下條 竜夫

Abstract The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I⁺ with phenyl or phenyl-like cations (C _n H or C _n H , with n  =  1 – 6 and x  =  1 – 5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

The dissociation dynamics of UV pumped iodothiophene molecules are investigated using velocity map ion imaging, in combination with site-selective extreme ultraviolet ionization of the iodine atom.

The multi-ionization processes of Kr and Xe in the K-edge photon energy region were investigated using short-pulse x-rays and time-of-flight apparatus. The generation of Kr13+ ions in this photon energy region was observed for the first time. The distribution characteristics observed in the experiment, such as the production of Kr13+ ions, high production ratios of Kr4+, Kr5+, and Kr6+ ions, and the existence of a small peak for Kr8+, were quantitatively reproduced by the Monte Carlo simulation for the charge distribution of Kr ions following 1s of inner shell ionization. The highly charged ions were suppressed in the experiments compared with those in the simulation, probably owing to the inhibition of certain Coster-Kronig processes in the highly charged ions, which were not considered for analysis. The charge distribution of Xe ions following 1s of inner shell ionization was also investigated. A similar charge distribution was observed when the photon energy was located between the K and L edges. This is because the Xe 1s hole state mainly undergoes ultrafast relaxation to the 2p hole state owing to the strong dipole transition moment between the 1s and 2p states.

The LMM Auger spectra of krypton are measured using the photon energies hν=1709 eV, 1792 eV, 1950 eV, and 13 keV. This approach allows separating the contributions from the various core holes L1, L2, and L3. Previously unobserved transitions are presented. Complementary theoretical work is performed allowing the assignment of the spectral features. The L2,3Y-MMY (Y=M4,5,N1,2,3) Auger transitions of Kr2+ formed via Coster-Kronig Auger decay of the core holes L1 and L2 are also investigated. These spectra comprise about 4000 and 13 000 transitions, respectively, so that only general statements on the assignment, such as the configurations involved in the transitions, can be given.

The dissociation and ionization dynamics of CF3I and CH3I molecules were investigated using a pump-and-probe technique that employs a soft x-ray free-electron laser (SACLA) in Japan. First, time-resolved inner-shell photoelectron spectroscopy was employed to observe the ultrafast reaction of CF3I by monitoring iodine 4d electrons. The change in the I 4d state observed in the photoelectron spectra is found to occur with a rise time τ of approximately 40 fs after a pump laser pulse, which is faster than that observed when an ultrafast gas-phase electron diffraction technique is employed. This implies that the inner-shell photoelectron spectroscopy is more sensitive to the potential surface near the Franck–Condon region. Second, a strong laser intensity at 266 nm, corresponding to a power density of 1.9 × 1014 W cm−2, can easily ionize CH3I molecules via multiphoton ionization processes, and the time dependence of the valence photoelectron spectra clearly shows that at the picosecond timescale, this pump laser pulse causes spectral peaks to shift owing to space-charge effects in response to the large amount of ions generated. Thus, the SACLA can be a useful tool to investigate not only the dynamical process of molecular dissociation but also the ionization process through the shift in the peaks of photoelectron spectra.

An experimental and computational investigation of the space-charge effects occurring in ultrafast photoelectron spectroscopy from the gas phase is presented. The target sample CF3I is excited by ultrashort (100 fs) far-ultraviolet radiation pulses produced by a free-electron laser. The modification of the energy distribution of the photoelectrons, i.e. the shift and broadening of the spectral structures, is monitored as a function of the pulse intensity. The experimental results are compared with computational simulations which employ a Barnes-Hut algorithm to calculate the effect of individual Coulomb forces acting among the particles. In the presented model, a survey spectrum acquired at low radiation fluence is used to determine the initial energy distribution of the electrons after the photoemission event. The spectrum modified by the space-charge effects is then reproduced by N-body calculations that simulate the dynamics of the photoelectrons subject to the individual mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method accounts for the space-charge effects on the energy distribution and allows to reproduce the complete photoelectron spectrum and not just a specific photoemission structure. The simulations also provide information on the time evolution of the space-charge effects on the picosecond scale. Differences with the case of photoemission from solid samples are highlighted and discussed. The presented simulation procedure, although it omits the analysis of angular distribution, constitutes an effective simplified model that allows to predict and account for space-charge effects on the photoelectron energy spectrum in time-resolved photoemission experiments with high-intensity pulsed sources.

