  We describe a theoretical framework for the interpretation of time-resolved
phonon absorption experiments carried out in the fractional quantum Hall regime
of a magnetically quantized two-dimensional electron system (2des). The only
phonons which can be absorbed at low temperature are those whose energies
exceed the magnetoroton gap predicted by Girvin, MacDonald and Platzman. The
rate of energy transfer from the phonons to the electron liquid is entirely
controlled by the creation of these collective excitations. Using simple
isotropic approximations for the phonon propagation and electron-phonon
coupling we obtain analytic results for the regime in which the electron
temperature and the characteristic temperature of the phonons are much less
than the gap and identify the way in which the dispersion curve of the
magnetorotons could be extracted from time and angle resolved experiments.
