Microwave absorption of surface-state electrons on liquid ³He

We have investigated the intersubband transitions of surface-state electrons (SSE) on liquid ³He induced by microwave radiation at temperatures from 1.1K down to 0.01 K. Above 0.4 K, the transition linewidth is proportional to the density of ³He vapor atoms. This proportionality is explained well by Ando's theory, in which the linewidth is determined by the electron-vapor atom scattering. However, the linewidth is larger than the calculated result by a factor of 2.1. This discrepancy strongly indicates that the theory underestimates the electron-vapor atom scattering rate. At lower temperatures, the absorption spectrum splits into several peaks. The multiple-peak structure is partly attributed to the spatial inhomogeneity of the static holding electric field perpendicular to the electron sheet.