Probing
electron momentum densities in molecules using a new multichannel (e,2e)
spectrometer
Masahiko Takahashi
and Yasuo Udagawa
Institute of
Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai
980-8577, Japan (masahiko@tagen.tohoku.ac.jp)
Electron momentum
spectroscopy (EMS), based on the (e,2e) reaction under the Bethe ridge
conditions, has been developed as a powerful means for investigations of
electronic structure of atoms and molecules [1]. Within the plane-wave impulse
approximation (PWIA), the EMS cross section for a gaseous target is directly
related to the spherically averaged momentum density or momentum profile of the
Dyson orbital for the ionization transition. Furthermore, EMS can give unique
and versatile information for the detailed study of electronic structure, such
as symmetries, binding energies and pole strengths of the states involved.
Nevertheless, the
potential of EMS has not been fully achieved as yet due mainly to the small
cross sections involved, which cause large statistical uncertainty and poor
energy resolution. Very recently, we have developed a new spectrometer that
employs a spherical analyzer and position sensitive detectors [2]. By taking an
advantage of simultaneous detection in energy and momentum, collection
efficiency for the two outgoing electrons has been significantly improved
compared with our previous apparatus [3], opening up the possibility of considerably
more precise measurements.
In this
contribution we report details and performance of the newly developed
spectrometer. Our recent studies of several targets will also be presented.
Applications to be discussed involve (1) electron momentum profiles for the
complete valence shell of CO and (2) simultaneous ionization excitation
processes of H2, together with comparison between experiment and
theory. Finally, prospects for probing directional momentum densities or
looking at molecular orbitals in the three-dimensional momentum-space will be
discussed for gaseous targets.
1
Weigold, E. and
McCarthy, I. E. Electron Momentum Spectroscopy (Kluwer Academic/Plenum Publishers, New York,
1999).
2
Takahashi, M.,
Saito, T., Matsuo, M., and Udagawa, Y. (2002) Rev. Sci. Instrum. 73, 2242-2248.
3
Takahashi, M.,
Nagasaka H., and Udagawa, Y. (1997) J. Phys. Chem. A 101, 528-532.