Mobility of electrons on the surface of liquid helium

A time-of-flight mobility measurement ($1.17<T<\mathrm{}2.7\mathrm{}$ K) is made on low-density electron pulses ($n<\mathrm{}{10}^4$ ${\mathrm{cm}}^{-1\mathrm{}}$) moving above the surface of liquid helium with the electrons constrained against the liquid surface by a net dc electric field. Below 1.45 K the mobility starts to level off in the manner predicted by Cole although at the lowest temperatures the experimental value is slightly higher than Cole's value. This is direct evidence that the electron-ripplon interaction is important at these temperatures. The mobility at high drift fields is also reported.