  Using non-linear response theory the time signals relevant for nonresonant
spectral hole burning are calculated. The step-reponse function following the
application of a high amplitude ac field (pump) and an intermediate waiting
period is shown to be the sum of the equilibrium integrated response and a
modification due to the preparation via ac irradiation. Both components are
calculated for a class of stochastic dipole reorientation models. The results
indicate that the method can be used for a clearcut distinction of
homogeneously and heterogeneously broadened susceptibilities as they occur in
the relaxation of supercooled liquids or other disordered materials. This is
because only in the heterogeneous case is a frequency selective modification of
the response possible.
