Inverse photoemission spectroscopy, the time-reversed counterpart of direct photoemission spectroscopy (also called photoelectron spectroscopy, PES), plays an important role in study of the unoccupied states of materials. In a typical experiment, a monochromatic electron beam impinges on a surface, resulting in the emission of photons whose energies are analyzed. The incident electrons couple to unoccupied states of the material and decay to lower states via both radiative and nonradiative pathways and the energies of the radiated photons provides a map of the unoccupied state architecture. In a simpler implementation of the experiment, the so-called isochromat mode, the energy of the incident electrons (Ei) is varied, while photons are detected at a fixed energy (hn), with a narrow bandpass on the order of a 100 meV.