss recent developments in experiments, performed with soft X-ray FELs and HHG sources, operating near the carbon K-absorption edge, being a key atomic component in biosystems and soft materials. Different spectroscopy methods such as time-resolved pump-probe techniques, nonlinear spectroscopies and photoelectron spectroscopy studies have been addressed in an attempt to better understand fundamental physico-chemical processes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Anewsynchrotron-based study of the photoelectron spectrum (PES) of difluoromethane is interpreted by an ab initio analysis of the ionic states, which includes Franck-Condon (FC) factors. Double differentiation of the spectrum leads to significant spectral sharpening; the vibrational structure observed is now measured with greater accuracy than in previous studies. Several electronic structure methods are used, including equation of motion coupled cluster calculations with single and double excitations (EOM-CCSD), its ionization potential variant EOM-IP-CCSD, 4th order Møller-Plesset perturbation theory (MP4SDQ) configuration interaction (CI), and complete active space self-consistent-field (CASSCF) methods. The adiabatic ionization energies (AIEs) confirm the assignments as band I, one state 1²B1 (12.671 eV); band II, three states, 1²B2 (14.259) < 1²A1 (15.030) < 1²A2 (15.478 eV); and band III, three states, 2²B2 (18.055) < 2²A1 (18.257) < 22B1 (18.808 eV). The three ionizations in each of the bands II and III lead to selective line broadening of the PES structure, which is attributed to vibronic overlap. The apparent lack of a vibrational structure attributable to both the 1²A1 and 2²A1 states in the PES arises from line broadening with the preceding states 1²B2 and 2²B2, respectively. Although these ²A1 states clearly overlap with their adjacent higher IE, some vibrational structure is observed on the higher IE. The effects of vibronic coupling are evident since the observed structure does not fit closely with the calculated Born-Oppenheimer FC profiles. Correlation of the lowest group of four AIEs in the PES of other members of the CH2X2 group, where X = F, Cl, Br, and I, clearly indicate these effects are more general. [ABSTRACT FROM AUTHOR]

A new ultraviolet (UV) and vacuum ultraviolet (VUV) spectrum for iodopentafluorobenzene (C6F5I) using synchrotron radiation is reported. The measurements have been combined with those from a recent high-resolution photoelectron spectroscopic study. A major theoretical study, which includes both Franck-Condon (FC) and Herzberg-Teller (HT) analyses, leads to conclusions, which are compatible with both experimental studies. Our observation that the VUV multiplet at 7.926 eV in the VUV spectrum is a Rydberg state rather than a valence state leads to a fundamental reassignment of the VUV Rydberg spectrum over previous studies and removes an anomaly where some previously assigned Rydberg states were to optically forbidden states. Adiabatic excitation energies (AEEs) were determined from equations-of-motion coupled cluster with singles and doubles excitation; these were combined with time dependent density functional theoretical methods. Frequencies from these two methods are very similar, and this enabled the evaluation of both FC and HT contributions in the lower valence states. Multi-reference multi-root configuration interaction gave a satisfactory account of the principal UV+VUV spectral profile of C6F5I, with vertical band positions and intensities. The UV spectral onset consists of two very weak transitions assigned to 1¹B1 (πσ*) and 1¹B2 (&#963&#963*) symmetries. The lowest unoccupied molecular orbital of a &#963*(a1) symmetry has a significant C-I* antibonding character. This results in considerable lengthening of the C-I bond for both these excited states. The vibrational intensity of the lowest 1¹B1 state is dominated by HT contributions; the 1¹B2 state contains both HT and FC contributions; the third band, which contains three states, two ππ*(1¹A1, 2¹B2) and one πσ*(2¹B1), is dominated by FC contributions in the ¹A1 state. In this ¹A1 state, and the spectrally dominant bands near 6.7 (¹A1) and 7.3 eV (¹A1 + ¹B2), the C-I bond length is in the normal range, and FC components dominate. [ABSTRACT FROM AUTHOR]