An inverse photoemission spectroscopy apparatus is configured to detect a light generated by the relaxation of electrons to an unoccupied state of a sample. The apparatus includes an electron source for generating electrons with which a sample is irradiated, a wavelength selector for extracting a light having a certain wavelength from the light generated in the sample, a photodetector for detecting the light extracted by the wavelength selector; and a focusing optics disposed between the sample and the photodetector. The electron source contains yttrium oxide as a thermionic emission material.
