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The first experimental study of the X-ray absorption spectrum (XAS) of the allyl free radical, CH(2)CHCH(2), is reported. A supersonic He seeded beam of hyperthermal allyl radicals was crossed by a high resolution synchrotron radiation (SR) in the focus of a 3D ion momentum imaging time-of-flight (TOF) spectrometer to investigate the soft X-ray absorption and fragmentation processes. The XAS, recorded as Total-Ion-Yield (TIY), is dominated by C1s electron excitations from either the central carbon atom, C(C), or the two terminal carbon atoms, C(T), to the frontier orbitals, the semi-occupied-molecular-orbital (SOMO) and the lowest-unoccupied-molecular-orbital (LUMO). All of the intense features in the XAS could only be assigned with the aid of ab initio spectral simulation at the Multi-Configuration Self-Consistent-Field (MCSCF) level of theory, this level being required because of the multi-reference nature of the core-excited state wavefunctions of the open shell molecule. The ionization energies (IEs) of the singlet and triplet states of the C1s ionized allyl radical (XPS) were also calculated at the MCSCF level.

An experimental and theoretical investigation of the reactive properties of the O( 1 D) + CF 3 Br system has been carried out using crossed molecular beam scattering measurements and quasiclassical trajectory calculations. From the crossed beam experiment energy and angular distributions of the BrO reaction product were obtained. By carrying out trajectory calculations on a potential energy surface that we derived from spectroscopic and ab initio data, rate coefficients and scattering properties were estimated. The calculated properties are in reasonable agreement with experimental findings. Trajectory calculations were also performed to evaluate the temperature dependence of the rate coefficient of the processes leading to CF 3 O + Br and BrO + CF 3 .

The electronic structure of epichlorohydrin is investigated in the whole valence region by a combined experimental and theoretical study. The issue of controversial assignments of the molecular electronic structure is here addressed. Photoelectron spectra (PES) and Threshold Photoelectron spectra (TPES) of room temperature molecules in the gas phase are recorded. Geometries and energies of the stable conformers due to internal rotation of the C-C-C-Cl dihedral angle, gauche-II (g-II), gauche-I (g-I), and cis, are calculated, and the effect of the conformational flexibility on the photoionization energetics is studied by DFT and 2h-1p Configuration Interaction (CI) methods. Strong breakdown of the Koopmans Theorem (KT) is obtained for the four outermost ionizations, which are further investigated by higher level ab initio calculations. The full assignment of the spectrum is put on a firm basis by the combination of experimental and theoretical results. The orbital composition from correlated calculations is found closer to the DFT orbitals, which are then used to analyze the electronic structure of the molecule. The Highest Occupied Molecular Orbital (HOMO) and HOMO--2 are n(O)/n(Cl) mixed orbitals. The nature of each valence MO is generally preserved in all the conformers, although the magnitude of the n(O)/n(Cl) mixing in HOMO and HOMO--2 varies to some extent with the C-C-C-Cl dihedral angle. The low energy part of the HOMO PE band is predicted to be substantially affected by the conformational flexibility, as experimentally observed in the spectra. The rest of the spectrum is described in terms of the dominant conformer g-II, and a good agreement between experiment and theory is found. The inner-valence PE spectrum is characterized by satellite structures, due to electron correlation effects, which are interpreted by means of 2h-1p CI calculations.

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon?photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core?valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300?650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400?500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon?photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2+. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV?O+ coincidence peak in the mass spectrum carries some information on the initial core excitation

A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH(3)(+) + C(5)H(3)(+), C(2)H(3)(+) + C(4)H(3)(+) and C(2)H(2)(+) + C(4)H(4)(+) ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 ± 0.1, 29.5 ± 0.1, and 30.2 ± 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C(6)H(6)(2+) dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center‐of‐mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty‐two percent of the available energy is released as product translational energy.

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

The photoionization of the ground state and 6p laser-excited caesium atoms was studied above their 4d ionization thresholds. The 4d photoelectron spectrum of 6p laser-excited atoms shows a stronger excitation of satellites upon ionization than its ground state counterpart. The relative intensities of satellite and main photolines show a slow variation with the incoming photon energy for both the ground state and the 6p laser-excited states. An assignment of the excited state spectra, supported by recently published ground state photoionization spectra and calculations, is given and a preliminary analysis of the 4d Auger spectrum of laser-excited atoms is also presented.