Electronic relaxation of atomic Kr below and above the 1s ionization threshold is investigated experimentally using hard x-ray photoelectron spectroscopy. The experimental results are interpreted with the aid of relativistic Dirac-Fock calculations. The 1s orbital core-hole lifetime is extracted and the satellite's structures accompanying the photoelectron main line are assigned. Auger spectra recorded below and above the K edge are also investigated. In particular, the Auger cascade originating from vacancies of different origin in the L shell is analyzed in great detail. Competition between radiative KL versus nonradiative KLL Auger emission is emphasized.

We measured the Kr photoelectron spectrum in the region close to the 3p ionization threshold. Our high-resolution measurements allowed a clear observation of spectral structures due to electron correlation effects. Analysis based on relativistic multiconfiguration calculations could explain these observed peaks as due to strong configuration interactions between the 3p(-1) state and 3d(-2) nl states. Calculated and experimental data for peak assignments and intensity distributions are in good agreement. In addition, we measured the anisotropy parameter beta, which also agreed well with theory. These findings provide a detailed view of strong configuration interactions between the 3(-1) and 3d(-2)nl inner-shell hole states.

近年、真空中での溶液あるいは液体状態の分光が注目され、水（液体）、アルコール（液体）、溶液の光電子スペクトル、軟Ｘ線発光スペクトル、内殻吸収スペクトルなどが測定されてきた。本研究では超音波振動による微少液滴生成技術を利用し、水溶液内のアミノ酸の軟Ｘ線吸収スペクトル測定を試みた。その結果、軟Ｘ線領域では、液滴の窓への付着、超音波噴霧の不安定性により、十分なＳ／Ｎ測定には、様々な改良が必要なことがわかった。

We have recently initiated hard x-ray photoelectron spectroscopy experiments on heavy atoms and heavy-element containing molecules in gas phase by using synchrotron radiation up to 35 keV at SPring-8 undulator beamlines. We have successfully measured deep inner-shell photoelectron spectra, as well as L-MM and M-NN Auger electron spectra excited below and above the K-edge of heavy elements. Target specimens utilized for the preliminary experiments are Ar, Kr and Xe atoms, and also iodine in iodomethane (CH3I) and trifluoroiodomethane (CF3I) molecules, respectively. We show some selected results on the extracted core-hole lifetime broadenings for the iodine 1s core level of the CH3I molecule and also for the Xe 2s, 2p core levels, to compare with theoretical values. The L-MM Auger electron spectra of Kr recorded at 13 and 16.6 keV excitation energies are also shown as typical examples, and the spectrum measured above the K-edge, i.e. 14.327 keV, is analyzed based on theoretical calculations using the Hartree-Fock method. As a result, we give a tentative assignment for the double-core-hole hyper-satellite LL-LMM Auger transitions of the Kr atom.

Hard X-ray electron spectroscopic study of iodine 1s and 2s photoionization of iodomethane (CH3I) and trifluoroiodomethane (CF3I) molecules is presented. The experiment was carried out at the SPring-8 synchrotron radiation facility in Japan. The results are analyzed with the aid of relativistic molecular and atomic calculations. It is shown that charge redistribution within the molecule is experimentally observable even for very deep levels and is a function of the number of electron vacancies. We also show that the analysis of Auger spectra subsequent to hard X-ray photoionization can be used to provide insight into charge distribution in molecules and highlight the necessity of quantum electrodynamics corrections in the prediction of core shell binding energies in molecules that contain heavy atoms.

