Organic photochemistry utilizes electronic excitation with electromagnetic radiation in the region between 1 electron volt (eV) and 8 eV, corresponding formally to a wavelength region between 1240 nanometers (nm) and 155 nm. Short-wavelength excitation can lead to direct electron detachment or to photoinduced electron transfer if substrates are excited in the presence of appropriate electron donors or acceptors. This property is demonstrated experimentally by a 308 nm excitation of an aqueous solution of an electron acceptor-donor pair by a xenon chloride (XeCl) excimer radiation source in a falling-film reactor. On the other hand, in the case of the long-wavelength excitation region, direct excitation is unlikely and sensitization by energy-transfer sensitizers is often used. This effect is demonstrated experimentally by excitation of a solution of an organic dye in the presence of oxygen by means of visible light, which leads to the formation of singlet oxygen with an excitation energy of less than 1 eV. Singlet oxygen shows high reactivity with unsaturated organic substrates and is used to produce organic peroxides.