We report the experimental investigation of spectral hole burning and four-wave mixing at the interface of glass and a dense cesium vapor using two tunable diode lasers in a pump-probe experiment. Narrow spectral features were observed in the center of the strongly self-broadened
D2 line at Cs number density from 9\ifmmode\times\else\texttimes\fi{}
1015 to 2.8\ifmmode\times\else\texttimes\fi{}
1016${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}3}$. The origin of the resonances is associated with the coherent scattering of laser beams at a self-induced population grating, and their spectral widths reflect the relaxation rate of the population difference. A simple asymptotic expression is used to fit the spectral line shapes and to infer a population relaxation time of 3.7(2) ns, which is almost an order of magnitude shorter than natural lifetime of the excited state.