O2(a1Δg) 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(a1Δg) luminescence. The O2(a1Δg) emission peak at 77 K was centered at 1281±1 nm and slightly red shifted to 1283±1 nm at 250 K. The emission decay time ranged from approximately 10 ms at 80 K to about 1 ms at 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(a1Δg) luminescence in solutions of C60 dissolved in CCl4 and in C60 films resulted in a quantum yield estimate for O2(a1Δg) production in the film of 0.15 and an O2(a1Δg) concentration estimate in the film of 0.18% relative to the C60 concentration. This set a lower limit for the oxygen concentration in the films. The low oxygen concentration relative to C60 concentration in the films led to saturation of the O2(a1Δg) emission intensity under 532 nm pulsed laser irradiation. Also, the O2(a1Δg) luminescence intensity was measured as a function of film thickness, from which an oxygen penetration distance of approximately 2500 A in the C60 films was inferred. Finally, the O2(a1Δg) emission was used to monitor the diffusion of oxygen out of C60 films. From the variation of the diffusion rate with temperature, the energy required to remove an oxygen molecule from the film was determined to be approximately 7 kcal/mol. (orig.)