Dynamics of oxygen Rydberg atom generation following O 1s inner-shell excitation of H2O.

The emission of low-energy electrons from H2O has been investigated at photon excitation energies in the vicinity of the O 1s ionization threshold. Neutral oxygen Rydberg atoms (O*) were found to form, and the correlation between the initial inner-shell excited state of H2O and the Rydberg state of O* was determined. The initially excited electron in a Rydberg orbital is shown to remain associated with O* even after the cleavage of two O-H bonds. We also show that the energy discrepancy between two Rydberg states of H2O and O* can be explained by the influence of the post-collision interaction, which becomes stronger as the excitation energy approaches the 1s ionization threshold.

Cascade Auger decays following Si KL23L23 Auger transitions in SiF4.

Cascade Si LVV Auger decays following KL(23)L(23) Auger transitions have been measured in SiF(4) molecule using an electron spectrometer combined with monochromatized undulator radiation. Molecular cascade processes from the two 2p holes states largely generate wide band structures in the spectra due to sequential electron emission leading to multiple valence holes. However, a peak with high yield is observed for the first time at about 103 eV, an energy being considerably higher than the energies of the normal LVV Auger electron, in the instance of the resonant excitation of Si 1s electron into the vacant molecular orbital. This peak is presumed to originate from the participator decay from the state with two 2p holes and one excited electron into the state with one 2p hole and one valence hole. A similar peak with less intensity is detected in the photoexcitation of the 1s electron into a Rydberg orbital. After the normal KL(23)L(23) Auger transition, the resultant cascade spectrum shows several peaks, e.g., 61 eV, 76 eV, and 82 eV. The former two peaks are assigned to the Auger transitions of Si atoms produced through molecular ion dissociation after cascade decays, and the latter is probably ascribed to the second step Auger decay into states having a 2p hole together with two valence holes.

Metastable states in NO2+ probed with Auger spectroscopy.

High-resolution N 1s and O 1s photoelectron spectra (PES) of NO are presented together with spectra of the subsequent Auger decay. The PES are analyzed by taking spin-orbit splitting of the (2)Π ground state into account providing detailed information on equilibrium distances, vibrational energies, and lifetime widths of the core-ionized states. In the Auger electron spectra (AES) transitions to five metastable dicationic final states are observed, with two of them previously unobserved. A Franck-Condon analysis of the vibrational progressions belonging to these transitions provides detailed information on the potential-energy curves of the dicationic final states as well as on the relative Auger rates. The present calculations of the potential-energy curves of NO(2+) agree well with the experimental results and allow an assignment of the two hitherto unresolved Auger transitions to excited states of NO(2+), C(2)Σ(+)and c(4)Π.

Three-electron interatomic Coulombic decay from the inner-valence double-vacancy states in NeAr.

We have unambiguously identified interatomic Coulombic decay in NeAr from the inner-valence double-vacancy state Ne-Ar(2+)(3s(-2)) to outer-valence triple-vacancy states Ne(+)(2p(-1))-Ar(2+)(3p(-2)) by momentum-resolved electron-ion multicoincidence. This is the first observation of interatomic Coulombic decay where three electrons (3e) participate. The results suggest that this 3e interatomic Coulombic decay is significantly faster than other competing processes like fluorescence decay and charge transfer via curve crossing.

Two-dimensional approach to fluorescence yield XANES measurement using a silicon drift detector.

The objective of this article is to describe the capability of a two-dimensional (2D) approach to X-ray absorption near-edge structure (XANES) measurement by means of a partial fluorescence yield (PFY) method. 2D-XANES measurements were achieved by using a silicon drift detector as an energy-dispersive fluorescence detector. The advantage of this technique is that it allows full surveys of X-ray fluorescence data that are lost in conventional PFY measurements. The availability of a map approach was demonstrated by applying it to XANES measurements in both a diluted (Mn-doped nano-diamond) and a concentrated (MnO crystal) manganese sample. The 2D approach clearly distinguished between the PFY spectra of Mn and O atoms, where absorption edges of both elements are close to each other. Further, the 2D approach extracted an unambiguous PFY spectrum of phosphorus in the XANES measurement of SS304 (P < 0.045 wt%).

