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The structure of a monolayer of C 60 molecules adsorbed on the Si(111)-)ϫ)-Ag surface has been investigated by scanning tunneling microscopy ͑STM͒ at room temperature and at low temperature ͑60 K͒. The C 60 molecules are arranged in a ͱ21ϫͱ21(RϮ10.9°) double domain structure and also in a 3) ϫ3)(R30°) structure in part. The intramolecular structures are observed only for the molecules adsorbed at step edges at room temperature, because the rotation of C 60 molecules is suppressed due to the strong interaction with the substrate at step edges, while the molecules adsorbed on terraces do not exhibit the internal structure because of their fast rotation. On the other hand, the internal structure of C 60 is resolved for every molecule adsorbed on terraces at low temperature because the rotation of C 60 is suppressed as in the C 60 bulk crystal. The orientations of the individual C 60 seem to be determined by the directions of underlying Si and Ag trimers of the)ϫ)-Ag surface. ͓S0163-1829͑99͒03140-9͔

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