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Gas phase C$_{70}$ molecules have been ionized with single photons of energies between 16 eV and 70 eV and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions from 55 eV. The low kinetic energy parts of the spectra are explained with thermal emission of transient hot electrons. Deviations at high photon energies are used to determine a value for the initial electron equilibration time. We propose a generally applicable mechanism, named Resonance Ionization Shadowing, for the creation of hot electrons by absorption of above-threshold energy photons., 9 pages, 7 figures

The potential for selective bond breaking of a small molecule was investigated with electron spectroscopy and electron–ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photoionization and resonant core excitation at the N 1s- and O 1s-edges, followed by the emission of resonant-Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states. The states are of special relevance for investigating how distinct electronic configurations influence selective bond breaking. The electron–ion coincidence measurements provided branching fractions of the produced ion fragments as a function of electron binding energy. They explicitly demonstrate how the final electronic states created after photoionization and RA decay influence fragmentation. In particular, we observed a significantly different branching fraction for spectator states compared with participator states. In addition, it was also observed that the bonds broken for the spectator states correlate with the antibonding nature of the spectator–electron orbital.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17–90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom., In this work, the time-dependent density functional theory is used to calculate the photoionization dynamics of the valence and core ionizations of OsO4 and RuO4 complexes, which are chosen as model systems of small-size mononuclear heavy-metal complexes. Accurate experimental measurements of the branching ratios and photoelectron asymmetry parameters of outer valence ionizations of OsO4 provide a sound benchmark of the computational methodology.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17-90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom.

We present a theoretical and experimental study of the electronic structure of N-methyltrifluoroacetamide (FNMA) and examine the resonant Auger (RA) decay upon site- and state-specific excitation at the C 1s, N 1s, and O is edges. Based on the calculated energies for cationic states of 1 h and 2h-lp configurations, we assign the peaks of recorded VUV photoelectron and RA spectra. Specifically, we identify the spectral features associated with participator and spectator RA decay. To aid in a deeper understanding of previous observations of limited site-specific bond breaking in FNMA, the influence of the site-specific excitation and the character of the Auger decay on the fragmentation is investigated using Auger electron ion coincidence experiments. Moreover, to investigate the potentially different capacity for selective bond cleavage of participator and spectator Auger states, we apply this technique to the spectator Auger state with the main electronic configuration of (31a')(-1) (32e '')(-1) (34a '')(1) which is well resolved in the experimental spectrum. Finally, the influence of the molecular excess energy and the electronic character of the RA final states on fragmentation is addressed. Based on our findings, we argue that the excess energy the FNMA molecule gains upon RA decay is the dominant factor determining the fragmentation pattern.

Experimental determinations of the Kinetic Energy Released (KER) for ions, originating by Coulomb explosion of CO22+dications produced by double photoionization of CO2, are reported. The KER are measured by using synchrotron radiation in the range of 34–50eV coupled with electron-ion-ion coincidence technique.For O+and CO+, originating by Coulomb explosion of CO22+, KER are ranging between 1.0–5.0 and 0.4–3.0eV, respectively, being large enough to allow these ions in participating in the atmospheric escape from Mars and Titan into space. An explanation for the observed behavior of O+and CO22+ion density profiles on Mars atmosphere is proposed.

An experimental investigation on the fragmentation dynamics following the double photoionization of simple organic molecules of astrochemical interest, propylene oxide and N-methylformamide molecules, induced by VUV photons has been reported. Experiments used linearly polarized light in the 18-37 eV (propylene oxide) and 26-45 eV (N-methylformamide) photon energy range at the ELETTRA Synchrotron Facility of Trieste (Italy), coupling ion imaging and electron-ion-ion coincidence techniques with time-of-flight mass spectrometry. In the case of propylene oxide, six different two-body fragmentation processes have been recorded with the formation of CH2+/C2H4O+, CH3+/C2H3O+, O+/C3H6+, OH+/C3H5+, C2H3+/CH3O+, C2H4+/CH(2)O(+ )ion pairs. On the other hand, the double photoionization of N-methylformamide occurs producing two main fragmentation reactions, forming CH3+ + CH2NO+ and H+ + C2H4NO+. The relative cross sections and the threshold energies for all fragmentation channels are recorded as a function of the photon energy. Furthermore, in the case of the double photoionization of propylene oxide, the measure of the kinetic energy released distribution for the CH3+/(C2HO+)-O-3 final ions with their angular distributions allowed the identification of a bimodal behavior indicating the possible formation of two different stable isomers of C2H3O+: acetyl and oxiranyl cations. The obtained results are important to clarify the physical chemistry of the elementary processes induced by the interaction of ionizing radiations with simple organic molecules of astrochemical interest: propylene oxide and N-methylformamide.

