Valence and inner shell non-dipole excitation spectroscopy of polyatomic molecules by angle-resolved inelastic electron scattering at high energy

Abstract

Abstract High energy (>2 keV) electron scattering has been used extensively in the past three decades as an effective method to simulate equivalent photon-impact experiments under dipole-dominated small-angle scattering conditions (the so-called pseudo-photon experiments). More recent studies have focused on its application as a unique spectroscopic tool that provides distinct information about those parts of the electronic structure that are not easily accessible by optical techniques. The capability to excite not just the dipole-allowed transitions but also the dipole-forbidden ones gives electron impact a special advantage for probing electronic excitations and for studying non-dipole phenomena. Indeed, the control of the scattering angle, or equivalently the momentum transfer, provides a powerful means to manipulate electron-induced chemical processes if the excited-state electronic structure can be better understood. This review will give a description of angle-resolved electron energy loss spectroscopy and highlight some of its spectroscopic applications that offer specific information about the electronic structure of the valence and inner shells in polyatomic molecules. Some of the novel aspects of this non-optical technique, including the determination of absolute generalized oscillator strength for non-dipole transitions, will be illustrated by examples drawn from recent studies on CF 3 Cl and other related chlorofluorocarbons and hydrochlorofluorocarbons.