We present spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed 1,3Po series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s 3Se) and singlet (from 1s2s 1Se) manifolds. The intensity ratios are drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlets are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account.

This paper reports a study by ion and electron spectroscopy of two-photon excited resonances of Cs and Rb at the 4d and 3d edges, respectively. The experimental spectra were simulated using multiconfiguration Dirac–Fock calculations. The excitation spectrum of laser-excited Cs atoms is predicted by single configuration calculations whereas a large set of correlated configurations is needed for proper description of the core-excitation of laser-excited Rb atoms.

The double photoionization of CO(2) molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO(2)(2+) molecular dication, (ii) the production of a metastable (CO(2)(2+))* that dissociates, with an apparent lifetime of 3.1 micros, giving rise to CO(+) and O(+) ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 micros. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO(2), for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO(+) together with a neutral autoionizing oxygen atom.

Titanium interstitial hydrides are well-known compounds, which have a very low hydrogen pressure below 250°C. The H,-Ti system has been extensively studied over a wide range of temperatures and pressures [ll. At room temperature, the equilibrium pressure of hydrogen of TiH, has been estimated to be of the order of lo-l6 atm [2]. Titanium hydride has been used as an electrode material in the temperature range 300-450 ’ C, using a melt containing LiH as electrolyte, and in relation to its hydrogen content reversible potentials have been obtained [3]. In our laboratory TiH, and TiH, have been used as the reference electrode for a hydrogen sensor tested from room temperature up to 200°C 14-51. The present paper deals with the electrochemical properties of hydride-proton conductor interfaces at room temperature, in order to ascertain their degree of reversibility.

An accurate experimental and theoretical study of the lowest core excitation of CH(3) and CD(3) methyl radicals is presented. The complex vibrational structure of the lowest band of the x-ray absorption spectrum (XAS) is due to the large variation of the molecular geometry, which is planar in the ground state and pyramidal in the core-excited state. The XAS spectra of the two radicals were recorded at high resolution and assigned by theoretical simulations of the spectra, taking into account the coupling of symmetrical stretching and symmetrical bending (umbrellalike) deformations of the radicals. An excellent agreement between experimental and theoretical spectral profiles allowed us to accurately characterize the vibrational structure of the electronic transition. The similarities, as well as the differences, of the peculiar vibrational progression observed for the two radicals are explained by the strong anharmonicity along the umbrella coordinate and by the isotopic variation, leading to a different probing of the double-well potential energy surface of the core excited state during the nuclear motion.

The double photoionization of N2O molecules, in the 30–50 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N+ + NO+ or N 2 + + O+ has been observed. These two processes appear to occur also below the vertical thresh-old of 35.8 eV, where the double ionization should be indirect. In the range between 35.8 and 38.5 eV. The two processes occur instead by direct coulomb explosion of the N2O2+ dication. Above 38.5 eV, the dissociation leading to NO+ + N+ is also promoted by the formation of a dication metastable state, which decays by fluorescence to the ground state and then dissociates.

A mode-locked tunable Ti:Sapphire oscillator has been synchronized with the time structure of a storage ring and used to study the photoionization of noble gases. In multi-bunch operation of the ring the setup permits the observation of the dynamics from a few nanoseconds down to the tens of picoseconds range. The characteristics of the setup are demonstrated by following the two-color ionization of helium (via the 1s3p 1Po state) and argon (via the 3p5 (2P3/2) 3d state). In the CW mode we have also examined the two-color ionization of neon via the 2p5 (2P1/2) 3d′ state. In neon the nf′ Rydberg series was followed up to n∼50, and the quantum defect was determined.

A detailed study of the umbrellalike vibration in inner-shell spectroscopy is presented. The high-resolution x-ray absorption spectrum for the lowest lying core excitation of the CH{sub 3} free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion time of 149 fs.

The oxygen K -edge x-ray-emission-threshold-electron coincidence spectrum of CO2 is presented. A two-step model, describing the result as a threshold-photoelectron spectrum free from post-collision ...

