We present the first direct spectroscopic study of electrons bound in image-potential-induced surface states outside liquid helium. We have observed absorption lines which show the expected linear Stark effect in the presence of an electric field. Zero-field splittings extrapolated from the data are 125.9 and 148.6 GHz for electronic transitions from the ground state to the first two excited states in the image-potential well.

• Received 26 November 1973

DOI:https://doi.org/10.1103/PhysRevLett.32.280