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, 2e) electron momentum spectroscopy has made it possible to study the phase, spatial extent, and chemical bonding nature of the molecular orbital of H2 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, 2e) electron momentum spectroscopy measurements of short-lived transient species are feasible, opening the door to time-resolved orbital imaging in momentum space.
2017 Japan Society for Molecular Science