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The spectroscopic characteristics and the decay mechanisms of the Rydberg states arising from valence shell excitations in carbon monoxide have been investigated by using a double ion chamber and synchrotron radiation to measure the absolute photoabsorption, photoionization and photodissociation cross sections, and the photoionization quantum efficiency, in the energy range between the ionization threshold and 36 eV. Guided by earlier theoretical predictions, the prominent absorption bands due to Rydberg states belonging to series converging onto the A 2Π state ionization threshold have been assigned, in accord with previous studies, to the ndδ and the [(n + 1)s + nd]σ transitions. Apart from the (4s + 3d)σ state which decays predominantly by predissociation into neutral fragments, most of the Rydberg states belonging to these series decay preferentially by autoionization. Another Rydberg series converging onto the A 2Π state limit has been tentatively assigned to the npσ transitions, even though these transitions are predicted to be weak. The v′ = 0 and v′ = 1 vibrational levels of the 4pσ state decay preferentially by predissociation into neutral fragments. The [(n + 1)s − nd]σ transitions are also calculated to be weak but may be associated with some previously unassigned absorption bands. Structure observed in the absorption spectrum between ∼19.5 and 23.5 eV may be due to Rydberg states associated with series converging onto the D 2Π or the 3 2Σ+ state limits. This structure is discussed in relation to dispersed fluorescence spectra and negative ion yields recorded over the same energy range. [ABSTRACT FROM AUTHOR]

The effect of vibrational autoionization on the H2+ X 2Σg+ v+ = 3, N+ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. In photon excitation regions free from the influence of autoionizing Rydberg states, where direct ionization dominates, the photoelectron anisotropy parameter associated with the X 1Σg+ v″ = 0, N″ = 1 → X 2Σg+ v+ = 3, N+ = 1 transition has a value close to the theoretical maximum. However, in the vicinity of a Rydberg state, vibrational autoionization leads to a substantial reduction in anisotropy. The value of the anisotropy parameter associated with the S-branch of the photoelectron spectrum is found to be considerably higher than that predicted under the assumption that the outgoing electron can be represented solely as a p-wave. This suggests that the f-wave contribution must be taken into account to obtain a proper description of the photoionization dynamics. The observed variations in the rotationally resolved branching ratios, in the vicinity of an autoionizing resonance, depend upon the rotational level of the Rydberg state. The rotationally averaged photoelectron anisotropy parameters have been compared with the corresponding, previously calculated, theoretical results and reasonable agreement has been found. The influence of vibrational autoionization on the H2+ X 2Σg+ v+ = 0, 1, 2, 3 vibrational branching ratios has also been investigated, and the experimental results show that, in energy regions encompassing Rydberg states, these ratios deviate strongly from the Franck–Condon factors for direct ionization. [ABSTRACT FROM AUTHOR]

The valence shell electronic states of pyridazine have been studied experimentally, by recording the photoabsorption spectrum, and theoretically, by calculating oscillator strengths and excitation energies. The absolute photoabsorption cross section has been measured between 4 and 40 eV, using synchrotron radiation, and is dominated by prominent bands associated with intravalence transitions. In contrast, structure due to Rydberg excitations is weak. One Rydberg state, belonging to a series converging onto the &Xtilde; ²B2 state limit has been observed and assigned. The accompanying vibrational structure has been characterized by analogy with that in the corresponding photoelectron band. Vibrational progressions associated with Rydberg states belonging to one or more series converging onto the à ²A2 state limit have also been observed. The absorption structure associated with these series is complex and only tentative assignments have been proposed for the Rydberg states. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet-singlet valence transitions and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. The valence shell photoionization dynamics have been investigated theoretically by calculating photoelectron angular distributions and photoionization partial cross sections of the four outermost orbitals. In addition, the ground state outer valence electronic configuration has been obtained at the complete active space self-consistent field and the N-electron valence state perturbation theory to second-order levels of theory. [ABSTRACT FROM AUTHOR]

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+-N5+ channel for nitrogen and up to the I8+-I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers. [ABSTRACT FROM AUTHOR]