An experimental and computational investigation characterizing the processes following the double photoionization of the methyloxirane and N-methylformamide molecules has been reported. The double photoionization experiments have been performed at the Elettra Synchrotron Facility of Trieste (Italy). Preliminary data show: (i) in the case of methyloxirane, six different two-body fragmentation processes leading to \( {\text{CH}}_{2}^{ + } /{\text{C}}_{2} {\text{H}}_{4} {\text{O}}^{ + } ,{\text{CH}}_{3}^{ + } /{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}^{ + } ,{\text{O}}^{ + } /{\text{C}}_{ 3} {\text{H}}_{6}^{ + } , {\text{ OH}}^{ + } / {\text{C}}_{ 3} {\text{H}}_{5}^{ + } ,{\text{C}}_{ 2} {\text{H}}_{3}^{ + } /{\text{CH}}_{ 3} {\text{O}}^{ + } ,{\text{C}}_{ 2} {\text{H}}_{4}^{ + } /{\text{CH}}_{ 2} {\text{O}}^{ + } \) pairs of final ions; (ii) in the case of N-methylformamide, two main two-body fragmentation processes, leading to \( {\text{CH}}_{3}^{ + } + {\text{CH}}_{2} {\text{NO}}^{ + } \) and \( {\text{H}}^{ + } + {\text{C}}_{2} {\text{H}}_{4} {\text{NO}}^{ + } \). The threshold’s energy for each dissociation channel is determined with the relative cross sections as a function of the investigated photon energy range. A careful analysis of recorded electron-ion-ion coincidence spectra mainly based on a Monte Carlo trajectory simulation is able to provide also the kinetic energy released (KER) distribution for the final ions of the investigated fragmentation reactions. These important experimental data are mandatory information to unravel the physical chemistry of the elementary processes induced by the interaction of photons, with simple relevant organic molecules: (i) the methyloxirane of astrochemical interest, being the first chiral molecule recently discovered in interstellar cloud Sagittarius B2; (ii) the N-methylformamide, being an important simple molecule containing the peptide bond, recently detected in the interstellar medium, in order to investigate its selective cleavage induced by UV photons. In the latter case, this can improve a deeper definition of formation/destruction routes in astrochemical environments of the more abundant formamide molecule.

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV energy range, permits: (i)-to observe six double-ionization fragmentation channels, their relative yields being accounted for about two-thirds by the couple (C2H4+, CH2O+), one-fifth by (C2H3+, CH3O+); (ii)-to measure thresholds for their openings as a function of photon energy; (iii)-to unravel a pronounced bimodality for a kinetic-energy-released distribution, fingerprint of competitive non-adiabatic mechanisms., 27 pages, 6 figures

We provide experimental evidence for interatomic Coulombic decay (ICD) in mixed rare gas dimers following resonant Auger decay. A velocity map imaging apparatus together with a cooled supersonic beam containing Ar2 and NeAr dimers was used to record the energy and angular distributions of electrons in coincidence with two mass selected ions following the excitation of a number of resonances converging to the Ar 2p3/2-1 threshold using synchrotron radiation. It is shown that the ICD process can be controlled by the choice of the partner atom in the dimer or of the resonance that triggers the resonant Auger decay

The X-ray absorption spectra (XAS) of Ar-2 and ArNe dimers and small Ar clusters in the L-2,L-3 region (244-252 eV) of the Ar atom have been recorded using synchrotron light and a combination of coincidence methods and kinetic energy discrimination of energetic ions. The absorption peaks in the spectra of the dimers and clusters were found to be shifted and broadened relative to the peaks in the spectrum of the Ar atom. In order to unambiguously relate these chemical shifts to the electronic structure of the core excited states in dimers, we performed ab initio calculations of the XAS spectra. Implications of the results for the use of XAS as a structure determination method in large rare gas clusters are discussed.

We report on experiments which show that C_{60} can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 10^{4} K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed.

The hitherto unexplored two-photon doubly excited states [Ne∗(2p-13s)]2 were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne2+ ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

We report on experiments which show that C_{60} can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 10^{4} K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed.

