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  1. (e,2e) ionization-excitation of H2
    M. Takahashi, Y. Khajuria, Y. Udagawa
    Physical Review A, vol.68, p042710(7 pages), 2003.
    DOI:10.1103/PhysRevA.68.042710
    Abstract:Binary (e,2e) measurements are reported for ionization-excitation processes of H2. The experiments were performed at impact energies of 1200, 1600, and 2000 eV using an energy- and momentum-dispersive spectrometer. Momentum profiles for transitions to the 2sσg and 2pσu excited final ion states are presented as normalized intensities relative to the cross section of the primary ionization to the 1sσg ground ion state. The results are compared with theoretical calculations of Lermer et al. [Phys. Rev. A 56, 1393 (1997)] using the first-order plane-wave impulse approximation. Certain features of the discrepancies between experiment and theory can be explained by incorporating contributions from the second-order two-step mechanisms into the (e,2e) cross sections. Furthermore, the present results suggest that 2sσg and 2pσu cross sections approach their high-energy limits in different ways.
  2. Observation of a molecular frame (e,2e) cross section: an (e,2e+M) triple coincidence study on H2
    M. Takahashi, N. Watanabe, Y. Khajuria, Y. Udagawa, J.H.D. Eland
    Physical Review Letters, vol.94, p213202(4 pages), 2005.
    DOI:10.1103/PhysRevLett.94.213202
    Abstract:We report the first experimental results showing transition-specific anisotropy of molecular frame (e,2e) cross sections. Vector correlations between the two outgoing electrons and the fragment ion have been measured for specific ionization-excitation processes of H2. The results enable us to obtain molecular frame (e,2e) cross sections for transitions to the 2sσg and 2pσu excited states of H2+, thereby making stereodynamics of the electron-molecule collisions directly visible.
  3. Looking at molecular orbitals in three-dimensional form: from dream to reality
    M. Takahashi
    Bulletin of the Chemical Society of Japan, vol.82, p751-777, 2009.
    DOI:10.1246/bcsj.82.751
    Abstract:Over the last four decades an experimental method has been developed for looking at electron orbitals in momentum space. The method, called electron momentum spectroscopy (EMS), is based on the electron-impact ionizing reaction near the Bethe ridge at incident electron energies of the order of 1 keV or higher. This account reviews frontiers of the field, involving the first approach to molecular frame EMS that enables one to look at molecular orbitals in three-dimensional form.
  4. Laser-assisted electron momentum spectroscopy
    K.A. Kouzakov, Yu.V. Popov, M. Takahashi
    Physical Review A, vol.82, p023410(14 pages), 2010.
    DOI:10.1103/PhysRevA.82.023410
    Abstract:We consider theoretically ionization of an atomic target by fast electron impact at large energy and momentum transfer and in the presence of laser radiation. The laser electric-field amplitude is weak compared to the typical field in the target. Two frequency regimes are investigated according to whether the laser frequency is (i) much smaller than or (ii) resonant to the frequency of the transition from the ground to the first excited target state. Fast incident, scattered, and ejected electrons are described using Volkov solutions. The dressing of the bound-electron state by the laser field is accounted for within time-dependent perturbation theory in the case of the low-frequency regime and within the rotating wave approximation in the case of a resonant one. The interaction of the incident electron with the target is treated in the first Born approximation. For atomic hydrogen embedded in a linearly or circularly polarized laser field, we discuss how the polarization-vector orientation influences the momentum-dependent (e,2e) differential cross sections assisted by exchange of few photons between the colliding system and the field. In addition, we inspect the dependence of the cross sections on the dressing of the hydrogen state.
  5. Interference effects on (e, 2e) electron momentum profiles of CF4
    N.Watanabe, X. J. Chen, and M. Takahashi
    Phys. Rev. Lett.108, 173201 (5 pages), 2012.