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron anisotropy parameters and branching ratios for the N2+X²Σg+ state have been measured between the ionization threshold and ~16.7 eV to study the influence of autoionization on the photoionization dynamics. In this energy range the ion yield curve exhibits extensive structure due to autoionizing Rydberg or valence states. Some of these Rydberg states belong to series converging onto the A ²Πu limit but, close to the ionization threshold, states belonging to series converging onto vibrationally excited levels of the X²Σg+ state are also observed. At photon energies free from the influence of neutral excited states, the measured vibrational branching ratios correlate with the Franck-Condon factors for direct ionization. However, strong deviations are observed when the excitation energy corresponds with a transition into an autoionizing state. An attempt to interpret these variations using Franck-Condon factors connecting the neutral excited state and the final ionic state proved unsuccessful. This failure suggests that the intermediate states might not be pure Rydberg states. Electronic and, close to threshold, vibrational autoionization affects the vibrationally resolved photoelectron anisotropy parameters, leading to a significant reduction in the fairly high β-values determined for non-resonant ionization. Photoelectron images of the N2+X²Σg+ state recorded close to the v+ = 1 or the v+ = 2 ionization thresholds suggest that electronic autoionization results in rotational branch populations that differ from those due to direct, non-resonant, photoionization. [ABSTRACT FROM AUTHOR]

Dispersed fluorescence spectroscopy has been employed to study vibronic coupling in neutral and ionic states of carbon disulphide. The vibrationally resolved excitation spectra of the {\rm CS}_{\rm 2} ^{\rm + } \,{\rm \tilde{B}}( {{\rm 0}\,{\rm 0}\,{\rm 1}}){\,}^{\rm 2}{\rm \Sigma }_{\rm g} ^{\rm + } \to {\rm \tilde{X}}( {{\rm 0}\,{\rm 0}\,{\rm 1}})\,{}^{\rm 2}{\rm \Pi }_{\rm u} , {\rm \tilde{B}}( {{\rm 0}\,{\rm 0}\,{\rm 2}})\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } \to {\rm \tilde{X}}( {{\rm 0}\,{\rm 0}\,{\rm 2}})\,{}^{\rm 2}{\rm \Pi }_{\rm g} , {\rm \tilde{B}}( {{\rm 1}\,{\rm 0}\,{\rm 0}})\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } \to {\rm \tilde{X}}( {{\rm 0}\,{\rm 0}\,{\rm 0}})\,{}^{\rm 2}{\rm \Pi }_{\rm g} and {\rm \tilde{B}}( {{\rm 0}\,{\rm 0}\,{\rm 0}})\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } \to {\rm \tilde{X}}( {{\rm 0}\,{\rm 0}\,{\rm 0}})\,{}^{\rm 2}{\rm \Pi }_{\rm g} transitions have revealed new structure and possible assignments are discussed. Autoionization from vibrationally unexcited Rydberg states belonging to series converging onto the \tilde{\rm C}\,{}^2\Sigma _{\rm g} ^ + limit appears to result in the formation of vibrationally excited {\rm \tilde{B}}\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } state ions, including those with one or two quanta of the antisymmetric stretching mode. Most of the bands in the {\rm CS}_{\rm 2} ^{\rm + } \,{\rm \tilde{B}}\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } \to {\rm \tilde{X}}\,{}^{\rm 2}{\rm \Pi }_{\rm g} dispersed fluorescence spectrum have been assigned using energy levels determined in photoelectron spectroscopic studies. The four subbands associated with the Renner-Teller split {\rm \tilde{B}}( {{\rm 0}\,{\rm 1}\,{\rm 0}})\,{}^{\rm 2}{\rm \Pi }_{\rm g} \to {\rm \tilde{X}}( {{\rm 0}\,{\rm 1}\,{\rm 0}})\,{}^{\rm 2}{\rm \Sigma }_{\rm u} ^{\rm + } vibronic transition have been identified. [ABSTRACT FROM AUTHOR]

The complete valence shell photoelectron spectrum of hexafluorobenzene has been recorded with synchrotron radiation and the observed structure has been interpreted using ab initioionization energies and relative spectral intensities. The theoretical predictions for the single-hole ionic states due to outer valence shell ionization agree satisfactorily with the experimental results. Ionization from the inner valence, essential F 2s, orbitals is strongly influenced by many-body effects and the intensity is spread amongst numerous satellites. Photoelectron angular distributions and branching ratios have been determined both experimentally and theoretically, and demonstrate that shape resonances affect the valence shell photoionization dynamics. Some of the shape resonances have been associated with virtual valence orbitals. An assessment of the perfluoro effect on the occupied and virtual valence orbitals of hexafluorobenzene has been carried out by comparing the present results for C6F6with similar data for C6H6. [ABSTRACT FROM AUTHOR]

