A well-collimated beam of electrons of a well defined energy (< 20 eV) is directed at the sample.
Due to the low energy of the incident electrons, their penetration depth is only a few atomic layers, making inverse photoemission a particularly surface sensitive technique.
Typically, an I2 gas filled Geiger-Müller tube with an entrance window of either SrF2 or CaF2 is used as the photon detector.
The combination of window and filling gas determines the detected photon energy, and for I2 gas and either a SrF2 or CaF2 window, the photons energies are ~ 9.5 eV and ~ 9.7 eV, respectively.
A diffraction grating is used to disperse the emitted photons that are in turn detected with a two-dimensional position sensitive detector.
One advantage of spectrograph mode is the ability to acquire IPES spectra over a wide range of photon energies simultaneously.
Although the grating spectrometer is very stable over time, the set-up can be very complex and its maintenance can be very expensive.
The advantages of isochromat mode are its low cost, simple design and higher count rates.