Temperature-Dependent Energy Levels Of Electrons On Liquid Helium

We present measurements of the resonant microwave absorption between 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 temperature dependent. The experiments are in agreement with recent theoretical calculations of the renormalization of the electron energy levels due to zero-point and thermal ripplons. The temperature-dependent contribution to the line width gamma(T) for excitation to the first excited state at 189.6 GHz is compared with other measurements and theoretical predictions.