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.

 

References

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.