星野 正光

Absolute differential and integral cross section measurements for electron impact excitation of the dissociative (a) over tilde B-3(1), (b) over tilde (3)A(1) and (B) over tilde (1)A(1) electronic states of water are described. These are very important channels for the production of OH (X2 Pi,v) radicals, as well as for understanding the origin of the atmospheric Meinel bands. The energy range of our measurements is 20 - 50 eV, with the angular range of the differential cross section data being 10 degrees - 90 degrees. Where possible comparison of the present cross sections with the corresponding results from theoretical calculations is made, the level of agreement between them being found to be quite marginal.

The authors report absolute differential and integral cross section measurements for electron-impact excitation of the A (1)B(1) electronic state of water. This is an important channel for the production of the OH (X (2)Pi) radical, as well as for understanding the origin of the atmospheric Meinel [Astrophys. J. 111, 555 (1950)] bands. The incident energy range of our measurements is 20-200 eV, while the angular range of the differential cross section data is 3.5 degrees-90 degrees. This is the first time such data are reported in the literature and, where possible, comparison to existing theoretical work, and new scaled Born cross sections calculated as a part of the current study, is made. The scaled Born cross sections are in good agreement with the integral cross sections deduced from the experimental differential cross sections. In addition they report (experimental) generalized oscillator strength data at the incident energies of 100 and 200 eV. These data are used to derive a value for the optical oscillator strength which is found to be in excellent agreement with that from an earlier dipole (e,e) experiment and an earlier photoabsorption experiment. (c) 2007 American Institute of Physics.

Angle-resolved energetic-ion yield spectra have been measured in the 0 Is excitation region of N2O. Franck-Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states reproduces well the observed vibrational excitations specific to the individual Rydberg states. The irregular Rydberg behavior in the Sigma-symmetry absorption spectrum is attributed the valence-Rydberg coupling in light of the second moment analysis. (c) 2006 Elsevier B.V. All rights reserved.

Site-specific fragmentation caused by C KVV and F KVV Auger decay of free CH3F molecules has been studied. Energy-resolved Auger electrons and pairs of mass-resolved ions were detected in coincidence. Strong enhancements for the production of CH3+-F+ and H+-CF+ ion pairs were found for the F KVV and C KVV Auger decays, respectively. (c) 2007 Elsevier B.V. All rights reserved.

The doubly differential cross sections (DDCS) of double electron capture in collisions of slow C4+ with He were measured in the energy range of 240, 320, and 440 eV (in laboratory frame) using a crossed-beam apparatus and represented as a two-dimensional contour map. The double-electron capture into the C2+(1s(2) 2s(21)S) state was found to be dominant in the present energy range. The Stuckelberg oscillation structures, which result from interference among two different paths on the interaction potential curves, were clearly observed in the DDCS at the present low impact energies. The differential cross sections were also calculated in an ab initio molecular orbital framework for (CHe)(4+), and show a good agreement with the present experimental results.

The doubly differential cross sections (DDCS) of double electron capture in collisions of slow C4+ with He were measured in the energy range of 240, 320, and 440 eV (in laboratory frame) using a crossed-beam apparatus and represented as a two-dimensional contour map. The double-electron capture into the C2+ (1 s2 2 s2 S1) state was found to be dominant in the present energy range. The Stückelberg oscillation structures, which result from interference among two different paths on the interaction potential curves, were clearly observed in the DDCS at the present low impact energies. The differential cross sections were also calculated in an ab initio molecular orbital framework for (CHe)4+, and show a good agreement with the present experimental results. © 2007 The American Physical Society.

