Vibrational Energy Dependence of the Triplet Lifetime in Isolated, Photoexcited C60

Abstract

We have measured the lifetime τ of the lowest triplet state T1 in free C60 by a pump−probe experiment using lasers with nanosecond pulse durations. At low pump fluence the population of T1 decays with a distinct, narrow distribution of lifetimes. τ depends on the pump wavelength (λ = 532, 355, or 266 nm) as well as the temperature of the source from which C60 is vaporized (420 ≤ T ≤ 510 °C); it ranges from 2 μs to 0.3 μs. At high pump fluence an additional lifetime as short as 40 ns is observed. A consistent correlation of all observed lifetimes with the experimental parameters is found if τ is assigned to an ensemble of C60 (T1) that has absorbed either one or two pump photons with the excess energy being randomized over all vibrational modes. Thus, τ = 2 μs corresponds to a vibrational energy Evib = Etotal − Etriplet = 4.6 eV (one-photon absorption at λ = 532 nm, T = 420 °C) while τ = 40 ns corresponds to Evib = 9.6 eV (two-photon absorption at 355 nm, 480 °C). This result strongly suggests that delayed electrons that are emitted from highly excited C60 (Evib ≫ 10 eV) on the time scale of ≈10 ns to 1 ms are not affected by long-lived electronically excited states. The frequently questioned description of delayed electron emission from photoexcited C60 as thermionic emission is, therefore, warranted.