The measurements of the Kinetic Energy Release (KER) for various ionic species originating from two-body dissociations reactions, induced by double photoionization of CO 2 , C 2 H 2 and N 2 O neutral molecular precursors of interest in planetary atmospheres, are reported. The KER distributions as a function of the ultraviolet (UV) photon energy in the range of 30–65 eV (Vacuum and Extreme UV photons) are extracted from the electron–ion–ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques. This experimental method allows assessing the probability of escape for some simple ionic species in the upper atmosphere of Mars and Titan. In fact, the KER measured for H + , C + , CH + , CH 2 + , N + , O + , CO + , N 2 + and NO + fragment ions are ranging between 1.0 and 5.5 eV (only for H + the maximum value reaches 6.0 eV), and these translational energies are large enough to allow these ionic species in participating in the atmospheric escape from Mars and Titan into space (for CO + , N 2 + and NO + , the measured KER of 0.5–2.5 eV, 0.5–2.8 eV and 1.0–2.5 eV, respectively, allows the possible escape only from the Titan atmosphere).

In this work, hydrogen peroxide has been studied with threshold photoelectron (TPE) spectroscopy and photoelectron (PE) spectroscopy. The TPE spectrum has been recorded in the 10.0-21.0 eV ionization energy region, and the PE spectrum has been recorded at 21.22 eV photon energy. Five bands have been observed which have been assigned on the basis of UCCSD(T)-F12/VQZ-F12 and IP-EOM CCSD calculations. Vibrational structure has only been resolved in the TPE spectrum of the first band, associated with the (X) over tilde (2)Bg H2O2+

Experimental Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra of N-methyltrifluoroacetamide (FNMA), which is a peptide model system, measured at the C, N, O and F K-edges are reported. The features in the spectra have been assigned by Static-Exchange (STEX) calculations. Using the same method, we have also assigned previously measured NEXAFS spectra of another peptide model system, N-methylacetamide (NMA). To facilitate the NEXAFS feature assignments, X-ray Photoelectron Spectroscopy (XPS) measurements for NMA and FNMA have been carried out with the aim of obtaining the 1s electron ionization potentials, which are compared with the values predicted by our Hartree-Fock (Delta HF) and Multi Configuration Self Consistent Field (Delta MCSCF) calculations. We also demonstrate an approach to compensate for screening effects that are neglected in the STEX method. Ion yield measurements of FNMA associated with the excitation of several C, N, O, and F K-shell pre-edge resonances have revealed site-specific fragmentation in some cases which we interpret with the aid of our theoretical calculations. QC 20160303

Fundamental properties of multiply charged molecular ions, such as energetics, structure, stability, lifetime and fragmentation dynamics, are relevant to understand and to model the behavior of gaseous plasmas as well as ionosphere and astrophysical environments. Experimental determinations of the Kinetic Energy Released (KER) for ions originating from dissociations reactions, induced by Coulomb explosion of doubly charged molecular ions (molecular dications) produced by double photoionization of CO2, N2O and C2H2 molecules of interest in planetary atmospheres, are reported. The KER measurement as a function of the ultraviolet (UV) photon energy in the range of 28-65 eV are extracted from the electron-ion-ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques at the ELETTRA Synchrotron Light Laboratory Trieste, Italy. These experiments allow assessing the probability of escape for simple ionic species in the upper atmosphere of Mars, Venus and Titan. The measured KER in the case of H + , C + , CH + , CH2 + , N + , O + , CO + , N2 + and NO + fragment ions are ranging between 1.0 and 5.5 eV, being large enough to allow these ionic species in participating in the atmospheric escape from such planets into space.

The two-body dissociation reactions of the dication C2H2+2, initiated via double ionization of acetylene molecules by photons in the energy range 31.9-50.0 eV, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The angular distributions and kinetic energy of product ions, measured in the 31.9-50.0 eV energy range, exhibit significant differences for the three leading dissociation reactions with respect to a previous investigation carried out at a fixed energy of 39.0 eV, providing thus new information on the dynamical evolution of the system. The analysis of the results indicates that such dissociation reactions occur with a different mechanism. In particular, the symmetric dissociation in two CH+ ions is characterized by different dynamics, and the anisotropy of the angular distribution of ionic products increases with photon energy in a more pronounced way than the other two reactions. Moreover, the kinetic energy distribution of the symmetric dissociation reaction exhibits several components that change with photon energy. The new experimental findings cast light on the microscopic evolution of the system and can provide a laboratory reference for new theoretical calculations on specific features of the multidimensional potential energy surface, namely, the structure, energy and symmetry of dication states, the electronic state of dissociation products, energy barriers and their dependence on the geometry of the intermediate state. Published by AIP Publishing.

