  We argue that the time-resolved spectrum of selectively-excited resonance
fluorescence at low temperature provides a tool for probing the
quantum-mechanical level repulsion in the Lifshits tail of the electronic
density of states in a wide variety of disordered materials. The technique,
based on detecting the fast growth of a fluorescence peak that is red-shifted
relative to the excitation frequency, is demonstrated explicitly by simulations
on linear Frenkel exciton chains.