The double photoionization of N2O molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging technique and electron-ion-ion coincidence. For the two possible dissociative processes, leading to N++NO+ and O++N2+, angular distributions of ionic fragments have been measured, finding an evident anisotropy. This indicates that the molecules ionize when their axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. The analysis of results provides, in addition to the total kinetic energy of ionic fragments, crucial information about the double photoionization dynamics.

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K-edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

We have measured the NEXAFS spectrum and the relative photoionization cross section of the 3d spin - orbit components of the barium atom in the energy range up to 50 eV above the ionization threshold. As in Xe and Cs, also in barium the two spin - orbit components show strongly different energy dependence. A strong enhancement of the 3d(5/2) cross section near the 3d(3/2) ionization threshold, which is attributed to interchannel coupling, is observed. The experimental results are in good agreement with the relativistic time- dependent density functional theory calculations.

Fragmentation processes of SO2 following excitation of the six main O 1s pre-edge resonances, as well as above the ionization threshold and below the resonances, are studied using a position-sensitive time-of-flight ion imaging detector, and the associated dissociation branching ratios and break-up dynamics are determined. In order to distinguish between the O+ and S2+ fragments of equal mass-to-charge ratio, the measurements have been performed with the isotopically enriched (SO2)-O-18 sample. By analysis of the complete set of the fragment momentum vectors, the beta values for the fragments originating from the SO+ + O+ break-up and the kinetic energy release for fragmentation channels of both SO22+ and SO23+ parent ions are determined. We also present results on the three-body break-up dynamics. (C) 2015 AIP Publishing LLC.

We have measured the lifetimes of {sup 1}P{sup e} (n=9-12) doubly excited states in static electric fields (1-6 kV/cm) by observing the decay of the fluorescence signal as a function of time. The effects of the field on these helium states below the second ionization threshold are twofold: their excitation becomes possible due to the Stark mixing with the optically allowed {sup 1}P{sup o} series, and their lifetime is strongly modified by the opening of the autoionization channel, not accessible in zero field. Although the electric field represents only a tiny perturbation of the atomic potential, a substantial shortening of the lifetimes below 100 ps is observed. This is the simplest quantum system where the ratio of autoionization to fluorescence decay probability can be effectively controlled by an electric field of moderate strength.

An influence of static magnetic fields on the fluorescence yield spectrum of He in the vicinity of the N=2 thresholds has been observed. The experimental results are in excellent agreement with predictions based on multichannel quantum defect theory, and it is demonstrated that the Rydberg electron l mixing due to the diamagnetic interaction is essential for the description of the observed fluorescence yield intensity enhancement. © 2006 The American Physical Society.

Resonant Auger spectra following O $1s\ensuremath{-}^{1}\ensuremath{\Pi}_{u}$ excitation of metastable oxygen $a\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Delta}_{g}$ molecules have been measured at high resolution under resonant Raman conditions. By selectively monitoring various decay channels, the singlet manifold excitation spectrum has been separated from the dominating triplet excitation. The decay spectra have been analyzed using the lifetime-vibrational-interference model to give the spectroscopic parameters of the $1s$ excited $^{1}\ensuremath{\Pi}_{u}$ state of ${\mathrm{O}}_{2}$. Singlet and triplet manifold Auger decay rates are also compared.

The ionization cross-section of the 3d spin–orbit components of the Cs atom has been measured from about 12 to 70 eV above their respective thresholds. The measured relative ionization cross-section of the 3d5/2 channel exhibits a pronounced minimum above threshold followed by a second maximum near the 3d3/2 ionization onset and thus qualitatively confirms the theoretical predictions of a spin–orbit activated interchannel coupling (Amusia et al 2002 Phys. Rev. Lett 88 093002).

The double photoionization of N2O molecules, in the 28-40 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N+ + NO+ or N-2(+) + O+ has been observed. Below similar to 35.5 eV, these two processes involve the formation of some antoionizing states. In the range between similar to 35.5 and similar to 38.5 eV the two processes occur instead by a direct coulomb explosion of the N2O2+ dication. Above similar to 38.5 eV the dissociation leading to NO+ + N+ is also promoted by the formation of a dication metastable state which decays by fluorescence to the ground state and then dissociates. (c) 2006 Elsevier B.V. All rights reserved.