Near-edge X-ray absorption fine-structure (NEXAFS) spectra measured at the C, N, and O K-edges for three molecules containing the amide moiety, N-methylformamide (HCONHCH3), N,N-dimethylformamide (HCON(CH3)(2)), and N,N-dimethylacetamide (CH3CON(CH3)(2)) are presented. These molecules have similar structures and differ by the number of methyl groups located at the molecular ends. The fragmentation of these molecules after resonant excitation at different K-edge resonances is also investigated, using a 3D-ion imaging time-of-flight spectrometer. A comparison between the molecules with respect to the relative contributions of the fragments created upon excitation at distinct resonances reveals site-specific fragmentation. Further information about the character of the core-excitation and dissociation process is obtained from the angular distributions of the ion fragments. Published by AIP Publishing.

The fragmentation of ClNO upon resonant core-electron excitation to the LUMO and LUMO+1 orbitals at the N and O K-edges is investigated. The produced fragment ions were detected in coincidence with a position sensitive ion time-of-flight detector which enables deduction of the angular distribution of the ions. This facilitates a comparison between the two resonances and the two K-edges with respect to fragmentation time, transition dipole moment orientation, fragment yield of single-ion and ion-pair channels, and fragmentation mechanisms. We observe significant correlations between the core-excited site and the location of the bonds that are broken, as well as the dissociation time. Moreover, we observe preferential cleavage of specific bonds upon excitation to the LUMO and LUMO+1 states which can be attributed to their orbital character.

The electronic structure of nitrosyl chloride (ClNO) has been investigated in the gas phase by X-ray Photoelectron (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the Cl 2p, Cl 2s, N 1s and O 1s edges in a combined experimental and theoretical study. The theoretical calculations at different levels of approximation predict ionization potential values in good agreement with the experimental data and allow us to assign the main features of the absorption spectra. An unexpected failure of the density functional model is, however, observed in the calculation of the Cl 2s binding energy, which is related to a large self-interaction error. Largely different photoabsorption cross-section patterns are experimentally observed in core excitations from the investigated quantum shells (n = 1, 2). This finding is confirmed by the oscillator strength distributions calculated at different absorption edges; in the case of the n = 2 shell the bands below the threshold are extremely weak and most of the absorption intensity is due to excitations in the continuum.

A description of the LDM beamline of FERMI is given, with a detailed description of the photon transport., The Low Density Matter (LDM) beamline has been built as part of the FERMI free-electron laser (FEL) facility to serve the atomic, molecular and cluster physics community. After the commissioning phase, it received the first external users at the end of 2012. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline.

We present a comprehensive analysis of autoionization processes in Ne clusters (similar to 5000 atoms) after multiple valence excitations by free electron laser radiation. The evolution from 2-body interatomic Coulombic decay (ICD) to 3-body ICD is demonstrated when changing from surface to bulk Frenkel exciton excitation. Super Coster-Kronig type 2-body ICD is observed at Wannier exciton which quenches the main ICD channel.

In this work we have studied the 4d photoexcitation of atomic cesium vapor at high resolution in the energy range 75–90 eV. Below threshold, the two series of the 4d 10 6s → 4d 9 6s6p 3,1 P discrete transitions are observed to dominate the measured ion-yield spectra. The high photon energy resolution allowed us to selectively excite specific resonances. In this way the decay of the different excited states lying close to each other could be followed with a minimum of interference from overlapping transitions. Here we present the resonant Auger spectra recorded at the lowest energy 4d excited states and a preliminary analysis based on previous experimental and theoretical results.

