The dependence of surface photovoltages on the wavelength of light are obtained for the first time from an atomic model and ab initio calculations. Photovoltages follow from a time-dependent density matrix treatment using a basis set of Kohn-Sham orbitals and a steady state solution for the time-dependent density matrix. An application to a H-terminated Si(111) surface gives the main features of calculated photovoltage versus incident photon energies in agreement with experimental trends. Our treatment can be implemented for a wide class of photo-electronic materials relevant to solar energy capture. (C) 2008 Elsevier B.V. All rights reserved.