Vibrational energy dependence of the triplet lifetime in isolated, photoexcited C-60


We have measured the lifetime tau of the lowest triplet state T-1 in free C-60 by a pump-probe experiment using lasers with nanosecond pulse durations. At low pump fluence the population of T-1 decays with a distinct, narrow distribution of lifetimes. tau depends on the pump wavelength ( lambda = 532, 355, or 266 nm) as well as the temperature of the source from which C-60 is vaporized (420 less than or equal to T less than or equal to 510degreesC); it ranges from 2 mus to 0.3 mus. 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 tau is assigned to an ensemble Of C-60 (T-1) that has absorbed either one or two pump photons with the excess energy being randomized over all vibrational modes. Thus, tau = 2 mus corresponds to a vibrational energy E-vib = E-total E-triplet = 4.6 eV (one-photon absorption at A = 532 nm, T = 420degreesC) while tau = 40 ns corresponds to Evib 9.6 eV (two-photon absorption at 355 nm, 480 degreesC). This result strongly suggests that delayed electrons that are emitted from highly excited C-60 (E-vib much greater than 10 eV) on the time scale of approximate to10 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.



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Journal of Physical Chemistry a



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