    DOI:10.1103/PhysRevLett.108.173201
    Abstract:Interference effects on electron momentum profiles have been studied using binary (e, 2e) spectroscopy for the three outermost molecular orbitals of CF4, which are composed of the F 2p nonbonding atomic orbitals. An analysis of the measured spherically averaged electron momentum densities has clearly shown the presence of oscillatory structures having direct information about the internuclear distance between the F atoms. Furthermore, it is demonstrated that the phase of the oscillatory structures depends upon the orientation in space of the constituent atomic orbitals.
  6. Vibrational effects on valence electron momentum distributions of ethylene
    N. Watanabe, M. Yamazaki, M. Takahashi
    The Journal of Chemical Physics, vol.137, p114301(8 pages), 2012.
    DOI:10.1063/1.4752653
    Abstract:We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions for the outer valence orbitals of ethylene (C2H4). The symmetric noncoplanar (e,2e) experiment has been conducted at an impact energy of 1.2 keV. Furthermore, a theoretical method of calculating electron momentum distributions for polyatomic molecules has been developed with vibrational effects being involved. It is shown from comparisons between experiment and theory that taking into account effects of the CH2 asymmetric stretching and CH2 rocking vibrational modes of C2H4 is essential for a proper understanding of the electron momentum distribution of the 1b3g molecular orbital.
  7. Molecular orbital imaging of the acetone S2 excited state using time-resolved (e, 2e) electron momentum spectroscopy
    M. Yamazaki, K. Oishi, H. Nakazawa, C. Zhu, M. Takahashi
    Physical Review Letters, vol.114, p103005(5 pages), 2015.
    DOI:10.1103/PhysRevLett.114.103005
    Abstract:We report a time-resolved (e, 2e) experiment on the deuterated acetone molecule in the S2 Rydberg state with a lifetime of 13.5 ps. The acetone S2 state was prepared by a 195 nm pump laser and probed with electron momentum spectroscopy using a 1.2 keV incident electron beam of 1 ps temporal width. In spite of the low data statistics as well as of the limited time resolution (±35  ps) due to velocity mismatch, the experimental results clearly demonstrate that electron momentum spectroscopy measurements of short-lived transient species are feasible, opening the door to time-resolved orbital imaging in momentum space.
  8. Development of an electron-ion coincidence apparatus for molecular-frame electron energy loss spectroscopy studies
    N. Watanabe、T. Hirayama、S. Yamada、M. Takahashi
    Review of Scientific Instruments, vol.89, p043105(10 pages), 2018.
    DOI:10.1063/1.5025773
    Abstract:We report details of an electron-ion coincidence apparatus, which has been developed for molecular-frame electron energy loss spectroscopy studies. The apparatus is mainly composed of a pulsed electron gun, an energy-dispersive electron spectrometer, and an ion momentum imaging spectrometer. Molecular-orientation dependence of the high-energy electron scattering cross section can be examined by conducting measurements of vector correlation between the momenta of the scattered electron and fragment ion. Background due to false coincidences is significantly reduced by introducing a pulsed electron beam and pulsing scheme of ion extraction. The experimental setup has been tested by measuring the inner-shell excitation of N2 at an incident electron energy of 1.5 keV and a scattering angle of 10.2°.
  9. Direct observation of intramolecular atomic motion in H2 and D2 by using electron-atom Compton scattering
    Y. Tachibana, M. Yamazaki, M. Takahashi
    Physical Review A, vol.100, 032506(5 pages), 2019.
    DOI:10.1103/PhysRevA.100.032506
    Abstract:We report electron-atom Compton scattering experiments on the H2 and D2 molecules. Energy-loss spectra of electrons quasielastically backscattered at an angle of 135° are measured as a function of azimuthal angle at an incident electron energy of 2.0 keV. Momentum distributions of the H and D atoms due to molecular vibration are extracted from the experimental data by using a protocol that we propose here. The results are successfully compared with theoretical ones predicted by the molecular vibrational wave functions. It is shown that electron-atom Compton scattering has a unique ability to provide direct information about intramolecular motion of each atom with different mass numbers.