Electron spectroscopy combined with a scattering experiment offers a powerful means to study spatial characteristics of a one-electron wavefunction in molecules or in continuum states. Here we review recent developments in such multi-dimensional electron spectroscopy studies which utilize atomic, photonic, and electronic collisions. Particular emphasis is placed on the following three techniques. Firstly, it has been shown that two-dimensional Penning ionization electron spectroscopy, with the help of classical trajectory simulations, leads to a determination of electron distributions of outer valence orbitals extending outside the molecular surface of diatomics. Secondly, an extension of core-level photoelectron angular distribution studies to oriented triatomic non-linear molecules investigated unimolecular photoelectron scattering and diffraction phenomena which can visualize directional information of a photoelectron wave ejected from the core orbital. Thirdly, a recent instrumental development in (
e, 2
e) electron momentum spectroscopy has made it possible to study the phase, spatial extent, and chemical bonding nature of the molecular orbital of H
2 in momentum-space. In addition to the above-mentioned topics, we review our recent efforts towards development of time-resolved electron momentum spectroscopy that employs femtosecond laser and picosecond electron pulses in a pump-probe scheme. In spite of the low data statistics as well as of the limited experimental resolutions, it has been clearly demonstrated that (
e, 2
e) electron momentum spectroscopy measurements of short-lived transient species are feasible, opening the door to time-resolved orbital imaging in momentum space.
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