Yoshiro Azuma

Dynamics of the Dissociating Uracil Anion Following Resonant Electron Attachment

We report a combined experimental and theoretical investigation of dissociative electron attachment (DEA) to the nucleobase uracil. Using ion momentum imaging experiments employing a DEA reaction microscope we have measured 3-dimensional momentum distributions of specific anionic fragments following DEA to uracil by 6 eV electrons. From the measured anion fragment kinetic energy we determine the possible dissociation pathways and the total kinetic energy release. We employ electronic structure and electron scattering calculations to determine the probability for electron attachment in the molecular frame. Combining these calculations with the imaging measurements, we reveal several key features of the coupled electronic and nuclear dynamics of DEA.

Long-lived, highly excited neutral hydrogen atom production following oxygen 1s photoexcitation of gas-phase water molecules

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.

PCI effects and the gradual formation of Rydberg series due to photoelectron recapture, in the Auger satellite lines upon Xe 4d?15/2 photoionization

The Xe (N5O2,3O2,3) Auger electron spectra originating from 4d−15/2 inner-shell photoionization were measured, with photon energy tuned close to the ionization threshold. As the photon energy approaches the threshold from above the 4d−15/2 photoionization threshold, Rydberg series structures are formed within the Auger electron peak by the recapture of the photoelectron into high-lying ion orbitals. They emerge in the tail on the higher energy side of the post–collision interaction (PCI) profile of the Auger electron. Discrete Rydberg peaks replace the continuous PCI tail and gradually form a series with intensity distribution emulating the intensity profile of the continuous tail. Structures due to the Xe+5p4(1S0, 1D2, 3P2,1,0) ml series were observed and assigned.

Probing electron correlation through radiative lifetime measurements upon inner-valence photoionization of Ne and Ar

This work demonstrates that electron correlation can have a strong effect on the radiative lifetime of atoms. We report measurements of the radiative lifetimes of inner-valence hole states, the 3s3p6 2S1/2 state of Ar+ and the 2s2p6 2S1/2 state of Ne+ by using the time-correlated single photon counting technique combined with photoionization by synchrotron radiation. Theoretical calculations utilizing the multi-configuration Dirac–Fock method agreed well with the experimental results. In particular, the radiative lifetime was found to depend very sensitively on the mixing of valence excited state configurations. While the Ne+ 2s2p6 2S1/2 state only has relatively weak inter-shell correlation, Ar+ 3s3p6 2S1/2 state has strong intra-shell correlation within the M-shell. This intra-shell correlation enhances configuration mixing and causes the radiative lifetime of the Ar+ 3s3p6 2S1/2 state to become very much longer than that of the Ne+ 2s2p6 2S1/2 state.

Photoion spectroscopy on isolated Mn atoms in the 2p???3d excitation region: I. Total photoion-yield spectrum

With a time-of-flight (TOF) ion analyzer combined with synchrotron radiation, total photoionyield spectrum has been measured on isolated Mn atoms in the 2p →3d excitation region (632 eV to 657 eV) with an estimated resolution 0.065 eV. The analysis of the observed spectrum was performed with the help of ab initio calculations of the excited states using the computer code GRASP92 for the MCDF calculation and of oscillator strengths for the excitation using the related REOS program. The calculations were performed for two outer-shell configuration cases: one considering the single reference-configuration 3d m 4s 2 and another with the multiple reference-configuration 3d m 4s 2 +3d m + 1 4s+3d m+2 (m=5 for the ground state and m=6 for the 2p →3d excited states). This two-step calculation showed the CI effect in the case of the excitation of a deep inner-shell.

Photoion spectroscopy on isolated Mn atoms in the 2p → 3d excitation region. II. Decay processes of the excited states

A series of time-of-flight (TOF) spectra of isolated Mn atoms were measured in the vicinity of 2p → 3d excitation states (635 to 656 eV) at a high-energy resolution. Synchrotron radiation was used as a light source. From this spectral data, charge-separated photoion-yield spectra and charge-state distribution as functions of the photon energy were obtained. The photoions resulting from the 2p → 3d excitation states are distributed from Mn+ to Mn6+, with the largest component being Mn3+ ions. Based on these spectra, the relaxation processes of the 2p → 3d excitation states are semi-quantitatively considered. For this purpose, highly excited states above the next ionization limit were calculated using the GRASP92 atomic code. It is found that single and double Auger shake-off processes play a central role in the formation of Mn5+ and Mn6+ ions. From the behavior of the charge-state distribution, the lowest 2p ionization limit has been determined to be 647.0 ± 0.3 eV.

Photoion-yield study of the 3p–3d giant resonance excitation region of isolated Cr, Mn and Fe atoms

Using the charge-separated photoion-yield method combined with monochromatized synchrotron radiation, single (M+) and double (M2+) photoionization spectra in the 3p excitation region were observed for isolated atoms M = Cr, Mn and Fe. The relaxation processes of excited states to double ionization, as well as to single ionization, were considered with the help of existing photoabsorption and photoelectron data and some calculations of highly-excited states of M+ using the atomic code GRASP92. In the 3p→3d giant resonance region before the 3p→n(s, d) Rydberg series starts, the spectral features in the M2+ channel are similar to those in the M+ channel. The ratios M2+/M+ in this region are rather constant accordingly against the photon energy with values 0.045 ± 0.005 for Cr, 0.17 ± 0.03 for Mn and 0.30 ± 0.08 for Fe at the giant resonance. The increase in the M2+/M+ ratio with increasing the atomic number Z can be explained by the increasing contribution of two-step double ionization via highly-excited M+ states. It is expected that, at the 3p→3d giant resonance, the one-step double ionization process occurs within a few per cent of the entire decay.

Observation and analysis of 3s?np resonance excitation in Cr, Mn and Fe atoms

Photoabsorption and the subsequent decay processes of Cr, Mn and Fe atoms have been studied for the 3s-excitation region using a charge-separated-photoion-yield method combined with monochromatized synchrotron radiation. Two weak peak-type features, which can be attributed to the first members of 3s−1np resonance excitation series, are observed in the double ionization channel of Cr, in the double and triple ionization channels of Mn, and in the triple ionization channel of Fe. The observed resonances have been fitted to Beutler–Fano profiles to obtain resonance parameters, and compared with MCDF calculations to identify those resonance states. The decay processes of these resonance states are discussed. The spectator Auger process plays a central role in the decay of 3s−1np excitation states. In particular, the triple ionizations can be exclusively explained by spectator Auger processes.