炭酸分子の酸素1s、炭素1sしきいエネルギー領域の吸収スペクトル測定を光電子-光イオン-光イオン同時測定法を利用して行った。得られたスペクトルには1s→π*由来と考えられるピークがあらわれたが、水および二酸化炭素の単体分子からのシグナルによる寄与がきわめて大きく、炭酸分子由来のシグナル強度も弱かったため、明確なピーク位置は確定することはできなかった。

Xe 1s photoelectron spectra were measured at SPring-8, Japan. The core-hole lifetime broadening was found to be 9.6 eV, yielding a lifetime of similar to 68 as. The amount of radiative versus nonradiative decay was assessed by recording Auger LMM spectra below and above the K edge. Below the K edge, L vacancies are produced only by direct photoionization, while above the K edge some of these vacancies are mainly produced by KL emission following 1s photoionization. Due to the dipole selection rule for x-ray emission, the dominant role of the KL relaxation process is rather directly observed.

We have realized element-specific pump-probe time-resolved hard X-ray photoelectron spectroscopy (TR-HAXPES) for the study of ultrafast electron dynamics in condensed matter. TR-HAXPES has been applied for the first time to determine the temporal evolution of space-charge effects on Ti 1s core-level photoelectron spectra of SrTiO₃ at the SACLA X-ray free electron laser facility. We found that the temporal evolution of Ti 1s kinetic energy shifts can be well explained by a mean-field model for the electron propagation in the vacuum. We also report preliminary results of a real-time laser-assisted valence-transition in YbInCu₄, observed by measuring Yb 3d_5/2 core-level HAXPES spectra at BL19LXU of SPring-8, under ON/OFF conditions of an optical pump-laser. We observed drastic changes in intensities of the Yb²⁺ and Yb³⁺ spectral components induced by pump-laser irradiation.

X-ray absorption and emission spectra of carbonate ions in aqueous solution were measured at two concentrations. The signals from the ion were extracted via selective excitation and background subtraction. The obtained spectral shape for the emission of the ion was well explained by DFT calculations. (C) 2013 Elsevier B.V. All rights reserved.

The novel spectroscopic technique of time-resolved fluorescence-photoion coincidence spectroscopy (TFPICO) has been applied to the investigation of the decay processes of 2p inner-shell excited Ar atoms and clusters. For the Ar atom, only that fluorescence accompanying the production of Ar+ showed a strong dependence on excitation energy. This dependence is discussed in terms of competing Auger decay processes. For Ar clusters, the TFPICO spectra for dimer ions (Ar-2)(+) revealed long-lifetime fluorescence components which can be attributed to the 'third excimer continuum'. With this work we demonstrate the usefulness of this technique for investigating the decay processes of inner-shell excited atoms and clusters.

Rydberg-valence (RV) mixing and doublet-quartet (DQ) interchannel coupling in the core-excited states of triplet open-shell O-2 are experimentally and theoretically unveiled through the resonant inelastic x-ray scattering around the O 1s -> sigma(u)* resonance. Ab initio calculations explain complicated excitation energy dependence of the spectral profile by the interplay of the DQ interchannel coupling under the presence of the RV mixing. Complicated intermediate states with valence and/or Rydberg character are successfully analyzed through the filtering of pure valence and pure Rydberg final states.

Angle-resolved metastable fragments yields spectra have been measured in the N 1s ionization region of the N-2 and C 1s ionization region of CO. These spectra are compared with zero kinetic energy electron and photoelectron spectra. It has been shown that an isotropic metastable fragments yields spectra are almost identical with the ZEKE spectrum, whereas metastable fragments yields spectra with the Sigma-Sigma transition show similarity with photoelectron spectra. This means that these spectra clearly contain information about two shake-up mechanisms: conjugate and direct shake-up processes. All the peaks in the metastable photofragment spectra can be assigned as either satellite states or double/triple excitation states. Thus, it was shown that angle-resolved metastable photofragment spectroscopy could be used to help characterize multi-electron excitation states in general. (C) 2012 American Institute of Physics. [doi:10.1063/1.3680561]

We have studied the production of long-lived, highly excited, neutral hydrogen atom fragments following oxygen 1s inner-shell excitation/ionization of gas-phase water molecules using synchrotron radiation. Striking differences to fragment ion and low-n neutral H yields are observed close to threshold. To investigate the decay pathways involved, the neutral fragments were also detected in coincidence with H+, O2+, O+ and OH+ fragments.

