  Two preceding parts of a paper (cond-mat/0508457, cond-mat/0508460)
considered the heuristic values of recent experiments pointing to the nearly
universal occurrence of photovoltages across solid surfaces under short-circuit
conditions. These voltages arise by virtue of a variety of spectrally-dependent
mechanisms activated by incident photons. For the visible range, the
photovoltages are obliged to the photodetachment of ions which leave the
surface charged. In an attempt to learn more, we now study short-circuit
photovoltages in well-defined materials including high-Tc superconductors
within a broad temperature range down to liquid nitrogen. We believe our data
provide a new insight into the process.
