Electron momentum spectroscopy with fixed-in-space molecules

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Through the last three decades of studies of binary (e,2e) spectroscopy or electron momentum spectroscopy (EMS), it has been demonstrated that the ionization reaction near the Bethe ridge is a sensitive probe for electronic structure and electron correlation. In particular, the ability to look at individual molecular orbitals in momentum space is the remarkable feature of this technique. However, EMS has not yet reached the stage of full use of its ability for intensive investigation on electronic structure of molecules. The reason for this may have been twofold; (1) a more complete knowledge of the binary (e,2e) reaction mechanism is an ever-increasing necessity as sophistication of experiments increases, and (2) the present EMS experiments measure averages over all orientations of gaseous molecules, resulting in enormous loss of information on electronic structure, anisotropy of the target wavefunction in particular. However, small cross section involved in the binary (e,2e) reaction has hindered one from developing EMS satisfactorily.Under these circumstances, we have constructed an electron-electron-fragment ion triple coincidence spectrometer. The spectrometer features remarkably high sensitivity for the two outgoing electrons by simultaneous detection in energy and momentum, and further it enables us to carry out (e,2e) experiments with fixed-in-space molecules based on their axial recoil fragmentation. By taking advantage of multichannel detection in. the spectrometer, we have successfully applied it to studies on collision dynamics of ionization excitation processes of the hydrogen molecule and directional momentum densities of the oxygen molecule. These are the first observation of molecular frame (e,2e) cross sections.