Fragmentation of doubly charged ethanol clusters [(C(2)H(5)OH)(n)] following the O 1s ionization has been investigated by means of the photoelectron-photoion-photoion coincidence (PEPIPICO) method. The dominant fission channel of (C(2)H(5)OH)(n)(2+) was the formation of protonated cluster ion pairs [H(C(2)H(5)OH)(l)(+)/H(C(2)H(5)OH)(m)(+)]. The fragmentation mechanisms of these ion pairs were discussed based on the analysis of the PEPIPICO contour shape. It was clarified that the prominent fragmentation channel was a secondary decay mechanism, where neutral evaporation occurs after charge separation. On the other hand, the formation of small fragment ions was suppressed, excluding the formation of certain specific fragments (H(3)O(+), C(2)H(5)(+)/COH(+), and C(2)H(4)OH(+)). The formation of small fragment ions was suppressed due to the cooling effect caused by the neutral evaporation and the decrease in the electrostatic repulsive force caused by charge separation. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3590164]

In order to examine inner-shell electron excitation and induced chemical processes of hydrogen-bonded (HB) molecular clusters, time-of-flight (TOF) fragment-mass and excitation spectra of deuterium-labeled acetic-acid-d (MD) have been studied under the effusive and cluster beam conditions. With O1s(CO) -> pi* co excitation at 532 eV, the TOF spectra of AAD clusters were compared with those of the free molecule. Transient intensity-enhancement of CH(3)(+)/CH(3)CO(+) fragments and growth of mixed-cluster cations. M(n)D(+)/M(n)CH(3)CO(+) could be ascertained in the cluster beams, where MD molecule is denoted by M. Cluster-specific excitation spectra have been generated by monitoring partial-ion-yields of the mixed-cluster cations. Resonance transitions of O1s(CO/OH) -> pi* co were found to shift in energy by +0.37/-0.92 eV upon cluster formation. The production of M(n)D(+) can be attributed to a low-energy pathway of deuteron (proton) transfer following fast electronic decays within the clusters made up of a stable dimer-unit with doubly-bridged HBs. The CH(3)(+)/CH(3)CO(+) intensity variation in beam stagnation pressure clearly shows that a local-fragmentation mechanism as previously proposed is also applicable to the fragmentation processes of the small MD clusters. (C) 2010 Elsevier B.V. All rights reserved.

The emission of low energy and Auger electrons from H2O has been investigated following photo-excitation/ionization at photon excitation energies just below and just above the O is ionization threshold. It is found that neutral oxygen atoms in high Rydberg states are formed. Direct evidence of post-collision interaction (PCI) between the Auger electron and the photo-electron is observed. The initially-excited electron is captured into a Rydberg orbital of H2O+, and remains associated with the oxygen fragment even after the cleavage of both O-H bonds. This implies that the Rydberg states are stable, with the electron behaving as a spectator during the dissociation of H2O+.

Site-dependent C1s-core excitation spectra of small acetaldehyde (AAL) clusters have been studied under the cluster regime of beam conditions. The excitation spectra of the clusters were generated by monitoring the partial-ion-yields of cluster-origin fragments. Comparison of the cluster bands with the monomer bands revealed that the band intensities of C1s core-to-Rydberg transitions come to be strongly excitation-site dependent upon cluster formation showing significant reduction of C1s(Hco)-> Ryd bands relative to the monomer bands. Computer modeling of X-ray absorption spectra using a density functional theory calculation demonstrated that the intensity reduction is the outcome of the site- and geometry-specific CH center dot center dot center dot O interaction where hydrogen-bonding association between the aldehyde (HCO) sites of different AAL molecules takes place exclusively within the clusters. As the representatives of small AAL clusters (n >= 3), two types of molecular complexes (a planar cyclic trimer and a tetramer with a stack of two dimers) were derived to rationalize the site-dependence of the experimental C1s excitation spectra. Spectral simulation of the above complexes could reproduce the experimental site-dependence of the core-to-Rydberg band-features upon clusterization, indicative of significant contribution of these clusters under the present beam-stagnation condition.

