Photocurrent measurements at irradiated highly oriented pyrolytic graphite
basal plane electrodes with adsorbed quinone/hydroquinone (benzoquinone, 1,
4-naphthoquinone, and 9,10-phenantherenequinone) couples are reported. The
photocurrent was generated by modulated W-vis light. The potential dependen
ce of the photocurrent, the photocurrent light action spectra, and photocur
rent dependence on light modulation frequency are reported. The peak of the
anodic photocurrent curve was found close to the formal potential of the a
dsorbed redox couple. The peak is ascribed to the action of two opposite pr
ocesses: (1) oxidation of adsorbed hydroquinones by hot holes that are phot
ogenerated in the graphite; (2) relaxation of the adsorbed redox couple to
equilibrium by a dark reduction process. A model describing the dependence
of the photocurrent magnitude and phase angle on the light modulation frequ
ency as a function of parameters of the dark reduction process is presented
. Rate constants calculated by the proposed model for the dark process are
consistent with reported values.