The Effect of Oxygen Adsorption on Photo- and Semiconduction of β-Carotene

Abstract

The presence of adsorbed oxygen gas increases the magnitude of the surface semiconduction currents in crystalline powder β-carotene by a factor of about 103. The activation energy for the surface semiconduction process is 1.52 ev in argon gas and is decreased to 1.29 ev in oxygen. The room temperature photo-current is increased by a factor of about 25 in oxygen over the value in argon. The activation energy of the photoconduction process is not affected by oxygen. The increase in dark current in an oxygen ambient requires a time of about 2 hr to come to the equilibrium value at room temperature. Radiation is not required to cause the increase. The effect of the oxygen can be reversed completely by heating the β-carotene to 120°C in an inert atmosphere (dry nitrogen or argon). The increase in dark current is linearly proportional to the partial pressure of oxygen in the chamber. The rate of desorption of the oxygen increases with the temperature. The resulting data yield an activation energy of desorption of 0.17 ev. It is suggested that this is the binding energy for the formation of an oxygen-carotene complex, and that the decrease in semiconduction activation energy is correlated to this binding energy. An analysis is given which leads to the conclusion that the mechanism of the current increase with formation of the oxygen complex is twofold; a decrease in the activation energy for semiconduction, and an increase in the mobility of the charge carriers. There is no evidence for a change in the charge creation process. The experimental results are considered to be a partial verification of Platt's theory on the donor-carotene-acceptor complex in photosynthesis since the triplet level of chlorophyll b has a value of 1.43 ev, midway between the value of 1.52 ev for the free carotene semiconduction activation energy and 1.29 ev for the corresponding value of the oxygen-carotene complex. It is also suggested that a possible mechanism for the carotene protection of cells from destructive photo-oxidation may be the formation of the oxygen-carotene complex.