Using a penetration-field threshold photoelectron (TPE) analyzer and efficient photon detectors X-ray-emission-threshold-electron coincidence (XETECO) spectroscopy has been developed to a point where peak shape analysis becomes meaningful. The spectra can, on the present level of accuracy, be interpreted as conventional TPE spectra free from post collision interaction effects. For the neon atom and the nitrogen molecule we find a detailed correspondence with state-of-the-art photoemission spectra, both regarding peak positions, line shapes, and relative intensities. Intensity redistribution due to fast core vacancy rearrangement processes in systems with close-lying core levels, like O-2, N2O, and NO is briefly discussed. It is demonstrated that even the complex threshold behavior at the F K threshold in SF6 leads to a well-defined XETECO peak, directly corresponding to classical photoemission results. (C) 2004 Elsevier B.V. All rights reserved. (Less)

The O 1s photoabsorption (ion-yield) spectra of OH ((2)Pi(i),v"=0) and OD ((2)Pi(i),v"=0) radicals have been studied in the gas phase for the first time. Spectroscopic parameters for the O 1s-->pi excited (2)Sigma(+) state of OH and OD, observed at similar to525.8 eV, have been obtained from a Franck-Condon analysis comparing the spectra of both molecules. The lifetime width of this state has been measured as Gamma=147+/-11 meV and Gamma=156+/-7 meV for OH and OD, respectively. Excellent quantitative agreement is found between the experimental spectroscopic parameters r(e), omega(e), and T-0, measured in this work for the (2)Sigma(+) state and recent theoretical results obtained for OH by high-level ab initio calculations. The spectroscopic parameters predicted by the equivalent-core model are close to the experimental values within a few percent. This model therefore provides an accurate description of the lowest-lying excitation process in both OH and OD molecules. (C) 2002 American Institute of Physics.

Pump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of “matched filter”. We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset.

The fragmentation pattern of the peptide model system, N-methylacetamide, is investigated using ion time-of-flight (TOF) spectroscopy after resonant K-shell excitation. Corresponding near-edge X-ray absorption fine structure (NEXAFS) spectra recorded at high resolution at the C1s, N1s and O1s edges are presented. Analysis of the ion TOF data reveals a multitude of fragmentation channels and dissociation pathways. Comparison between the excitation of six different resonances in the vicinity of the C1s, N1s and O1s edges suggests evidence for site-selective bond breaking. In particular the breaking of the peptide bond and the N-C-alpha bond show a clear correlation with resonant excitation at the N1s edge. Also, stronger tendencies towards site-selective bond breaking are found for the generation of single ions compared with ion pairs. Analysis of angular distributions of ions from breakage of the peptide bond yields a fragmentation time of

We made use of synchrotron radiation to perform near edge X-ray absorption fine structure spectroscopy, NEXAFS, at the carbon K-edge of perylene and perylene-tetracarboxylic-diimide, PTCDI. Reference spectra measured for isolated molecules in the gas phase are compared with polarization dependent NEXAFS spectra measured on highly oriented thin films in order to study the symmetry of the molecular orbitals. The molecular overlayers are grown onto the rutile TiO2(110) surface for which the large anisotropic corrugation effectively drives the molecular orientation, while its dielectric nature prevents the rehybridization of the molecular orbitals. We employed density functional theory, DFT, calculations to disentangle the contribution of specific carbon atoms to the molecular density of states. Numerical simulations correctly predict the observed NEXAFS azimuthal dichroism of the σ* resonances above the ionization threshold, from which we determine the full geometric orientation of the overlayer molecules. A discrepancy observed for the spectral contribution of the imide carbon atom to the calculated unoccupied molecular orbitals has been explained in terms of initial state effects, as determined by Hartree-Fock corrections and in full agreement with the corresponding shift of the C 1s core level measured by X-ray photoelectron spectroscopy, XPS. © the Partner Organisations 2014.

The Kinetic Energy Release (KER) for various ionic species coming from two-body dissociations reactions, induced by double photoionization of CO2, C2H2 and N2O neutral precursors of interest in planetary atmospheres, are reported. The KER distributions as a function of the VUV photon energy in the range of 30-65 eV are extracted from the coincidences spectra obtained by using tunable synchrotron radiation and electron-ion-ion coincidences coupled with ion imaging techniques. This experimental method, coupled with a computational analysis of the data based on a Monte Carlo trajectoriy simulation, allows to discuss about the probability for some ionic species to escape the upper atmosphere of Mars and Titan. In fact, the KER measured for H + , C + , CH + , CH\(_{\rm 2}^{\rm +}\), N + , O + and CO + fragment ions are ranging between 1.0 and 6.0 eV, and these translational energy contents are large enough to allow these ionic species in participating in the atmospheric escape from Mars and Titan into space.

