Microwave study of surface electrons on helium films in a magnetic field

We report preliminary results of a cyclotron resonance study of surface electrons (SE) on saturated helium films covering a PMMA substrate at T > 1 K. The real and imaginary parts of the dielectric response epsilon(k, omega) of the SE are measured at fixed k and omega in B-fields up to 10 T in a 12 GHz cavity. The cyclotron resonance of the SE is determined at different helium film thicknesses d(He) and at various electron densities. At small d(He) we find significant anomalies in the cyclotron resonance lineshape and position. As d(He) increases the lineshape becomes progressively more symmetric and its peak moves towards the cyclotron field value expected for a free electron. To fit these data we have modified the classical Drude expression, introducing two different relaxation times for the low and high B-field regions. The phenomenological formulas fit the data quite well. A systematic theoretical analysis of these results is in progress.