Ejecting Electrons from Molecular Anions via Shine, Shake/Rattle, and Roll

The periodically oscillating electromagnetic potential of a photon can, in an electric-dipole transition, "shine" an electron from an anion's bound-state orbital directly into a continuum-state orbital. This occurs in photoelectron and photodetachment spectroscopy, both of which provide much information about the electronic structure of the anion. Alternatively, a molecular anion containing sufficient vibrational energy to "shake/rattle" an electron out of a bound-state orbital can induce electron detachment via a vibration-to-electronic nonadiabatic transition. In this case, the electron binding energy in the anion must be smaller than the vibrational energy-level spacing, so these processes involve anion states of low binding energy, and they eject electrons having low kinetic energy. If the anion's electron binding energy is even smaller, it is possible for a rotation-to-electronic energy transfer to "roll" an electron from the bound-state orbital into the continuum. For each of these mechanisms by which electron detachment can occur, there are different selection rules governing the angular distribution in which the electrons are ejected, and this manuscript discusses these rules, their origins, and their utility when using spectroscopic tools to probe the anion's electronic structure. Several examples of the shine-, shake/rattle-, and role-ejection of electrons from a range of experimental conditions are discussed as are similarities and differences among the corresponding selection rules. Of special novelty are the effects arising when electron ejection occurs from orbitals having very low electron binding energies and thus large radial extent.