We report threshold electron energy-loss spectra for the fluorohalomethanes CF3X (X=Cl,Br). Measurements were made at incident electron energies of 30 and 100 eV in energy-loss range of 4-14 eV, and at scattering angles of 4 degrees and 15 degrees. Several new electronic transitions are observed which are ascribable to excitation of low-lying states as well as are intrinsically overlapped in the molecules themselves. Assignments of these electronic transitions are suggested. These assignments are based on present spectroscopic and cross-section measurements, high-energy scattering spectra, and ab initio molecular orbital calculations. The calculated potential curves along the C-X bond show repulsive nature, suggesting that these transitions may lead to dissociation of the C-X bond. The present results are also compared with the previous ones for CF3H, CF4, and CF3I. (c) 2007 American Institute of Physics.

We have developed methods to focus slow highly charged ions, MeV ions, and muon beams with various glass-made beam optics. (1) A focusing effect for an Ar8+ beam of 8 keV through a cm-long tapered capillary was obtained with a density enhancement of the transmitted beam compared with that of the input beam, which increases from 1 to 6 as the input current decreases from 30 pA to 0.8 pA. To study the stability of the transmitted beams through a glass capillary, we have measured the transmission of an 104 keV Ar8+ beam through a gap between a pair of parallel glass plates, and observed a precisely vibrational output current. (2) For 4 MeV He2+ beam, a 100 times density enhanced beam by a cm-long tapered capillary with a closed outlet was utilized to irradiate a cell in liquid. The range of the beam was controlled by the closed outlet with accuracy of similar to 1 mu m. (3) Using 40 cm-long tapered glass tubes, a density enhancement of a factor of similar or equal to 2 was observed for both positive and negative muon beams with an energy of 13 MeV.

An apparatus combining the cross-beam and threshold-ionization techniques has been used to measure absolute cross sections for electron impact dissociation of CH4 molecules into the CH3 neutral radicals at energies from threshold up to 13 eV. The threshold energy for the lowest-lying T-3(2) state has been observed to be 7.5 +/- 0.3 eV, and attributed to neutral CH3 formation. Peaks have been observed at similar to 9.6 and 11.5 eV, in agreement with electronic excitation and photon impact neutral dissociation literature data. The current results indicate that all excited states of CH4 predominantly result in dissociation via the CH3 neutral radical channel below 12.5 eV.

O K-shell photoionization of the CO2 molecule causes antisymmetric vibrations v '(3) in the core-ionized molecule. The v '(3)-resolved partial cross sections sigma(+)(v ' 3) and asymmetry parameters beta(v ' 3) are measured in the Sigma(u) shape resonance region above the O K-shell ionization threshold of the CO2 molecule. Neither the shape resonance energy in sigma(+)(v ' 3) nor the value of beta(v ' 3) shows v '(3)- dependence. This v '(3)- independent nature is attributed to the fact that v '(3) vibrations do not cause any change of the size of the potential box responsible for the formation of the shape resonance. Multiple scattering calculations employing the v '(3)-dependent fixed-nuclei approximation reproduce well the measured v '(3)-resolved partial cross sections.

We have measured the vibrationally resolved partial cross sections sigma(nu'1) and asymmetry parameters beta(nu'1) for C K-shell photoionization of the CO2 molecule in the Sigma(u) shape resonance region above the C K-shell ionization threshold. The positions of both the maxima of sigma(nu'1) and the minima of beta(nu'1) move towards the C K-shell threshold with increasing symmetric stretching vibrational excitation nu('1) in the C 1s single-hole state. Calculations employing the relaxed-core Hartree-Fock approach reproduce the observed vibrational effects.

Electron impact vibrational excitation cross sections are presented for carbonyl fluoride (F2CO). Measurements have been done for two of the six fundamental modes; the C=O stretching and the F-C-F symmetric vibrational modes. In the comparison of the vibrational C=O stretching mode in F2CO with those in H2CO and CO molecules, the resonance energy at 20 eV in CO was observed shifted to 17 eV in F2CO and to 16 eV in H2CO, a phenomenon explained as due to the increasing C=O bond length, in the order CO < F2CO < H2CO. (c) 2006 Elsevier B.V. All rights reserved.

