  The effect of an ac perturbation on the shot noise of a fractional quantum
Hall fluid is studied at finite temperature. For a normal metal, it is known
that the zero-frequency noise derivative exhibits steps as a function of bias
voltage. In contrast, at Laughlin fractions, the backscattering noise exhibits
evenly spaced singularities, which are reminiscent of tunneling
density-of-states singularities for quasiparticles. The spacing is determined
by the quasiparticle charge $\nu e$ and the ratio of the dc bias with respect
to the drive frequency. Photo-assisted transport can thus be considered as a
probe for effective charges of the quantum Hall effect.
