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J. M. Holden and Y. Wang Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506

P. Zhou and P. C. Eklund Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511

C. C. Eloi and J. D. Robertson Department of Chemistry, University of Kentucky, Lexington, Kentucky, 40506 and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511 (Received 2 April 1993; accepted 23 April 1993)

C60 films simultaneously exposed at T = 300 K to oxygen and visible-ultraviolet light are found to exhibit rapid diffusion of oxygen into the deep bulk of the film. The diffusion rate is found to be ~ 10 times faster in the presence of light. Vibrational and optical absorption spectroscopy indicate that the photoassisted reaction of O2 with solid C6o involves two coupled reactions: (i) the photoassisted diffusion of molecular oxygen into solid C60 and (ii) oxidation of C 60 .

I. INTRODUCTION In this paper, we present results of vibrational spectroscopy and optical absorption studies on oxygen-doped C60 films prepared by simultaneous exposure of pristine C60 to visible or ultraviolet (UV) light and O 2 . Direct measurement of the O 2 uptake by resonant a-particle scattering reveals a dramatic, 10-fold increase in the diffusion rate of O 2 into the fee C60 lattice in the presence of visible or UV light. Our spectroscopic results indicate that O2 initially diffuses and resides physisorbed in the C6o lattice, followed by a slower, concurrent chemical reaction with the C6o molecules. The chemical stability of C60 has been a topic of interest since the proposal1 and structural studies2 of its truncated icosahedral symmetry with fully satisfied single and double carbon bonds. It is now well known that C6o reacts with a variety of reagents, which include the production of C 60 H 36 , 3 polyfluorination4 and polymethylation5 of Cm, formation of large adducts such as osmylated C6o,6'7 and Pt, Pd, and Ni-complexes attached to C6o,8'9 to mention only a few examples. Of particular interest here is the reactivity of C60 with oxygen that was first reported to occur in O2-saturated benzene in the presence of ultraviolet light by Taylor et al.w The reaction of dioxygen with solid C60 was first noted by Kroll et al.n in XPS studies of thin solid C60 films sublimed onto GaAs substrates. In these experiments, photoassisted reactions were observed which involved the surface formation of CO, CO 2 , carbonyl groups, and possibly amorphous carbon. Creegan et al.12 first reported the oxidation of C6o in oxygen-saturated J. Mater. Res., Vol. 8, No. 9, Sep 1993

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benzene to form the C6oO epoxide. Later, Elemes et al.13 have shown that a reaction of solid C6o with dimethyldioxirane also leads to the formation of the C6oO epoxide and a C6o- 1,3-dioxolane derivative. Vassallo et al.14 reported the observation of the thermal oxidation of