Photodissociation investigation of doubly charged ethanol clusters induced by inner-shell electron ionization.

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.

Photofragmentation of the K-shell excited perfluorocyclobutane: anisotropies in the fragments and breakdown pathways.

Total ion yield spectrum of perfluorocyclobutane (c-C(4)F(8)) has been measured in the C and F K-shell excitation regions. The peak assignments are presented based on angle-resolved photofragment ion mass spectrometry. The peaks at 291.34 and 688.5 eV are found to come from the transitions from the C 1s and the F 1s to the lowest unoccupied b(2)σ(CF)* orbital, respectively. A photoelectron-photoion-photoion coincidence spectrum is acquired at 700.1 eV for clarifying the breakdown pathways of c-C(4)F(8)(2+). Two series of the pathways are identified; fission of F atom(s) followed by charge separation and elimination of CF(2) or CF(3) followed by charge separation.

Electron-transfer-mediated decay and interatomic Coulombic decay from the triply ionized states in argon dimers.

We report the first observation of electron-transfer-mediated decay (ETMD) and interatomic Coulombic decay (ICD) from the triply charged states with an inner-valence vacancy, using the Ar dimer as an example. These ETMD and ICD processes, which lead to fragmentation of Ar(3+)-Ar into Ar(2+)-Ar(2+) and Ar(3+)-Ar+, respectively, are unambiguously identified by electron-ion-ion coincidence spectroscopy in which the kinetic energy of the ETMD or ICD electron and the kinetic energy release between the two fragment ions are measured in coincidence.

Projection of Si 1s photoexcited orbitals into resonant Auger electron spectra in KLL decays of Si(CH3)4 and SiF4.

Spectator resonant KL(23)L(23) Auger electron spectra have been measured in the Si 1s photoexcitation region of Si(CH(3))(4) using monochromatized undulator radiation combined with a hemispherical electron spectrometer. The broad peak with high intensity in a total ion yield spectrum, coming mainly from excitation of a 1s electron into the 6t(2) vacant orbital, induces a spectator Auger decay in which the excited electron remains in its excited orbital. The component on the higher energy side of this peak through 1s excitation into a Rydberg orbital produces resonant Auger decays in which the excited Rydberg electron moves into a slightly higher Rydberg orbital, or is partly shaken up to a significantly higher Rydberg orbital. These findings of Si(CH(3))(4) indicate a clear contrast to those for SiF(4), in which the 1s excitation into a Rydberg orbital induces a shake-down phenomenon as well as a shake-up one. The results of these molecules exhibit a clear splitting effect among excited orbitals which are smeared out by overlapping due to lifetime widths and due to densely populated levels in the 1s electron excitation spectrum. This is consistent with the calculation on photoexcitation within the framework of density functional theory.

Valence photoelectron spectroscopy of N2 and CO: recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals.

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.

Rotational analysis for the Doppler-free photoelectron spectrum of water using the spectator model.

A photoelectron spectrum of H(2)O has been recorded at a resolution of 2 meV under Doppler-free conditions. Complex rotational structures appear in the individual vibrational states of the electronic X̃(+ 2)B(1) and Ã(+ 2)A(2) states in H(2)O(+). The rotational structures are analyzed and well reproduced using a spectator orbital model developed for rotationally resolved photoelectron spectroscopy.

Dissociation of core-valence doubly excited states in NO followed by atomic Auger decay.

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined.

Development of a differential pumping system for soft X-ray beamlines for windowless experiments under normal atmospheric conditions.

A novel design for a differential pumping system has been investigated. This system allows windowless experiments in a soft X-ray beamline under normal atmospheric conditions. The new design consists of an aperture-based four-stage differential pumping system, based on a simple model calculation. A prototype system with a total length of 600 mm was constructed to confirm the validity of the design concept. Relatively short conductance-limiting components allow easy installation and alignment of the system on a synchrotron beamline. The fabricated system was installed on a beamline to test the transmission of soft X-rays through atmospheric helium.

Site-specific behavior in de-excitation spectra of F(3)SiCH(2)CH(2)Si(CH(3))(3) in the Si 1s excitation region.

