Evidence for electron localization in 2D at metal-dielectric interfaces is reported. A nondispersive peak, derived from the image-potential state and observed in angle-resolved two-photon photoemission, is assigned to localized excess electrons. The image electron on alkane monolayers is a free electron. Bilayers of chain or ring alkanes show both localized and delocalized states; electrons are localized on trilayer n-pentane. Bilayers and trilayers of neopentane show no localization. Our results link the geometry of adlayer molecules with localization of excess electrons at interfaces.

• Received 26 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.2243