Surface Science Letters 294 (1993) L964-L968 North-Holland i i i i!i i iiiiii iiiiiii!iii!ii !ii !ii !ii i i i i i! surface science letters Surface Science Letters C60 matrix-isolated in Xe: plasmon shifts and polarization effects T.R. Ohno, G.H. Kroll, J.H. Weaver Department of Materials Science and Chemical Engineering, University of Minnesota, Minneapolis, MN 55455, USA L.P.F. Chibante and R.E. Smalley Rice Quantum Institute and Departments of Chemistry and Physics, Rice University, Houston, TX 77251, USA Received 20 September 1992; accepted for publication 21 June 1993 Energy loss features for solid C60, matrix-isolated C60 in Xe, and for small amounts of C60 on Xe showed differences related to the rare gas environment. In contrast, losses related to on-site ,rr-~-* shake-up features were unchanged. The energy of the molecular it plasmon shifts ~ 0.8 eV to lower binding energy for matrix isolated C6o due to the dielectric response of the surrounding medium. The work function of solid C60 is 5.0 eV. Its ionization potential of 7.2 eV shows a reduction by 0.4 eV relative to the gas phase because of dielectric screening. Most investigations of the electronic properties of C6o have focused on isolated molecules and condensed films [1-14]. The molecular nature of condensed fullerenes has been evident from the separation of the highest-occupied and lowest-un- occupied levels as measured with photoemission and inverse photoemission, a result of localiza- tion in final states created by electron removal and electron addition [2]. Optical absorption measurements for solid C60 and C6o in solution have shown only small differences in the domi- nant spectral features, confirming the importance of on-site excitations [3-5]. Examinations of C60 interactions with K, Mg, Ti, Cr, Au, Ag, Bi, and n-type GaAs(ll0) have shown Fermi level align- ment and dipole formation with fractional occu- pation of the empty states of the molecule [6,7]. This paper examines intrinsic electronic differ- ences between C6o molecules and C6o solids, an investigation made possible by the formation of mixtures with Xe. Photoemission measurements for C60 clusters on Xe and C6o isolated within a Xe matrix reflect the evolution from the molecu- lar to the solid state. The results show a reduc- tion and shift in energy loss features associated with C ls photoelectrons and a reduced ioniza- tion potential relative to the gas phase. The ~-- electron collective excitation shows a shift in en- ergy for isolated molecules relative to thin films, confirming the molecular character of the plas- mon and showing its dependence on the dielec- tric constant of the medium. These studies were performed in an ultrahigh vacuum chamber equipped with a high resolution monochromatic AI Ka X-ray spectrometer (Sur- face Science SSX-100-03 having 0.6 eV instru- mental resolution). In all of the spectra reported here, the energies were referenced to the Fermi level of the system, E F. The sample temperature was varied from 25 to 400 K by using a two stage, closed-cycle He refrigerator and an internal fila- ment heater [15]. Metal substrates were prepared by Cr condensation onto vacuum-cleaved, highly- oriented pyrolytic graphite (HOPG). The temper- ature was held at 25 K during condensation to minimize Cr clustering. Cr films of 50 ,~ thickness showed the emission characteristics of the bulk, and emission from the HOPG was attenuated beyond the level of detectability. Ultrapure Xe was admitted through a precision leak valve at 0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved