The very short and high power far infrared pulses of the new Dutch Fre
e Electron laser FELIX at the FOM Institute ''Rijnhuizen'' have been u
sed for the first time to perform linear and non-linear spectrscopic e
xperiments in bulk n-GaAs. Preliminary results on a variety of effects
observed in the far infrared shallow donor spectrum in an applied mag
netic field are reported.