The near edge x-ray absorption spectra of CX4 (X = H, F, Cl) have been measured with high resolution at the 1s edges of carbon and fluorine, and the 2p edge of chlorine. The absolute photoabsorption cross section has been determined at the resonances and in the nearby continuum. In accord with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C 1s edge. We report the first detailed study of the CCl4 Rydberg states. The vibrational frequency of the CF4 Rydberg states is anomalously high, which we attribute to a Fermi resonance. The C 1s line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionization. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 > CH4 > CCl4.

The X-ray photoelectron spectra of gaseous 2,5-(2,2′-dithienyl)diethynyl-thiophene (TRIM) in the C 1s and S 2p core level regions have been recorded by means of synchrotron radiation (SR) at ELETTRA. For all the non-equivalent carbon atoms of TRIM, ΔSCF calculations of the C 1s photoelectron spectra, have also been performed. All the carbons of the aromatic rings are perturbed by an electron charge withdrawing. The ethyne carbon atoms are negatively shifted in energy. The relative broadening of the S 2p accounts for the presence of two non-equivalent sulphur atoms. The S 2p 3/2 ionization potential value is lowered with respect to thiophene by −0.63 eV.

We have observed, for the first time, $\mathrm{LS}$-forbidden triplet doubly excited states, in single photon excitation of ground state helium, below the second ionization threshold. These states are identified as $^{3}D^{o}$ and $^{3}P^{o}$ and their excitation is due to spin-orbit interaction that mixes them with the optically allowed $^{1}P^{o}$ states. This observation is possible due to the very high efficiency in detecting metastable atoms created after the fluorescence decay of the doubly excited states, and the new capabilities of third generation synchrotron vacuum ultraviolet sources with high resolution beam lines.

The K-edge photoionization spectra of metastable O 2 (a 1 Δ g ) molecules and O ( 3 P) atoms have been investigated at high resolution. Both species were prepared on-line using a microwave discharge. For the molecule the transition to the π* 1 Π state has been identified below the ionization threshold, using total-ion-yield spectroscopy. Above the edge the previously unobserved 2 Δ state of the O 2 + core-hole ion has been located by photoelectron spectroscopy. The results provide a link between the singlet and triplet manifolds of the core excited states of the oxygen molecule. The 1s excited states of the oxygen atom were also studied. The high resolution allowed the characterization of higher members of the Rydberg series and first linewidth measurements for those transitions.

The dynamics of the prototypical insertion reaction N(2D) + D2 have been investigated in a combined experimental and theoretical study. Angular and velocity distributions of the ND product have been obtained in crossed molecular beam experiments with mass spectrometric detection at two collision energies (Ec = 3.8 and 5.1 kcal mol-1). The center-of-mass product angular and translational energy distributions have been derived; at both Ec's, the former is found to be nearly backward−forward symmetric, reflecting an insertion dynamics, and the latter corresponds to a fraction of total available energy released in translation of 32%, indicating that the ND product is highly internally excited. Quasiclassical trajectory (QCT) calculations were performed on an accurate potential-energy surface obtained from large-scale ab initio electronic structure computations, and the results were compared to experiment. Generally good agreement was found between the experimental results and the theoretical predictions; howe...

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight (from the surface to the detector) distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: (i) desorption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl s...

ion mechanism.The main thrust of our CMB work has been that of (a)further investigating the reactive scattering of oxygenatoms with a special focus on electronically excitedO( 1 D) 27 and on the comparative study of O( 3 P) and O( 1 D)reaction dynamics, 15,17 (b) extending reactive scatteringstudies to nitrogen atoms, 21,22 (c) studying the dynamicsof simple insertion reactions such as O( 1 D) + H 2 and N( 2 D)+ H 2 in order to provide experimental data for comparisonwith theory, 21,28 (d) providing scattering data for Cl + H 2 , 26 which is the only other reaction for which exact quantumscattering calculations are feasible to date, beyond H +H 2 and F + H 2 , (e) extending reactive scattering studiesto the chemically very important hydroxyl radical, 15,29 and(f) exploring the reactive scattering of O( 3 P, 1 D) from aliquid surface. 30 In this Account, we review our recentaccomplishments on the first three topics. II. The Crossed Molecular Beams Technique The experiments were carried out in a CMB apparatus(Figure 1) which follows the classic design of Nobellaureates Lee and Herschbach

The dynamics of the astrophysically relevant reactions OH + H2,OH+CO,N(2D)+C2H2 and O(1D)+H2 are studied using the crossed beam scattering technique. Comparisons of the experimental results with those of dynamics calculations on ab initio and semi-empirical potential energy surfaces are discussed.