Odagiri Takeshi

We have developed three pulse selectors to obtain soft x-ray pulses with repetition rates suitable for electron coincidence measurements. The devices enable us to pick out the light pulse from the solo bunch located in the center of the 364-ns dark gap of the hybrid-filling operation. We report the development status and show some typical results of the coincidence measurements with the pulse selectors.

We report here a detailed experimental study on multiple Auger decay after core excitation into the π^* valence and Rydberg orbitals in N₂ by an electron-electron coincidence technique.

Dipole oscillator strengths for the formation of excited hydrogen atom from C₂H₂ were measured to investigate formation and decay of molecular superexcited states. It seems that non-adiabatic transition plays an important role in the formations of the hot hydrogen atom.

Multi-electron coincidence measurements have been performed at the photon energies for the core-to-valence (1s → π^*) and core-to-Rydberg (1s → 3sσ and 3pπ) resonant excitations in N₂ in order to investigate the dynamics of multiple Auger-electron emissions from these core-excited states in detail. Peaks due to slow electrons from superexcited atomic fragments are observed in the decay processes by emission of two or three Auger electrons, indicating stepwise (cascade) multiple Auger decays that involve faster dissociations than electronic relaxations. Energy partitions between the emitted electrons enable us to reveal the detailed decay mechanisms for these processes. Branching ratios among the decays by emission of one, two, or three Auger electrons and those between the simultaneous (direct) and stepwise (cascade) processes have been determined for each of the core-excited states. Branching ratios of decay channels resulting in molecular or fragment ions have also been substantiated.

The angular correlation functions (ACFs) of a pair of Lyman-α photons in photodissociation of H2 and D2 are measured with linearly polarized incident light at a 33.66-eV incident photon energy. Searching for reasonable 2p atom-pair states that reproduce the experimental ACFs to solve the issue, we show that hydrogen molecules are photoexcited to the Q2 1Πu (1) state in the Franck-Condon region and then the Q2 1Πu (1) state comes to superpose with the Q2 3Σu+(2) state as the internuclear distance increases to infinity. The 2p atom pairs turn out to be in the 1u superposition state, which is entangled.

A set of cross sections for the formation of a pair of 2p atoms on an absolute scale is determined against the incident photon energy in the double photoexcitation of the isotopomers H2, HD, and D2, incorporating the same cross sections of H2 and D2 obtained in our recent experiments, and the oscillator strengths for the formation of a pair of 2p atoms from the precursor Q2 1u (1) state are determined from the cross sections. The oscillator strength of HD is found to be larger than the value expected from H2/D2 considering the same decay mechanism for H2, HD, and D2 molecules photoexcited to the Q2 1u(1) state. The origin of the enhancement is attributed to nonadiabatic transitions between a gerade electronic state and an ungerade one through a term neglected in the Born-Oppenheimer approximation.

An absolute total cross-section for electron scattering from O₂ is obtained in the energy range from 20 eV down to 16 meV with a very narrow electron energy width of 7 meV, using the threshold-photoelectron source employing synchrotron radiation. The total cross-section obtained in the present study generally agrees well with those obtained in previous experimental works in the energy region where the previous experimental total cross-sections agree with each other. At lower energies, several features are found, including the very large contribution of the ²Π_g resonance to the total cross-section. The resonance energies and the resonance widths of the individual vibrational states of the ²Π_g resonance are determined from the analysis of the measured absolute total cross-section. The resonance width of the ²Π_g resonance of O₂ is found to be much narrower than the value estimated from previous high-resolution experiments and theoretical calculations.

The cross sections for the formation of the H(2p) and H(2s) atoms, σ2p and σ2s, respectively, in photoexcitation of C2H2 were obtained in an absolute scale for studying formation and decay of superexcited states in the extreme ultraviolet range. Several superexcited states of C2H2 including multiply excited states were found in the curve of the σ2p cross sections as a function of the incident photon energy. The same states seem to contribute to the variation in the σ2s cross sections as well, which can be ascribed to the non-adiabatic transitions between the 2p and 2s channels. The Σ/Π symmetry-resolved cross sections for the H(2s) atom formation, σ2s(Σ) and σ2s(Π), were also obtained on an absolute scale. The coupling between the 1Σu+ and 1Πu states was found to be small.