Excitation (total ion yield) and de-excitation (resonant photoemission) spectra have been measured in the Si 1s photoexcitation region of the F(3)SiCH(2)CH(2)Si(CH(3))(3) molecule using monochromatized undulator radiation. Theoretical calculations within the framework of density functional theory have reproduced the observed total ion yield spectrum very well. The first peak at the lowest photon energy, coming from Si 1s excitation at the trimethyl side into a vacant orbital, induces spectator Auger decays in which the excited electron remains in its valence orbital. The second peak produced through excitation of Si 1s electron at the trifluoride side generates resonant Auger decays in which the excited valence electron remains predominantly also in the valence orbital or is partly shaken up into higher Rydberg orbitals. The third peak generated through Si 1s excitation at the trifluoride side produces resonant Auger decays in which the excited Rydberg electron remains or is partly shaken down to a lower lying valence molecular orbital. These findings exhibit a clear distinction between resonant Auger decays following photoexcitation of Si 1s electrons under different chemical environments.

Hydrogen bonding in acetone clusters probed by near-edge x-ray absorption fine structure spectroscopy in the carbon and oxygen K-edge regions.

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-->3ppi(CH(3)) excitation feature was found to be strongly suppressed. The selective suppression of the C(CH3) 1s-->3ppi(CH(3)) resonance transition can be explained by the change in the character of the 3ppi(CH(3)) orbital due to the C=O...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...H-C hydrogen bonding interaction.

Is CO carbon KVV Auger electron emission affected by the photoelectron?

Angular distributions (ADs) of O+ fragments from C 1s photoexcited CO detected in coincidence with carbon KVV Auger electrons emitted in the horizontal direction were measured at photon energies of 298, 305, 320, and 450 eV. At 450 eV, the ADs are polarization-independent and coincide with the molecular-frame Auger electron angular distribution. All measured ADs can be rationalized as a product of the same molecular-frame Auger electron angular distribution and the axial selectivity in the photoionization process. Thus the interaction between the photoelectron and the Auger electron for the normal Auger decay of CO can be neglected, and the two-step model is a good approximation.

Mechanisms of spontaneous two-electron emission from core-excited states of molecular CO.

We demonstrate that the observation of slow electrons emitted in the decay of molecular core-excited states can be a sensitive probe of the double Auger processes, and that in combination with electron-electron coincidence spectroscopy, it can provide clear insight into the mechanisms involved. The present study identifies all cascade Auger paths from the C1s-to-Rydberg states in CO to final states of CO2+. One pathway includes the first directly identified case of molecular level-to-level autoionization of a cation and shows remarkable selectivity for a specific final state.

X-ray absorption measured in the resonant Auger scattering mode.

We report both experimental and theoretical studies on x-ray absorption measured in the resonant Auger scattering mode of gas phase carbon monoxide near the O1s-->2pi region. Both experiment and theory display a crucial difference between the x-ray absorption profiles obtained in the conventional and resonant scattering modes. Lifetime vibrational interference is the main source of the difference. It is demonstrated that such interference, which arises from a coherent excitation to overlapping intermediate levels, ruins the idea for obtaining x-ray absorption spectra in a lifetime broadening free regime.

Hydrogen bonding in methanol clusters probed by inner-shell photoabsorption spectroscopy in the carbon and oxygen K-edge regions.

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.

Study of the dissociation of nitrous oxide following resonant excitation of the nitrogen and oxygen K-shells.

A photochemistry study on nitrous oxide making use of site-selective excitation of terminal nitrogen, central nitrogen, and oxygen 1s-->3pi excitations is presented. The resonant Auger decay which takes place following excitation can lead to dissociation of the N2O+ ion. To elucidate the nuclear dynamics, energy-resolved Auger electrons were detected in coincidence with the ionic dissociation products, and a strong dependence of the fragmentation pathways on the core-hole site was observed in the binding energy region of the first satellite states. A description based on the molecular orbitals as well as the correlation between the thermodynamical thresholds of ion formation and the first electronic states of N2O+ has been used to qualitatively explain the observed fragmentation patterns.