Exterior characteristics of molecular orbitals and molecular surfaces as studied by atomic probes

This is a brief survey of studies on exterior characteristics of molecules conducted by means of atomic probes, mostly by metastable states of rare gas atoms in collisional ionization experiments and partly by atoms from H to Ar in quantum chemical calculations. Spatial distributions of molecular orbitals (MO) outside the molecular surfaces have been studied to determine stereoelectronic properties related with partial ionization cross sections of molecules due to electrophilic attacks of metastable atoms. A newly developed collision-energy/electron-energy resolved two-dimensional electron spectroscopy has disclosed anisotropic and dynamic characteristics of molecular surfaces (MS) via collision-energy dependence of partial ionization cross sections. Starting from a simple theoretical model of exterior electron densities (EED), researchers have developed quantum chemical treatments of the ionization reactions to a level of trajectory calculations based on potential energy surfaces of both entrance and exit reaction channels together with electronic transition rates between the surfaces. The present stage of techniques can elucidate one of the two aspects, MO or MS, assuming knowledge of the other. Since experimental aspects may contain various features of MO and MS, a simultaneous determination of both MO and MS by experiments will allow further developments in experimental techniques.