Superexcited states of H2S have been investigated with determining the cross sections for emission of dispersed and nondispersed atomic fluorescence against the incident photon energy in the range 11–40 eV to address the ionization and excitation of the valence electrons. This method enables us to extract the discrete electronic state from the superposition with continuous electronic states. The cross sections for H(2p) formation have been put on an absolute scale. Ten superexcited states have been found, two in the range 13–15 eV are singly excited 2b2^−1(mo) states with a single configuration and the other eight states in the range 16–25 eV are doubly excited states with multiple configurations. State-resolved dipole oscillator strengths for H(2p) formation in the photoexcitation of H2S have been determined. Similar experiments have been performed for H2O. The state-resolved dipole oscillator strengths for H(2p) formation in the photoexcitation of H2S are ∼10^−3, whereas those in the photoexcitation of H2O range from ∼10^−3 to ∼10^−2.

The dynamics of doubly excited molecules mediated by the absorption of a single photon are a subject of current interest. The key to observing the doubly excited states is measuring cross sections free of ionization as a function of incident photon energy. In the present investigation, we measured the absolute values of the cross section for the formation of a 2p atom pair in the photoexcitation of H₂ and D₂ against the incident photon energy in the range of doubly excited states by means of the coincidence detection of a pair of Lyman-α photons. It turns out that the cross-section curves are attributed only to the contribution of the doubly excited Q₂ ¹Π_u (1) state. Using the present results and previous ones obtained by our group [1], the dissociation dynamics of the Q₂ ¹Π_u (1) state are comprehensively revealed.

The single, double, and triple Auger decays from the 1s shake-up states of O-2 have been studied using a multi-electron coincidence method. Efficient populations of two-hole final states are observed in single Auger decays of the pi-pi* shake-up states, which is understood as a characteristic property of the Auger transitions from shake-up states of an open-shell molecule. The O-2(3+) populations formed by double Auger decays show similar profiles for both the O1s(-1) and shake-up states, which is due to the contributions from cascade double Auger processes. While the cascade contributions to the double Auger decays increase with the initial shake-up energy, the probability of direct double Auger processes remains unchanged between the O1s(-1) and shake-up states, which implies a weak influence of the excited electron on the double Auger emission that originates from the electron correlation effect. Published by AIP Publishing.

We have measured the coincidence time spectra for the pair of the Lyman-α and Lyman-β photons produced in the photodissociation of H2 in the range of the incident photon energy 32.5 - 42 eV. The result shows that the pair of H(2p) and H(3p) fragments is produced in the photodossociation of H2 molecules.

Cross sections for the formation of H(2p) atom in photoexcitation were measured for H2 in the lowest rotational level in the energy range of the doubly excited states for examining contribution of the non-adiabatic transition between the 1Σu - 1Πu doubly excited states.

Double-electron excitations in electron collisions with NH3 are investigated in the range of the outer valence and inner valence excitations by means of angle-resolved electron-photon coincidence measurements. Double-electron and single-electron excitations are termed double and single excitations, respectively, in this article. It is found that the dissociative double excitation resulting in H(2p) formation becomes more dominant over the dissociative single excitation at 100-eV incident energy as the electron scattering angle increases just from 8 degrees. to 15 degrees. This remarkable domination is not expected from the independent particle model and strongly supports the double-excitation mechanism through the electron correlation induced by the penetration of the incident electron depth into the valence orbitals of a NH3 molecule.

Absolute grand total cross sections for electron scattering from N-2 are obtained in the energy range from 20 eV down to 5 meV with very narrow electron energy width of 9 meV using the threshold-photoelectron source. Total cross sections obtained in the present study are compared with the previous experimentally obtained results. At the very-low energy region below 50 meV, the present total cross sections are somewhat smaller than those reported by the Aarhus group [S.V. Hoffmann et al., Rev. Sci. Instrum. 73, 4157 (2002)], which has been the only experimental work that provided the total cross sections in the very-low energy region. The energy positions of the peaks in the total cross sections due to the (2)Pi(g) shape resonance are obtained with higher accuracy, due to the improved uncertainty of the energy position in the present measurement compared to the previous works. The resonance structure in the total cross sections due to the Feshbach resonances of N-2 at around 11.5 eV are also observed. Analysis of the resonant structure was carried out in order to determine the values of resonance width of Feshbach resonances of N-2.

