A review is presented of photoelectrochemical systems which at p-type
semiconductor electrodes give a quantum efficiency greater than one. T
he use of intensity modulated photocurrent spectroscopy (IMPS) for stu
dying such systems is considered. Special attention is devoted to the
mechanism of current-doubling at p-GaAs caused by oxidizing agents whi
ch also etch the semiconductor chemically. New data are presented for
the photocathodic reduction of iodate in alkaline solution at p-GaAs.
At low light intensity this system gives a quantum efficiency (Q) clos
e to 6.