Reversible Photochemical Bleaching in Frozen Aqueous Systems at 77° K

Abstract

IT is known that some colourless glasses containing certain impurity centres become coloured on exposure to visible light at room temperature and get colourless again in the dark, evidently due to some thermal back-reaction. We have found some systems which show a similar behaviour at temperatures of about 77° K. The systems referred to are transparent matrices of ice containing certain solutes such as sulphuric or phosphoric acid at concentrations of 1–10 mole per cent. If these glassy ice matrices are irradiated with ionizing radiations (γ-rays, X-rays, etc.) they become coloured, deep yellow in the case of sulphuric acid and pink in the case of phosphoric acid, respectively. If now these coloured glasses are exposed to a source of visible light for several minutes, for example, a 500-W tungsten lamp, this colour disappears. However, on annealing these glasses in the absence of light the colour is again restored, for example, at a temperature of about 120° K the original colour is very rapidly and completely restored in a very short time. These processes can be followed by both spectrophotometric as well as electron spin resonance techniques since the colour centres formed on irradiation as well as the species formed on optical bleaching of the colour centres show characteristic absorption and electron-spin resonance spectra. The results of these investigations, which will be published more fully elsewhere, have led to the following conclusions.