We have measured the direct absorption of ballistic phonons by a two-d
imensional electron system (2DES) in the fractional quantum Hall regim
e. We show experimentally that the electron-phonon interaction in our
heterojunction sample is dominated by coupling to TA phonons. At odd i
nteger denominator fractional filling factors a strong non-equilibrium
heating of the 2DES is observed which can be attributed to excitation
s across the magneto-roton gap. Away from fractional minima, in partic
ular at a filling factor, v = 1/2, a much smaller temperature increase
is found. (C) 1998 Elsevier Science B.V. All rights reserved.