Unoccupied States in C₆₀ Thin Films Probed by Two-Photon Photoemission

Abstract

Unoccupied electronic states in C 60 thin films on Cu(111) have been measured by laser two-photon photoemission (2PPE) spectroscopy. These measurements allow the quantitative determination of energetic positions for the lowest unoccupied molecular orbital (LUMO), LUMO + 1, LUMO + 2, image potential states, as well as the highest occupied molecular orbital (HOMO). The transiently populated LUMO and LUMO + 1 levels are stabilized by the on-molecule charge correlation energy during the 2PPE process. Compared to previous measurements using one-photon photoemission and inverse photoemission, the HOMO-LUMO gap determined by 2PPE is stabilized by half of the Hubbard U. Both intramolecular excitation and a minor metal-to-molecule electron-transfer excitation channel are observed.

Time-resolved photoelectron spectroscopy of low-energy excitations of 4×4 C60/Cu(111)

“…The Hubbard U of fullerite can be estimated to U = 0.8 eV from the conduction band gap and the lowest exciton energy. Zhu and coworkers [74] showed that going from 20 to 2 ML fullerite on Cu(111) U decreases to 0.3 eV, which is in agreement with figure 2.7. In the surface layer U is 1.0 eV [40,100].…”

“…Integration of the charge redistribution yields the net charge transfer into C 60 as a function of the definition of the precise position of the interface between Cu(111) and C 60 . The range of values of this integral explains the range of the reported charge transfers which is 0.8 [73] to 2.0 [74] electrons per C 60 molecule. Eberhardt and coworkers [38] observed the LUMO+1, the LUMO and the singlet exciton in a 3 nm thick fullerite film on Ag(100).…”

Orientation of individualC60molecules adsorbed on Cu(111): Low-temperature scanning tunneling microscopy and density functional calculations

“…This is in contradiction with the monolayer structure typically found for annealing at higher temperature. 8,9,[11][12][13][14][15][16][17][18][19][20] It could well be that the alternating azimuthal orientations correspond to the two alternating binding sites that we have found for the isolated C 60 molecules on Cu͑111͒. The preferred spacing of the fullerenes in a monolayer could favor an alternating pattern of binding-site occupations that are also associated with different preferred azimuthal orientations.…”

“…This is in full agreement with the ͑4 ϫ 4͒ structure found in many previous studies after annealing. 8,9,[11][12][13][14][15][16][17][18][19][20] The annealed films show an increased number of substrate steps underneath the C 60 film-an indication that the annealing induces a surface reconstruction of the Cu͑111͒ surface. 23 We therefore believe that the resulting geometry is in this case not related to the monomer experiments.…”

“…[8][9][10][11] Therefore, a similar surface preparation has been applied in most subsequent studies to obtain well-ordered monolayer films and a variety of experimental methods have then been applied to study these films. 8,9,[11][12][13][14][15][16][17][18][19][20] From the corresponding STM images, 8,9,11,[20][21][22] it was concluded from the threefold molecular symmetry that the fullerenes were oriented with a six-membered ring toward the surface. Binding was argued to be on hollow sites of the surface from an observed lateral shift between neighboring domains.…”