The red luminescence of bismuth-doped alkaline-earth-metal sulfates is ascribed to the presence of divalent bismuth. The optical transitions are due to transitions within the 6p shell.

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It has been found that porous GaP photoanodes show considerable photocurrent when illuminated with sub‐bandgap light, in contrast to flat electrodes. The origin of this enhanced response to sub‐bandgap light was investigated. Due to the morphology of the porous structure, photons are more effectively absorbed. Light absorption by surface electronic levels is found to be an important source of sub‐bandgap photocurrent.

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The main mechanisms of electron−hole generation in a semiconductor induced by absorption of a sub-band gap photon are considered. Electron−hole generation with sub-band gap light involves optical and thermal transitions between delocalized valence or conduction band states and localized band gap states. A semiconductor device with a retrieval layer in which electron−hole pairs are effectively separated is considered. For each mechanism, the steady-state photocurrent quantum yield and the photocurrent response to a harmonic modulation of the intensity of sub-band gap light is calculated. It is shown that sub-band gap intensity modulated photocurrent spectroscopy makes it possible to resolve different contributions to the sub-band gap photocurrent and to study the kinetics of free carrier generation induced by sub-band gap light absorption. Experimental results obtained with nanoporous GaP photoanodes are analyzed; two different mechanisms are responsible for sub-band gap photocurrent flow. The potential of sub-band gap intensity modulated photocurrent spectroscopy for the characterization of (surface) localized band gap states is discussed and illustrated with results obtained with nanoporous GaP.