Inverse photoemission
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Inverse photoemission is a surface science technique used to study the unoccupied electronic structure of surfaces, thin films and adsorbates. As inverse photoemission probes the electronic states above the Fermi energy of the system, it is a complementary technique to photoemission spectroscopy. In inverse photoemission, a well-collimated beam of electrons of a well defined energy (< 20 eV) are directed at the sample. These electrons couple to high-lying unoccupied electronic states and decay to low-lying unocupied states, with a subset of these transitions being radiative. The photons emitted in the decay process are detected and an energy spectrum, photon counts vs. incident electron energy, is generated. 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.
There are two modes, isochromat mode and spectrograph mode, in which an inverse photoemission experiment can be performed.
[edit] Isochromat Mode
In the isochromat mode, the incident electron energy is ramped and the emitted photons are detected at a fixed energy that is determined by the photon detector. 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.
[edit] Spectrograph Mode
In the spectrograph mode, the energy of the incident electron remains fixed and a grating spectrometer is used to the detect the emitted photons over a range of photon energies. A diffraction grating is used to disperse the emitted photons that are in turn detected with a two-dimensional position sensitive detector.
[edit] Comparison of Modes
One advantage of the spectrograph mode is the ability to acquire IPS spectra over a wide range of photon energies simultaneously. Additionally, the incident electron energy remains fixed which allows better focusing of the electron beam on the sample. Furthermore, by changing the incident electron energy the electronic structure can be studied in great detail. 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.
