Superatom orbitals of C 60 on Ag(111): Two-photon photoemission and scanning tunneling spectroscopy

The unoccupied electronic structure of C${}_{60}$ layers on Ag(111) was investigated with two-photon photoemission (2PPE) and scanning tunneling spectroscopy (STS). The focus, in addition to providing unoccupied level assignments for C${}_{60}$ monolayers, is on identifying signatures of C${}_{60}$ superatom molecular orbitals (SAMOs), diffuse unoccupied levels weakly bound by the spherical potential of the fullerene shell, in the two complementary techniques. The s-SAMO is identified 3.3 eV above the Fermi level in 2PPE and at a bias of 3.5 eV in STS. Possible contributions from $p$-SAMO levels are also discussed for both measurement techniques. The results are compared with recent low-temperature scanning tunneling microscopy (LT-STM) work that investigated C${}_{60}$ SAMOs on Cu(111) and oxygen-covered Cu(110) surfaces and with previous 2PPE measurements for C${}_{60}$ monolayers on Au(111) and Cu(111).