小田切 丈

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.