The photoelectron spectrum of selenophene has been recorded using synchrotron radiation in the photon energy range 20-80 eV and the inner valence region has been studied in detail for the first time. Green's function methods have been employed to evaluate the energies and spectral intensities of all valence shell ionization transitions and the results have facilitated an interpretation of the experimental spectra. Strong configuration interaction results in a redistribution of the intensity associated with the low lying ?1(1b1) orbital amongst several satellite states located in the outer valence region. The continuum multiple scattering approach has been used to calculate photoelectron asymmetry parameters for selenophene, thiophene and hydrogen sulphide, and these theoretical predictions have been compared with the corresponding experimental data to assess the influence of Cooper minima and shape resonances. The comparison indicates that the Se 4p and the S 3p Cooper minima have little effect on the valence shell photoionization dynamics of selenophene and thiophene, respectively. This outcome is discussed in connection with the closely related hydrogen selenide and hydrogen sulphide molecules where strong resonant phenomena are observed. [ABSTRACT FROM AUTHOR]

The absolute photoabsorption cross-section of cyclopropane has been measured between threshold and 30 eV using monochromated synchrotron radiation. The spectrum is dominated by prominent broad peaks arising from electronic transitions into valence-excited states. Much weaker structure ascribed to Rydberg states associated with excitation from the 3e' or 1a?2orbitals is also discernible. Some of these Rydberg states display vibrational progressions which can be correlated with similar structure observed in the corresponding photoelectron spectrum. Ab initiomultireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the observed absorption bands. [ABSTRACT FROM AUTHOR]

The valence shell electronic structure of hexafluorobenzene (C6F6) has been studied both experimentally and theoretically. The absolute photoabsorption cross section has been measured between 6 and 42 eV, using synchrotron radiation, and is dominated by prominent broad bands associated with intravalence transitions. In contrast, the structure due to Rydberg excitations is weak, but series have been observed converging onto the {\rm \tilde X}\,{}^2{\rm E}_{{\rm 1g}}, {\rm \tilde A}\,{}^2{\rm A}_{{\rm 2u}}, {\rm \tilde B}\,{}^2{\rm B}_{{\rm 2u}} or {\rm \tilde C}\,{}^2{\rm E}_{{\rm 2g}} limits. At photon energies above 13.7 eV the absorption bands become strongly perturbed, possibly as a result of Rydberg/valence state mixing, and Rydberg series can no longer be identified. The TDDFT approach has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet-singlet valence transitions, and also to obtain the excitation energies for electric-dipole forbidden and/or spin forbidden transitions. These theoretical results have allowed many of the experimentally observed bands to be assigned and have proved particularly useful in clarifying the identifications of several of the weak absorption peaks appearing below 7 eV. The valence shell threshold photoelectron spectrum has been recorded to help characterize the vibrational structure and assign autoionizing Rydberg states. Evidence has been found for the participation of the Jahn-Teller active n+17 and n+18 modes in the {\rm \tilde X}\,{}^2{\rm E}_{{\rm 1g}} and the {\rm \tilde C}\,{}^2{\rm E}_{{\rm 2g}} state photoelectron bands. [ABSTRACT FROM AUTHOR]

The absolute photoabsorption spectrum of ethylene oxide (C2H4O) has been measured between onset and 30 eV, using monochromated synchrotron radiation. Below the ionization threshold (10.56 eV) the spectrum is dominated by sharp peaks related to excitation of Rydberg series converging on the first ionization energy. Above the ionization threshold, valence-excited states, which give rise to broad bands, predominate. Underlying Rydberg states are signalled in weak vibrational structure on the valence bands. Ab initio multireference configuration interaction (MRDCI) calculations have been carried out to assist in assignment of the excited states, both valence and Rydberg. The lowest-lying valence state is electric-dipole forbidden; the first optically allowed valence state lies close to the ionization onset. The spectrum of the oxide is compared with those of cyclopropane (C3H6) and ethylene sulphide (C2H4S). [ABSTRACT FROM AUTHOR]