Formation and decay kinetics of light-induced dye radicals on spectrally sensitized AgBr microcrystals measured by electron spin resonance spectroscopy: A quantitative model

Abstract

During optical excitation of spectrally sensitized silver bromide microcrystals, dye radicals are generated at the surface of the crystals. For the dyes studied in this work the radicals are oxidized dye molecules, which have a hole trapped at their doubly degenerate highest occupied molecular orbital. The dye positive holes can be detected using electron spin resonance (ESR) spectroscopy. After optical excitation the oxidized form of the dye starts to decay. A new method to acquire kinetic data of formation and decay of dye positive holes is introduced, taking into account the variation in linewidth of the ESR signal. By comparing measurements in ambient air, dry air and dry nitrogen gas it is found that moisture and not oxygen influences the kinetic behavior of the radicals. A quantitative model is suggested which is able to describe the formation of dye positive holes during optical excitation. This model allows one to predict the decay of the oxidized dye molecules, using the fitting parameters of the formation curves.