  We present measurements of the resonant microwave absorption by the Rydberg
energy levels of surface state electrons on the surface of superfluid liquid
helium, in the frequency range 165 - 220 GHz. The resonant frequency was
strongly temperature dependent from 0.1 to 2 K. The experiments are in
agreement with recent theoretical calculations of the renormalisation of the
electron energy levels due to zero-point and thermal ripplons, analogous to a
condensed matter Lamb shift. The temperature-dependent contribution to the
linewidth for excitation to the first excited state at 189.6 GHz is compared
with other measurements and theoretical predictions.
