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

Journal of Physical Chemistry A, ISSN: 1089-5639, Vol: 108, Issue: 34, Page: 6944-6952
2004
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We have measured the lifetime τ of the lowest triplet state T in free C by a pump-probe experiment using lasers with nanosecond pulse durations. At low pump fluence the population of T 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 C 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 C (T ) 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 E = E - E = 4.6 eV (one-photon absorption at λ = 532 nm, T = 420 °C) while τ = 40 ns corresponds to E = 9.6 eV (two-photon absorption at 355 nm, 480 °C). This result strongly suggests that delayed electrons that are emitted from highly excited CSG (E ≫ 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 C as thermionic emission is, therefore, warranted.

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