Soft X-ray emission spectroscopy is an old experimental technique that has seen a strong revival since the advent of new high brilliance soft X-ray sources in the shape of synchrotrons. This thesis covers a range of applications where soft x-ray techniques have been applied. I have looked at fluorescence yield from doubly excited helium in field free conditions as well as in the presence of weak electric and magnetic fields. Using soft X-ray emission spectroscopy (SXES) I have explored scattering mechanisms in molecular systems. Quenching of symmetry breaking transitions by detuning of the excitation energy was observed in CO2. Bond-length dependence was seen for symmetry breaking transitions in a series of hydrocarbons. Dissociation dynamics was explored in OCS and HCl. I have also applied SXES to liquid water exploring the properties of hydrogen bonding. Buried layers of AlAs under 100 A GaAs(100) was studied and thickness dependence and interface effects observed. The technique was also used to study the electronic structure of yttrium and vanadium hydrides. dd-excitations and accompanying spin flip excitations was observed in Cu M resonant x-ray Raman emission from Sr2CuO2Cl2.

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients Di(É) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

X-ray-emission threshold-electron coincidence spectroscopy has been applied to N2, O2, and N2O. The main features of the spectra can be interpreted as conventional core level threshold electron spectra, free from postcollision interaction effects. The relative cross sections for adiabatic ionization of close-lying core hole states and vibrational substates are presented. The results are compared to theoretical predictions and state-of-the-art photoelectron spectra and are discussed in terms of direct threshold ionization and core vacancy rearrangement processes.

The C K-edge photoabsorption and 1s core-level photoemission spectra of pentacene (C22H14) molecules in the gas phase are experimentally determined, and calculated by Self Consistent Field (SCF) and Static-Exchange Approximation (STEX) ab-initio methods.The contribution of the six non-equivalent C atoms present in the molecule to the C 1s photoemission spectrum, is clearly brought to light. We observe and interpret the complex near-edge structures of the carbon K-edge absorption spectrum, with two main groups of discrete transitions between 283eV and 288eV photon energy, as due to absorption to pi* virtual orbitals, while the broader structures at higher energy involve sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the threshold for the C 1s ionization, demonstrating strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted core orbitals. These results constitute an important basis and prerequisite

A threshold-photoelectron-coincidence spectrum of HBr has been recorded in the 32.2-35.8 eV photon energy range, with a resolution of approximately 0.01 eV, using a synchrotron radiation source. The X (3)Sigma(-) and a (1)Delta(2) states of the HBr(2+) dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the b (1)Sigma(+) electronic state. For the first two states, the vibrational states v=0-3 have been resolved, while for the ground X (3)Sigma(-) state also spin-orbit splitting has been detected. The results appear in good agreement with previous experimental observations. A comparison with theoretical predictions indicates the role of "noncovalent" contributions to the interaction between the two atomic partners for the formation of metastable states.

The present study describes the characterization of energy and structure of HBr2+ in its low-lying electronic states, achieved through an extension of a new empirical method [Chem. Phys. Lett. 379, 139 (2003)] recently introduced to evaluate the interatomic interaction in the HX2+ (X=F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck-Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results. (C) 2004 American Institute of Physics.

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory.

We report a joint experimental and theoretical study of circular dichroism in the valence photoelectron spectra of a free chiral molecule. The circular dichroism in photoelectron spectroscopy is measured at the magic angle for various valence states of $R(+)$ and $S(\ensuremath{-})$ methyl-oxirane enantiomers in the vapor phase. The maximum dichroism measured is about $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$. Experimental and theoretical results are in agreement. The ab initio calculation employs a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian.

