PROTONIC PHOTOCONDUCTIVITY OF ICE

Abstract
A number of attempts have been made to find the protonic photoconductivity of ice (abbreviated as PPC hereafter), but most of them were not successful. Camp and Spears (1) tried to excite PPC of pure ice single crystals by a xenon lamp and concluded that the probability of photodissociation of ice, if any, is very small. De Haas et al. (2) and Itagaki et al. (3) used gamma-rays, X-rays, or electron beams, and found a change in the electrical conductivity. Very high-energy beams were used in these experiments, however, which yielded radiation-induced damage and defects in the bulk specimen. PPC was observed in ice doped with o- nitrobenzoaldehyde (abbreviated as NBAld) ; ice was grown from aqueous solution of NBAld at the concentration of 6.6 x 10^-3 M (moles per liter) and cut into a plate of 4 mn in thickness. Network or disk electrodes were fixed to the ice sample, to which ultraviolet rays were emitted by using a xenon lamp of 300 W. Conductivities and capacitances were measured by using a bridge and null detector at frequencies between 30 Hz and 30 kHz. Both the conductance and capacitance were found to increase when the ice sample was illuminated by the ultraviolet light, however the increase in conductance was noted to be larger than that in capacitance. It was also found that the increase in the limiting high-frequency conductivity was much larger than that in the low-frequency conductivity. The increasing rate of conductance with time showed a maximum at the wavelength between 390 and 400 nm. A theoretical model of PPC in NBAld-doped ice was introduced ; the model is based on the observation of Konstantinov et al. (4) that protons are released from NBAld by the ultraviolet illumination. It is further assumed that a photo-proton released creates an H₃O ion and a D-defect. Theoretically estimated increases in the conductivities were in good agreement with those observed. The observed time-dependence of PPC was explained by the formation of small clusters of NBAld molecules in ice ; estimated sizes of the clusters were roughly 1µm.