Multielectron coincidence spectroscopy has been performed for Ar at a photon energy of only 0.2 eV above the 2p(1/2) threshold. It is revealed that a postcollision interaction induced by doubleAuger decay leads to photoelectron recapture, followed by reemission of the captured electron, where the recapture of the slow photoelectron forms theAr(2+) Rydberg-excited stateswhich subsequently undergo autoionization. The energy correlation of the emitted electrons discloses that both direct and cascade paths in the double Auger decay contribute to the photoelectron recapture.

The absolute values of the cross section for formation of a 2p atom pair in the photoexcitation of H-2 and D-2 are measured against the incident photon energy in the range of doubly excited states by means of the coincidence detection of two Lyman-alpha photons. The cross-section curves are explained only by the contribution of the doubly excited Q(2) (1)Pi(u)(1) state. The isotope effect on the oscillator strengths of 2p + 2p pair formation for H-2 and D-2 from the Q(2) (1)Pi(u)(1) state is almost the same as that on the oscillator strengths of 2s + 2p pair formation from the Q(2) (1)Pi(u)(1) state obtained by our group [T. Odagiri et al., Phys. Rev. A 84, 053401 (2011)]. This channel independence indicates that both isotope effects are dominated by the early dynamics of the Q(2) (1)Pi(u)(1) state, before reaching the branching point into 2p + 2p pair formation and 2s + 2p pair formation.

Absolute total cross sections for electron scattering from He, Ne, Ar, Kr and Xe at very low electron energies are presented. The cross sections were obtained using the threshold-photoelectron source, which employs a combination of the penetrating field technique together with the threshold photoionization of atoms by synchrotron radiation. Obtained total cross sections for electron scattering from these noble gas atoms generally agree well with those obtained in the previous experiments above 100 meV, where several experimental works have been reported. Comparison of the measured cross section for He with that of theoretical ones shows very good agreement at very low energies even below 10 meV, which confirms the validity of theoretical cross sections of He which have been regarded as the 'standard' cross sections. Scattering lengths for the e(-) - noble gas scatterings determined from the our cross sections using the modified effective range theory (MERT) showed that scattering lengths for He and Ne agree well with the values obtained in the previous experimental and theoretical studies. On the other hand, in case of heavier noble gas atoms, significant discrepancies were found between the scattering lengths derived from MERT analysis to our total cross sections and those reported in previous studies.

The angular correlation functions of a pair of Lyman-alpha photons in the photodissociation of H-2 are measured more precisely than in the early experiments [Nakanishi et. al., Phys. Rev. A. 90, 043405(2014)]. The linearly polarized light is used to excite the photodissociation via the doubly excited Q(2) (1)Pi(u)(1) state.

The angular correlation functions (ACFs) of a pair of Lyman-α photons emitted from H fragments in the photodissociation of a hydrogen molecule are measured at a 33.66-eV incident photon energy and at hydrogen gas pressures of approximately 0.1 and 1 Pa. The ACFs are measured for both opposite and nonopposite arrangements of the two photon detectors. It turns out that the experimental ACFs involve neither the contribution of the reactions H(n=2)+H2 nor the contribution of the cascade from H(n ≥ 3) to H(2p) fragments. Thus the experimental ACFs are those for primary H(2p) pairs following the photodissociation of H2. The experimental ACFs are compared with (i) the theoretical ACF for entangled pairs of H(2p) atoms, where the magnetic quantum number of each hydrogen atom is indefinite, and (ii) the theoretical ACF for H(2p) pairs with definite magnetic quantum number of each hydrogen atom relative to the internuclear axis [the former entangled state of H(2p) pairs is a sum of the latter pair states with definite magnetic quantum number]. In the theoretical ACF in (ii), the disentanglement in H(2p) pairs during the dissociation is considered. The experimental ACFs show a similar tendency in angular dependence to the theoretical ACF for entangled H(2p) pairs. However, there still remains a considerable difference in the variation magnitude between those experimental and theoretical ACFs. The experimental ACFs show the reverse tendency in angular dependence to the theoretical ACF for H(2p) pairs with definite magnetic quantum number of each hydrogen atom relative to the internuclear axis. We thus conclude that the pair of H(2p) atoms in the photodissociation of H2 is unlikely to be in the definite states of magnetic quantum number of each hydrogen atom relative to the internuclear axis, i.e., unlikely to be in the components of the entangled state of H(2p) pairs.

