In Einstein's photoelectric effect, the energy of a photon incident on a material is absorbed by an electron, which is subsequently excited and emitted from the material. In angle-resolved photoemission spectroscopy (ARPES), the energy and momentum of photoemitted electrons are measured, leading to a so-called "momentum space microscope." ARPES is a powerful tool for directly measuring the electronic structure of materials, including band dispersions, Fermi surfaces, and energy gaps.