We report here the measurements of vibrationally resolved resonant x-ray emission spectra of core-excited N2 and O2 molecules in the 1s → π* resonance region. The measured spectra were compared with the simulated spectra calculated based on the lifetime-vibrational interference theory. We find that the specific excitation to the v' = 0 vibrational level in the intermediate core-hole state of N2 molecule leads to clear vibrational structure (V" = 0 − 3) in both of the final electronic X 1Σ+g (participator band) and a 1Πg (spectator band) states. In contrast to that, the excitation to higher-v' vibrational levels gives rise to the complicated structure in the resultant soft x-ray emission spectra. In the case of O2 molecule, we have observed some structures arising from the vibrational progression, but we find that much higher resolution for the spectrometer is needed to observe distinct vibrational levels.

Hydrogen bonding in acetone clusters was investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of the cluster ions were measured as the NEXAFS spectra of acetone clusters. In the carbon K-edge region, the first resonance peak, which was assigned to the C(CO) 1s ->pi(*)(C=O) resonance transition, showed no substantial change in the PIY curves of the acetone clusters, while the C(CH3) 1s -> 3p pi(CH(3)) excitation feature was found to be strongly suppressed. The selective suppression of the C(CH3) 1s -> 3p pi(CH(3)) resonance transition can be explained by the change in the character of the 3p pi(CH(3)) orbital due to the C=O center dot H-C type of hydrogen-bonding interaction. On the other hand, the NEXAFS spectra of the acetone molecule and clusters were almost identical in the oxygen K-edge region, except for a small shift in the pi(*)(C=O) resonance of 0.13 eV, because the character of the pi(*)(C=O) orbital remained, regardless of the C=O center dot H-C hydrogen bonding interaction.

Ionic fragmentation following the inner-shell photoexcitation of [(CH3)(2)CO]Ar-n heteroclusters was investigated in the Ar L-23- and O K-edge regions. A partial ion yield (PIY) measurement revealed that Ar-n(+) and various acetone fragments (H-m(+), CHm+, C2Hm+, C3Hm+, and CHmCO+) were produced following the Ar L23- shell excitation. In the O K-edge region, the PIY of O+ exhibited a sharp resonance peak at the O 1s -> pi*(C=O) resonance excitation. The selective O+ formation was interpreted as the suppression of statistical fragmentation, which was dominant in isolated acetone, owing to the dissipation of excess energy into Ar clusters. (C) 2008 Elsevier B.V. All rights reserved.

The coordination of C atoms in diamond-like carbon (DLC) thin films formed by Ar gas cluster ion beam (GCIB) assisted deposition using fullerene as a carbon source was investigated using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. From the curve fitting analysis of XPS spectra of the C 1s core level, the absolute sp(2) and sp(3) contents in the DLC films formed by Ar GCIB-assisted deposition were evaluated for the first time. The absolute sp(3) content of DLC films formed by the GCIB-assisted deposition at an acceleration voltage of 5 kV was the highest compared with that formed at 7 and 9 kV. In addition, the absolute sp(2) contents evaluated from XPS spectra were compared to the relative sp(2) contents evaluated from NEXAFS spectra. [DOI: 10.1143/JJAP.47.3380]

Hydrogen bonding in methanol clusters has been investigated by using inner-shell photoabsorption spectroscopy and density functional theory (DFT) calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of H(CH(3)OH)(n)(+) were measured as the soft x-ray absorption spectra of methanol clusters. The first resonance peak in the PIY curves, which is assigned to the sigma*(O-H) resonance transition, exhibits a 1.20 eV blueshift relative to the total-ion-yield (TIY) curves of molecular methanol in the oxygen K-edge region, while it exhibits a shift of only 0.25 eV in the carbon K-edge region. Decreased intensities of the transitions to higher Rydberg orbitals were observed in the PIY curves of the clusters. The drastic change in the sigma*(O-H) resonance transition is interpreted by the change in the character of the sigma*(O-H) molecular orbital at the H-donating OH site due to the hydrogen-bonding interaction. (C) 2008 American Institute of Physics.

