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We have observed cyclotron resonance from electrons in two-dimensional states outside liquid helium. Their resonant frequency depends only on the perpendicular component of the applied magnetic field, and their observed mobility is that expected for electrons scattered parallel to the liquid surface in only two dimensions, In a recent Letter, Ostermeier and Schwarz 1 have presented data questioning the existence of image-potential-induced surface states for electrons outside of liquid helium. These states had been predicted for dielectrics generally by Cole and Cohen. 2 They are caused by the potential well formed by the attractive image potential outside a dielectric surface, and the repulsive barrier to electron penetration at the surface. For liquid helium, they are calculated to have a binding energy of 0.68 meV and a spatial extent outside the liquid surface of approximately 100 A. Observation of these states was reported

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PHYSICAL REVIEW LETTERS 27 DECEMBER 1976 dueed displacement of hydrogen is a strong function of the irradiating temperature, being higher at higher temperatures, and (3) hydrogen is displaced more readily than deuterium.