The C K-shell photoelectron mainline of C2H2 molecules have been measured at photon energies 295-350 eV. The gerade and ungerade symmetries of the core-ionized states, 1 sigma(g) and 1 sigma(u), are resolved. Resonance enhancement is found only in the log photoionization cross-section and assigned to the sigma(u)(*) shape resonance. The photoelectron asymmetry parameter takes a minimum value at the shape resonance energy. The shape resonance energy decreases with increase in the vibrational quantum number. No correlational shape resonance appears in the 1 sigma(u) photoionization channel, suggesting that the channel coupling is negligible. (c) 2006 Elsevier B.V. All rights reserved.

In charge-transfer collisions of C4+(1s(2 1)S) with He (1s(2) S-1), the process of double electron capture into the ground state C2+ is well-known to dominate other channels by an order of magnitude for projectile energies below 10 keV. This work presents a calculation of differential cross-sections resolved in the angle and energy gain variables, based on an ab initio treatment of electronic states, and compares with the measurements published in the literature (projectile energy E=270, 400, and 470 eV). We also briefly discuss the semi-empirical two-state models developed by experimentalists for this process.

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi(*) shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect. (c) 2006 American Institute of Physics.

Symmetry-resolved x-ray absorption spectroscopy has been first carried out on high-temperature molecules. From the angle-resolved ion yield spectra of CO2 both at room temperature and at 430 degrees C, symmetry-resolved absorption profiles of the C 1s(-1)2 pi(u) and O 1s(-1)2 pi(u) resonances have been extracted for the vibrational ground state molecules and bending-vibration excited ones. The profiles change dramatically between them, and the Renner-Teller effect becomes more evident for the vibrationally excited molecules. The effects of the multimode vibronic coupling are suggested for the O 1s(-1)2 pi(u) and O 1s(-1)3s sigma(g) resonances.

We have carried out a coincidence experiment between energy-resolved resonant Auger electrons and mass-resolved ions on CH3F molecules following F 1s and C 1s excitation to the lowest unoccupied C-F antibonding molecular orbital sigma(CF)(*). We found a strong enhancement of the F+ or CH+ ion production in coincidence with the F KVV and C KVV spectator Auger electrons, respectively, in the wide binding energy range of 28-36 eV. This site-selective ion production is interpreted as a consequence of the resonant Auger emission taking place in the transient region where the C-F elongation caused by the core excitation transforms the molecular valence orbitals gradually into nonoverlapping valence orbitals of each fragment.

We have carried out experimental and theoretical studies on electron scattering from the C3H6 isomers and C3F6 molecules and we report on total, differential as well as theoretical integral elastic cross-sections for these molecules. Vibrational excitation functions are also presented for the typical vibrational peaks in C3H6 and cyclo-C3H6 for the angle of 90 degrees, impact energy range of 1 - 16 eV and loss energies of 0.12 eV and 0.13 eV, respectively. In the cross-sections, clear differences in peak positions and magnitudes between the C3H6 isomers can be viewed as the isomer effect. The same is observed between C3H6 and C3F6 in a clear manifestation of the fluorination effect. The resemblance of the pi* shape resonance in the cross-sections, observed at about 2.2 eV for C3H6 and 3.5 eV for C3F6, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Theoretical analysis is performed to provide rationales for the scattering dynamics.

K X-rays emitted from 2.1 keV/u N ions transmitted through a thin Ni microcapillary foil were measured in coincidence with a final charge state q(f), using a Si(Li) X-ray detector in the cases of incident charge states of q(i) = 7 and 6. It was found that (1) the coincidence X-ray yield for (q(i),q(f)) = (7,6) decreased faster than those for other (q(i),q(f)) combinations, (2) the coincidence X-ray yields did not depend on the number of K-shell holes of the incident ions but on the final charge state q(f). (c) 2005 Elsevier B.V. All rights reserved.

