The first experimental study of the spectral dependence of the nucleation of cesium vapor caused by light absorption is carried out in a diffusion cloud chamber. The spectral dependence of the nucleation rate is compared with the absorption and ionization spectra of cesium vapor. The results evidence that the observed structure in the nucleation spectrum at photon energies above the ionization threshold is correlated to the light-induced ionization of cesium dimers Cs-2(+). At lower photon energies light-induced nucleation can be either due to generation of ions by direct ionization of cesium clusters, to various two-step ionization processes, or to optical excitation of cesium atoms. The spectral dependence of the nucleation rate provides a new and powerful tool to study ionization spectra of metal vapors. In addition to results obtained by conventional methods a more consolidated insight into energy levels of atoms and clusters under equilibrium conditions is possible.