The high-resolution absolute photoionization cross sections for Ar, Kr, Xe and N-2 in the inner-shell ionization region have been measured using a multi-electrode ion chamber and monochromatized synchrotron radiation. The energy ranges of the incident photons for the target gases were as follows: Ar: 242-252 eV (2p Rydberg excitation), Kr: 1650-1770 eV (near the 2p ionization thresholds), Xe: 665-720 eV (near the 3d ionization thresholds) and 880-1010 eV (near the 3p ionization thresholds), N-2: 400-425 eV (N Is excitation and ionization). It is the first time to measure the absolute ionization cross sections of Ar, Kr, Xe and N-2 over the present energy ranges with the energy resolution of over 10,000. The natural lifetime widths of 2p(3/2)(-1)4s, 2p(3/2)(-1)3d, 2p(3/2)(-1)4d and 2p(1/2)(-1)4s resonances for Ar, 3d(5/2)(-1)6p resonance for Xe, and 1s(-1) pi(g)* (v' = 0) resonance for N-2 have been obtained based on the cross sections determined. The ionization energies into the Ar+ (2p(3/2)(-1)), Ar+ (2p(1/2)(-1)) and Xe+ (3d(5/2)(-1)) ionic states are also determined using the Rydberg formula. (C) 2007 Elsevier B.V. All rights reserved.

Angle-resolved metastable fragment yields following photo-excitation have been measured in the photon energy region 405.7-430 eV in N-2. Structures corresponding to multiple excitations associated with parallel and perpendicular transitions are observed in the metastable yield curves recorded using detectors orientated at 0 degrees and 90 degrees to the electric field vector of the linearly polarized incident radiation. The double-excitation structures around 415 eV are interpreted in terms of the overlap of three broad peaks and vibrational progressions. Multiple excitation structures corresponding to high Rydberg states converging to N-2(+) shake-up satellites are seen above 418 eV. The present work demonstrates that the angle-resolved detection of metastable fragments is an effective spectroscopic tool for probing core-valence multiply excited states in molecules.

Time-resolved soft X-ray and vacuum ultraviolet fluorescence resulting from the decay processes of 2p inner-shell excited Ar clusters has been observed. Fluorescence with 3-10 ns lifetime is observed, and assigned as being cluster-specific. This fluorescence can be attributed to arising from the decay of excited states of either neutral or singly ionized atoms produced via energy transfer or electron transfer processes including the interatomic Coulombic decay mechanism. The time-uncorrelated neutral yield spectra arise from large Ar clusters. (c) 2006 Elsevier B.V. All rights reserved.

The high-resolution absolute photoabsorption cross section with an absolute photon energy scale for Ne in the energy region of 864-872 eV (1 s(-1) np Rydberg states) has been measured using a multi-electrode ionization chamber and monochromatized synchrotron radiation. The natural lifetime width of Ne 1 s(-1)3p resonance state has been obtained to be 252 +/- 5 meV. The Ne+ (1s(-1)) ionization potential is determined to be 870.16 +/- 0.04 eV by using the Rydberg formula. These absolute values are supposed to be more reliable than those previously reported.

Time-of-flight mass spectra have been measured for inner-shell ionization of ethanol molecules, its clusters, and methanol clusters at the oxygen K-edge regions. Only in the case of the ethanol clusters, a fairly intense signal corresponding to H3O+ has been observed. The measurements for ethanol-d(1) (C2H5OD) suggest that at least one H atom in the H3O+ ion originates from the C2H5 group of neighbouring ethanol. It is proposed that in addition to the O-H center dot center dot center dot OH type hydrogen bonding in the ethanol clusters, the H in the C2H5 group and OH interact with each other. (c) 2006 Elsevier B.V. All rights reserved.

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH center dot H(+) and H(3)O(+) cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C = O/OH) electrons into pi(*)(CO) orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C = O)->pi(*)(CO) transition by similar to 0.2 eV and a redshift of the O1s(OH)->pi(*)(CO) by similar to 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer and/or trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO(+) was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD(2)O(+) was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD center dot D(+) (protonated HCOOH center dot H(+)) cations within the clusters. (c) 2006 American Institute of Physics.

The high-resolution absolute photoabsorption cross sections with an absolute photon energy scale for Ne 1s2p/1s2s double excitation in the energy region of 900-940 eV have been measured using a multi-electrode ionization chamber and monochromatized synchrotron radiation. The Ne [1s2p](P-3,P-1)nln'l' doubly excited states observed in the energy range of 900-920 eV are analysed with the aid of theoretical calculations based on the multiconfiguration Dirac-Fock method. The second strongest peak in this energy region is assigned to be [1s2p](P-3)3p4p.

