The kinetic energy (KE) of this electron, the photoelectron, is measured, and the electron binding energy (BE) is calculated based on the photoelectric effect BE = hν -KE.( ) A histogram of the BEs emitted by the photoelectrons is plotted as a NIPES spectrum, from which the relative energies of the ground and excited states of both the anion and the corresponding neutral species can be determined. Photoelectron spectra can be compared with calculations to obtain structural information, but is typically combined with ab initio molecular dynamics calculations to determine the key binding interactions between cluster components.