O2(A(1)DELTA(G) PRODUCTION AND OXYGEN DIFFUSION IN C60 FILMS

Photoexcitation of C60 thin films by either 532 nm pulsed or 514.5 nm c.w. radiation in the presence of oxygen yielded O2(a1DELTA(g)) lumine scence. The O2(a1DELTA(g)) emission peak at 77 K was centered at 1281 +/- 1 nm and slightly red shifted to 1283 +/- 1 nm at 250 K. The emiss ion decay time ranged from approximately 10 ms at 80 K to about 1 ms a t 280 K and the steady state emission intensity dropped by an order of magnitude as the temperature was raised from 80 to 280 K. Comparison between O2(a1DELTA(g)) luminescence in solutions of C60 dissolved in C Cl4 and in C60 films resulted in a quantum yield estimate for O2(a1DEL TA(g)) production in the film of 0.15 and an O2(a1DELTA(g)) concentrat ion estimate in the film of 0.18% relative to the C60 concentration. T his set a lower limit for the oxygen concentration in the films. The l ow oxygen concentration relative to C60 concentration in the films led to saturation of the O2(a1DELTA(g)) emission intensity under 532 nm p ulsed laser irradiation. Also, the O2(a1DELTA(g)) luminescence intensi ty was measured as a function of film thickness, from which an oxygen penetration distance of approximately 2500 angstrom in the C60 films w as inferred. Finally, the O2(a1DELTA(g)) emission was used to monitor the diffusion of oxygen out of C60 films. From the variation of the di ffusion rate with temperature, the energy required to remove an oxygen molecule from the film was determined to be approximately 7 kcal/mol.