Two recent theoretical studies [C. Liu et al., Phys. Rev. A 64, 010501 (2001); M. \ifmmode \check{Z}\else \v{Z}\fi{}itnik et al., ibid. 65, 032520 (2002)] predict that the fluorescence lifetimes of helium doubly excited states converging to ${\mathrm{H}\mathrm{e}}^{+}$ $N=2$ should be longer than that of the ${\mathrm{H}\mathrm{e}}^{+}$ $2p$ ion state. This effect is caused by the electric field of the outer electron which, through Stark mixing, gives the inner fluorescing electron some series specific, stabilizing $2s$ character. We have obtained the first experimental evidence that confirms this effect by measuring the lifetime of the $2p3d(^{1}P^{0})$ doubly excited state. This was determined to be $190\ifmmode\pm\else\textpm\fi{}30\text{ }\text{ }\mathrm{p}\mathrm{s}$ compared to 100 ps for the ${\mathrm{H}\mathrm{e}}^{+}$ $2p$ ion state. The measurements were performed using short pulses of synchrotron radiation to form doubly excited states and recording the arrival time of photons from fluorescence.

High resolution synchrotron radiation has been used to make extensive measurements of the doubly excited states converging on the He+(N = 2) ionization threshold of helium. The excitation of two series of triplet doubly excited states, 2pnd(3Do) and sp2,n−(3Po), have recently been observed by Penent et al (2001 Phys. Rev. Lett. 86 2758). In this paper the excitation of a third weakly excited series of triplet doubly excited states, 2pnd(3Po), which was suggested by the previous measurements, is confirmed. The metastable atom yields, resulting from the radiative decay of these three observable series of triplet doubly excited series, are reported and compared to the recent theoretical calculations of Žitnik et al (2002 Phys. Rev. A 65 032520).

Krypton 3p excitations and their subsequent resonant Auger decay have been studied. The absolute photoabsorption cross section was determined in the region of the 3p resonances, and some structures were detected in the absorption spectrum. Hartree-Fock calculations including correlation between 3p{sup -1}nl and 3d{sup -2}n{sup '}l{sup '}nl configurations were used to interpret the structures of the 3p resonant excitation series and of the corresponding decay spectra. Furthermore, the decay spectra confirmed the assignment of the main structures in the absorption spectrum.

The asymmetry parameter P for fluorescence photons emitted during the radiative decay of the sp2,n+(P-1(o)), sp2,n-(P-1(o)), 2pnd(P-1(o)) and 2pnd(D-3(o)) series of helium doubly excited states, formed by the absorption of linearly polarized synchrotron radiation, has been measured. The results are compared with recent theoretical calculations (Zitnik et al 2002 Phys. Rev. A 65 032520). At high n the beta parameter is shown to increase and oscillate due to the excitation of triplet states, optically accessible from the helium ground state due to spin-orbit effects.

2-Mercaptobenzothiazole (C7H5NS2) has been investigated by synchrotron radiation photoelectron spectroscopy (PES) in the gas phase. The valence region, C 1s and S 2p photoemission spectra have been measured. Comparison of the experimental results with theoretical ab initio calculations, performed for two possible tautomeric forms of the molecule, shows that only the thione form is present in the gas phase. Vibrational structure has been clearly observed in the valence photoemission spectra. In the C 1s spectrum the seven components due to the inequivalent carbon atoms in the molecule have been assigned on the basis of ab initio calculations. S 2p components have also been assigned. The photoemission results have been compared with previously reported photoemission measurements for 2-mercaptobenzoxazole.

The double photoionization of HBr molecules, by synchrotron radiation in the energy range between 25 and 55 eV, has been studied in a mass spectrometric experiment. The HBr2+ and Br2+ product ions have been detected by a photoelectron-photoion-coincidence technique, while the H++Br+ formation, which follows the double ionization of HBr, has been studied by photoelectron-photoion-photoion-coincidence technique. HBr2+ ions are produced with a threshold of 32.4±0.4 eV, while the dissociative channel leading to H++Br+, shows a threshold around 33 eV. The production of H+Br2+ occurs with a threshold of 40.2±0.4 eV. These results appear to be in a fairly good agreement with earlier nonrelativistic calculations of potential energy curves and also with values indirectly obtained from experimental Auger spectra.