Absolute total cross sections for electron scattering from He and Ne are obtained in the energy range from 20 eV down to below 10 meV with a very narrow electron energy width of 6-8 meV using the threshold-photoelectron source. Total cross sections obtained in the present study generally agree well with those obtained in the previous experiments for both He and Ne above 100 meV, where several experimental works have been reported. Comparison of the present cross sections with the theoretical cross sections which have been regarded as the "standard" cross sections shows very good agreement even at very low energies below 10 meV, which confirms the validity of theoretical cross sections. The scattering lengths for electron scattering from He and Ne are also determined from the present total cross sections using the modified effective range theory. The resonant structures in the total cross sections due to Feshbach resonances of He and Ne are also observed. Analysis of the resonant structure was carried out based on the spin-dependent resonant scattering theory in order to determine the values of natural width of Feshbach resonance of Ne.

Absolute total cross sections for electron scattering from He and Ne at electron energies ranging from 6 meV to 20 eV were obtained with the experimental technique employing the threshold-photoelectron source. The measured total cross sections were compared with previous experimental and theoretical cross sections. © Published under licence by IOP Publishing Ltd.

A new experimental technique for the total cross section measurements of ultra-low energy electron collisions with atoms and molecules utilizing the synchrotron radiation is presented. The technique employs a combination of the penetrating field technique and the threshold photoionization of rare gas atoms using the synchrotron radiation as an electron source in order to produce a high resolution electron beam at very low energy. Absolute total cross sections for electron scattering from He, Ne, Ar, Kr, and Xe in the energy region from extremely low electron energy to 20 eV are presented.

The absolute cross sections for the formation of the H(2s) and H(2p) atoms, sigma(2s) and sigma(2p), respectively, in photoexcitation of CH4 and NH3 were measured in the range of the incident photon energy 15-48 eV for studying superexcited states of the molecules. The same superexcited states were found to contribute to the sigma(2s) and sigma(2p) cross sections. It was concluded that the non-adiabatic transitions play a significant role during the dissociation of the superexcited states and ionic states. (C) 2013 AIP Publishing LLC.

The cross sections for emission of two fluorescence photons from a pair of excited fragments in photoexcitation of H2O have been measured as a function of the incident photon energy using the photon-photon coincidence technique. The cross section increased in the range 30-45 eV, i.e. in the vicinity of the double ionization potential of H2O. The increase of the cross section was attributed to three-body neutral dissociations of a water molecule via multiply excited states: H2O** -> H(2p) + OH** -> H(2p) + H(2p) + O(P-3). Some multiply excited states of H2O were also found in the cross section curve around 65 eV.

Double Auger decay of O1s(-1) and its satellite states in H2O has been studied with a multi-electron coincidence method, and a process leading to autoionizing O* fragments has been revealed. The breaking of the two O-H bonds producing the autoionizing O* fragments occurs for highly excited H2O2+ populated by the initial Auger decay. The O* fragments are more favorably produced in the decay from the satellite states, resulting from the larger population of highly excited H2O2+ states inheriting the valence excitation in the initial state. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768213]

A new experimental technique for the measurements of the total cross section for electron scattering from atoms and molecules at very low energy is described. Momentum transfer cross sections for scattering from Ar, Kr and Xe at very low energies were carefully derived using the modified effective range theory from the recently measured total cross sections, which were obtained with a new experimental technique utilizing the threshold photoelectron source. A significant discrepancy between the momentum cross sections derived from the present analysis and those determined in the previous electron swarm studies was found at energies below 100 meV. The findings emphasize the need of further high precision experiments in the very low energy region as well as re-analysis of the previous swarm data.