A new experimental setup for probing multielectron processes in molecular inner-shell ionization regions has been developed. Symmetry-resolved zero-kinetic-energy (ZEKE) spectra have been measured by scanning the photon energy along with monitoring the intensity of the coincidence signals between ZEKE electrons and fragment ions detected at 0 degrees and 90 degrees relative to the electric vector of the light. The actual performance of the method is illustrated by using it to reveal the symmetry decomposition of the multielectron processes, such as double excitations and shake-up satellites, in the K-shell ionization region of nitrogen. (c) 2006 American Institute of Physics.

High resolution angle-resolved ion-yield spectra are reported for the C1s -> Rydberg excitations of acetylene. Vibronic coupling features are found in the energy regions of 3s sigma(g)/3 sigma(u)*, 3p sigma(u), and near threshold. By increasing retarding potentials for ion detectors to select more energetic fragmentation channels, the feature observed in the 90 degrees direction is assigned to the C1s -> 3 sigma(u)* valence state coupled with the C1s -> 1 pi(g)* excited state via cis bending (pi(u)) vibrational mode. (c) 2005 Elsevier B.V. All rights reserved.

Inner-shell excitation of ethanol cluster was investigated using a partial-ion-yield method at the oxygen K-edge. The resonance feature corresponding to the excitation to the 3sa' unoccupied orbital was not observed in the partial yield spectra of cluster ions, while it is clearly seen in the total ion yield spectrum. The absence of this feature is explained by the changes in local electronic structures owing to the hydrogen bonding in the clusters. (c) 2005 Elsevier B.V. All rights reserved.

Total photoabsorption cross section and peak assignments were presented for CF3CCH in the carbon K-shell region. Anisotropy parameters of fragment ions obtained by means of an angle-resolved mass spectrometer were helpful to the peak assignments of the K-shell excitation into pi* and sigma* states. Kinetic energy distributions of the H+, CH+, C2H+ and CF2+ fragment ions were fitted using a Gaussian function with one peak, while those of the CF3+ fragment ion were analyzed by use of two and three components for the K-shell excitation of the CF3 and C2H sides, respectively. The kinetic energy distribution of the CF3+ fragment ion was reasonably understood by the consideration that photofragmentation of the K-shell excited molecule probably competes with intramolecular energy relaxation in which the CF3 group works as an effective energy reservoir. The yields of the CFn+ (n = 2,3), C3FH2+ and C3F2H2+ ions were dependent on the site of excitation. (C) 2004 Elsevier B.V. All rights reserved.

Pump and probe experiments using storage ring free electron laser (SRFEL) and synchrotron radiation (SR) pulses have been tried to perform for the last decade, because the natural synchronization of the SRFEL pulse with the SR pulse makes it easier to do such experiments. Recently, we have successfully carried out the two-photon double-resonant excitation on Xe atoms, utilizing a SR pulse as a pump and an FEL pulse as a probe light. Here, we report the recent experimental results of the investigation for excited states of Xe dimers as well as Xe atoms. The results on the pressure-dependent measurements that were applied to the same system, strongly suggest the formation of Xe dimers in the intermediate states prepared by the primary pump processes. (C) 2004 Elsevier B.V. All rights reserved.

A new experimental setup for probing multi-electron processes in molecular inner-shell ionization regions has been developed at the UVSOR facility, IMS. A zero-kinetic-energy electron (ZEKE) analyzer is composed of a lens system on the basis of the penetrating field technique and an electrostatic analyzer. Two identical ion detectors with regarding grids are utilized for detecting the fragment-ions emitted at 0 and 90 degrees relative to the electric vector of the light, which correspond to the symmetry-resolved photoabsorption spectra. The symmetry-resolved ZEKE spectra have been measured by scanning the photon energy with monitoring the intensity of the coincidence signals between the ZEKE electrons and fragment-ions. The results obtained at SPring-8 using the new setup are reported.