  The quasiparticle relaxation time in superconducting films has been measured
as a function of temperature using the response of the complex conductivity to
photon flux. For tantalum and aluminium, chosen for their difference in
electron-phonon coupling strength, we find that at high temperatures the
relaxation time increases with decreasing temperature, as expected for
electron-phonon interaction. At low temperatures we find in both
superconducting materials a saturation of the relaxation time, suggesting the
presence of a second relaxation channel not due to electron-phonon interaction.
