Several techniques not closely related to photoelectron spectroscopy give results similar to some of those obtained from photoelectron spectroscopy. They may also give other types of information as well. In the following we describe a few of them, but only with respect to data that can be compared with photoelectron spectroscopy. Other useful results from these techniques are not mentioned. We finish this chapter with some results from the spectroscopies closely related to photoelectron spectroscopy: electron energy-loss spectroscopy, soft x-ray absorption, and soft x-ray emission.

Infrared spectroscopy

Infrared spectroscopy probes the sample with photons of energies below about 1 eV, down to perhaps 1 meV. The interaction Hamiltonian is the same as for photoexcitation, proportional to A · p + p · A, and similar selection rules apply. The electronic excitations usually are divided into intraband excitations, often called the Drude contribution, and interband excitations. Because infrared photons have such small wave vectors, an additional scattering mechanism, phonons or impurities, is required in the Drude contribution, leading to a second-order process, and often to a strong temperature dependence. However, this can still lead to intense absorption for systems with metallic electron densities. The Drude term is the high-frequency analog of the electrical conductivity. In a superconductor, one then expects a delta function in the optical conductivity at zero energy and, perhaps, zero absorption until an energy equal to the gap energy 2Δ, is reached.