We have studied the dissociation of the CO2 molecule into neutral fragments by observing coincidences between neutral metastable fragments and photoions at photon energies between 250 eV and 600 eV. We attribute the metastable signal to oxygen atoms in the 2s(2)2p(3)(S-4)3s(S-5) state and to fragments in high Rydberg states. Neutral metastable fragments are produced by the dissociation of valence-excited states of CO2+ that are populated by direct photoionization or, more efficiently, by core excitation and ionization followed by Auger decay. Metastable fragments were detected at all C 1s and O 1s single excitations, and their intensity was enhanced particularly just above the O 1s and C 1s ionization thresholds, where the recapture of slow photoelectrons is possible. They were also abundantly created at core-valence double excitations, at the thresholds of O 1s and C 1s in shake-up satellites, and to a lesser extent, also in regions of the photoionization continua where there are no resonances.

Near edge X-ray absorption fine structure (NEXAFS) spectra of 2-mercaptobenzothiazole have been investigated from experimental and theoretical points of view. High-resolution NEXAFS spectra (total-ion-yield) have been obtained in the gas phase near the carbon and nitrogen K-edges, and sulfur L-2,L-3-edges using synchrotron radiation. The experimental data are interpreted with the help of static-exchange (STEX) ab initio calculations for the different edges and compared with previously reported experimental and theoretical spectra of a similar molecule, the 2-mercaptobenzoxazole. The good agreement between experiment and theory allows for a detailed discussion and assignment of the various features observed. A vibrational progression at the C K-edge is also identified in the highly resolved experimental spectrum.

The K-edge photoionization spectra of metastable O2 (a1?g) molecules and O (3P) atoms have been investigated at high resolution. Both species were prepared on-line using a microwave discharge. For the molecule the transition to the ?* 1? state has been identified below the ionization threshold, using total-ion-yield spectroscopy. Above the edge the previously unobserved 2? state of the O2+ 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.

A recent experimental study [J.-E. Rubensson et al., Phys. Rev. Lett. 83, 947 (1999)] measured a significant fluorescence yield of the He( 2lnl(')) photoexcited resonances, showing major qualitative differences from nonrelativistic predictions. We present a further theoretical study of these states, and perform R-matrix multichannel quantum defect theory calculations to extract fluorescence and ionization cross sections. These theoretical results are in excellent agreement with newer, higher-resolution measurements. Radiative and spin-orbit effects are quantified and shown to play an important role in the overall characterization of highly excited states.

The reaction of excited nitrogen atoms, N(2D), with the simplest alkyne, C2H2, was investigated for the first time under single-collision conditions in crossed beam experiments with mass spectrometric detection. The experimental results combined with electronic structure calculations and RRKM predictions allow us to identify cyanomethylene (HCCN) as the main primary reaction product and to establish its formation dynamics. The identification of the N/H exchange channel suggests that the title reaction is a good candidate to explain formation of nitriles in the upper atmosphere of Titan.

The reaction of excited atomic nitrogen, N(2D), with the simplest alkene, C2H4, has been investigated under single-collision conditions in crossed beam experiments with mass spectrometric detection. The experimental results clearly show that the N/H exchange channel leading to compounds of gross formula C2H3N is the main reaction pathway and that the reaction proceeds through the formation of a bound intermediate. The title reaction may well be regarded as a good candidate to account for the formation of acetonitrile, CH3CN, in extraterrestrial environments such as the dense atmosphere of Saturn's moon Titan where the ultimate fate of product isomers other than acetonitrile itself, that is 2H-azirine, H2C(N)CH, and ketene imine, CH2CNH, is expected to be tautomerization to the most stable structure of acetonitrile.

In this paper we report angular distributions and time-of-flight spectra for the Cl+H2→HCl+H and Cl +D2→DCl+D reactions at 5.85 kcal mol−1 and 6.3 and 6.4 kcal mol−1 collision energies, respectively, obtained from high-resolution crossed molecular beam experiments. In addition, quasiclassical trajectory (QCT) and quantum mechanical (QM) dynamical calculations have been carried out for these reactions on the G3 potential energy surface (T. C. Allison et al., J. Phys. Chem., 1996, 100, 13575). Reaction probabilities, integral and differential cross sections have been calculated using the QCT and QM methodologies, and a comparison of these results is presented. The molecular beam results have been simulated using the theoretical calculations, and theory and experiment are in reasonably good agreement.