We have set up a coincidence apparatus consisting of a high-resolution electron spectrometer and a k-resolving ion spectrometer. As an example for measurements performed with this apparatus, we present ion mass spectra that are coincident with energy-resolved resonant Auger electrons emitted from F 1s excited SF6 molecules and resonant Auger electron spectra that are coincident with mass-selected fragment ions, as a function of the detuning from the F(1s(-1))a(1g)* resonance in SF6. Significant changes of these spectra due to detuning suggest that a substantial fraction of the excess photon energy goes into fragmentation rather than into electron kinetic energy. (c) 2005 Elsevier B.V. All rights reserved.

We have carried out angle-resolved measurement for the C 1s photoelectron shakeup satellites of gaseous CO molecules, at unprecedented energy resolution, and resolved the vibrational structures for one (2)Sigma(+) and two (2)Pi bands. The present ab initio calculations for the potential curves of these shakeup satellite states and the relevant Franck-Condon analysis well reproduce the vibrational structures of these bands and confirm the assignments.

The lowest-energy satellite bands accompanying the O 1s mainline Of CO2 molecule have been investigated, using angle-resolved photoelectron spectroscopy. The measurements were carried out at beamline 27 at SPring-8 in Japan with a high-resolution electron spectroscopy apparatus consisting of a hemispherical energy analyzer (SES-2002) combined with a gas cell. We found that the internal inelastic scattering is significant while the conjugate shake-up plays its role as well. (c) 2005 Elsevier B.V. All rights reserved.

High-resolution angle-resolved molecular resonant Auger emission from the F 1s excited states of the CF4, SiF4, and SF6 molecules are reported. A number of strong spectator lines resulting into the dissociative final states, whose anisotropy changes by changing the excitation photon energy, dominate the resonant Auger spectra. (c) 2005 Elsevier B.V. All rights reserved.

The vibrationally resolved C 1s single-hole ionization cross sections of H2CO are measured in the region of the sigma(*) shape resonance. The energy of the shape resonance is pushed down by the excitation of the C-O stretching vibration v(2)('). Our calculations, which take account of elastic multiple scattering within the C-O potential box, show that the downward shift of the shape resonance energy is originated from the elongation of the characteristic internuclear C-O distance in the core-ionized molecule with an increase in v(2)('). The characteristic internuclear distance varies from R+(0,0,0)=1.198 angstrom up to R+(0,2,0)=1.415 angstrom. In addition to the elastic multiple-scattering mechanism, the internal inelastic scattering of the C 1s photoelectron by valence electrons is suggested as a possible mechanism responsible for the enhancement of vibrational excitations through the shape resonance and near threshold.

We have made coincidence measurements between L X-rays and final charge states of highly-charged Ar ions passing through a Ni microcapillary thin foil to study the formation and relaxation processes of hollow atoms. By using N6+ projectile ions which have single K-shell hole, Ninomiya et al. [Phys. Rev. Lett. 78 (1997) 4557] showed the existence of long lived excited states formed by multiple electron transfer from a Ni microcapillary thin foil. To further study the formation and relaxation dynamics, such experiments were extended to Ar ions with multiple L-shell holes. The yields of L X-rays from Ar ions, which have one stabilized-electron, decrease faster than those from ions with multiple stabilized-electrons, when the projectiles have multiple inner shell holes. (c) 2005 Elsevier Ltd. All rights reserved.

The vibrationally resolved C 1s single-hole ionization cross sections of H2CO are measured in the region of the σ* shape resonance. The energy of the shape resonance is pushed down by the excitation of the C-O stretching vibration v2′. Our calculations, which take account of elastic multiple scattering within the C-O potential box, show that the downward shift of the shape resonance energy is originated from the elongation of the characteristic internuclear C-O distance in the core-ionized molecule with an increase in v2′. The characteristic internuclear distance varies from R+(0,0,0)=1.198 up to R+(0,2,0)=1.415. In addition to the elastic multiple-scattering mechanism, the internal inelastic scattering of the C 1s photoelectron by valence electrons is suggested as a possible mechanism responsible for the enhancement of vibrational excitations through the shape resonance and near threshold. © 2005 The American Physical Society.