High-power cw far-infrared laser magnetospectroscopy has been used to determine impurity and Landau-level lifetimes in n-type GaAs from saturation absorption measurements. Impurity lifetimes of 3050 ns for the 2p+ state and 500 ns for the 2p- state are obtained for pure uncompensated material. The optical magneto-impurity effect is shown to be characteristic of highly compensated material. At higher laser intensities, saturation cyclotron-resonance absorption has been measured, and well fitted on a three-level model. The carrier-density dependence of the N=1 Landau-level lifetime, 1, has been determined from this and cyclotron emission measurements, and compared to that of InSb. It is shown to be determined by carrier-carrier scattering, and is 10 times longer for n-type GaAs than for n-type InSb over the whole range. At densities of 1012 cm-3, required for possible cyclotron laser action, the measured lifetime is greater than 10 ns for n-type GaAs, implying that population inversion is achievable with interband pumping. Measurements of the intensity (carrier-density) dependence of cyclotron-resonance linewidth have been made, and are shown to be consistent with ionized-impurity scattering. © 1985 The American Physical Society.