We have measured the coincidence time spectra of two Lyman-alpha photons emitted by a pair of H(2p) atoms in the photodissociation of H-2 at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 Pa and 0.02 Pa, from which the angular distributions of two Lyman-alpha photons have been obtained. The experimental angular distributions indicate the generation of the entangled pair of H(2p) atoms as predicted by the theory of our group (Miyagi et al 2007 J. Phys. B 40 617) and the role of a new kind of reaction, i.e. the reaction of the entangled pair of H(2p) atoms with an H2 molecule that efficiently changes the entanglement. It has turned out that more entangled pairs of H(2p) atoms survive at 0.02 Pa than at 0.40 Pa. The decay time constant in the coincidence time spectrum at 0.02 Pa is approximately half the lifetime of a single H(2p) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H(2p) atom. It follows that the decay faster than the lifetime of a single H(2p) atom originates from the entanglement in the pair of H(2p) atoms.

Absolute total cross sections for electron scattering from Ar and Xe at electron energies ranging from 7 meV to 20 eV were obtained with the experimental technique employing the threshold-photoelectron source. The measured total cross sections are in good agreement with those obtained by other groups down to 100 meV, above which several experimental works have been reported. Scattering lengths for electron scattering from Ar, Kr, and Xe were determined from the present total cross sections and our recent results for Kr using the modified effective range theory. The values of the scattering length obtained in the present analysis differ from the values determined from the previous swarm experiments and beam experiments. The resonant structures in the total cross sections due to Feshbach resonances of Ar, Kr, and Xe with an improved energy resolution were also measured. Analyses of the resonant structure were carried out based on the spin-dependent resonant scattering theory in order to determine the values of the natural width of Feshbach resonances of Ar, Kr, and Xe precisely. © 2011 American Physical Society.

Absolute total cross sections for electron scattering from Ar and Xe at electron energies ranging from 7 meV to 20 eV were obtained with the experimental technique employing the threshold-photoelectron source. The measured total cross sections are in good agreement with those obtained by other groups down to 100 meV, above which several experimental works have been reported. Scattering lengths for electron scattering from Ar, Kr, and Xe were determined from the present total cross sections and our recent results for Kr using the modified effective range theory. The values of the scattering length obtained in the present analysis differ from the values determined from the previous swarm experiments and beam experiments. The resonant structures in the total cross sections due to Feshbach resonances of Ar, Kr, and Xe with an improved energy resolution were also measured. Analyses of the resonant structure were carried out based on the spin-dependent resonant scattering theory in order to determine the values of the natural width of Feshbach resonances of Ar, Kr, and Xe precisely.

The cross sections for the formation of the metastable atomic hydrogen in the 2s state in photoexcitation of H2 and D2 were measured as a function of the incident photon energy in the range of the doubly excited states with their symmetries of the electronic states, 1Σu+ or 1Πu, being resolved. It has turned out from the comparison with the cross-section curves for other dissociation processes and the theoretical calculation that the Q21Πu(1) doubly excited state of H2 dissociates into both H(2s) + H(2p) and H(2p) + H(2p). The dissociation dynamics of this state has been discussed in terms of the nonadiabatic transition during neutral dissociations. © 2011 American Physical Society.

The electron energy loss spectrum of H2O in coincidence with detecting Lyman-α photons (CoEELS) has been measured at the incident electron energy of 100 eV and electron scattering angle of 8° in the inner valence range in order to investigate the formation and decay of the doubly excited states. The present CoEELS has been compared with that at the infinite incident electron energy and 0° electron scattering angle, which was derived from the density of the dipole oscillator strength of H2O for the emission of the Lyman-α photons against the incident photon energy (Nakano et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 215206). It is remarkable that there exists a large difference in shape between these CoEELSs. This difference has turned out to be attributed to the noticeable contribution of the forbidden doubly excited states at 100 eV incident electron energy and 8° scattering angle. They lie at 25.0 and 27.4 eV and have been found out in this study. The differential cross sections for the excitation to the superexcited states resulting in H(2p) formation have been obtained at 100 eV and 8° and compared with those at the infinite energy and 0°. The electron collisions at 100 eV and 8° enhance the dissociative double excitation against the dissociative single excitation as compared with the electron collision at the infinite energy and 0°. © 2011 IOP Publishing Ltd.

In this Reply to the preceding Comment by Sancho and Plaja, we point out the difference between the coincidence time spectrum of two Lyman-α photons measured by us and their theoretical one. © 2011 American Physical Society.

In this Reply to the preceding Comment by Sancho and Plaja, we point out the difference between the<br /> coincidence time spectrum of two Lyman-α photons measured by us and their theoretical one.