Ultrafast Dynamics of Photoexcited States in C ₆₀

Abstract

We have measured the relaxation of photoexcited states in C60 using femtosecond excitation at energies corresponding to the h1u → t1u bandgap. Induced absorption with a fast 2 ps relaxation is observed, which has bimolecular kinetics indicative of exciton-exciton annihilation. The induced absorption is spectrally broad with peaks at 1.92 and 2.25 eV. These features are assigned to excited-state absorption from the t1u level to higher-lying hg excited states at ∼ 4.0 eV.

Excited-state photoelectron spectroscopy of excitons inC60and photopolymerizedC60Long1, Chase2, Kabler3 2001Phys. Rev. B16070View full textAdd to dashboardCite

“…This time is much less than the ϳ2.5-ns intersystem crossing time we measure at lower exciton concentrations. Similar behavior has been previously suggested 19 for incompletely characterized films of C 60 and has also been discovered in model compounds of conjugated polymers. 2 Excess T 1 production at high concentrations may have ramifications for potential devices operating at high concentrations, such as polymeric lasers.…”

“…Previously, to explain room-temperature measurements of transient absorption in C 60 films exposed to air, an increased rate of T 1 production at high fluence was proposed, but the roles of S 1 and T 1 could not be separated. 19 Recently, a similar preferential growth in T 1 states at high excitation density was reported for the conjugated oligomer para-hexaphenyl. 2 Accompanying the enhanced T 1 production at high excitation densities in C 60 and pp-C 60 , we find complex transient behavior, which includes an increase in the decay rates of both T 1 and S 1 with excited state concentration, always with S 1 decaying more rapidly.…”

“…2 Reaction ͑i͒ was previously suggested to operate in C 60 to account for a transient absorption signal that decayed more rapidly as excitation fluence increased but that left a long-lived transient residue attributed to T 1 absorption. 19 Those experiments were carried out on films exposed to air and were unable to separate overlapping S 1 and T 1 contributions, as our ESPES results do. Interaction ͑i͒ has also been invoked to explain increased T 1 production at high excitation densities observed by transient absorption in para-hexaphenyl films, in which separate absorptions from S 1 , T 1 , and polarons were followed.…”

Normal-mode selectivity in ultrafast Raman excitations inC60

“…Using a 2.03 eV pump beam and pulse duration of 100 fs, Thomas et al measured the relaxation of photoexcited states in C 60 by time-resolved optical absorption and found a 1 ps response, and when tuning to the forbidden h 1u to t 1u transition, they found a 2 ps relaxation. 4 Ishihara et al used a 100 fs, 0.2 mJ, and 628 nm laser to find that the decay times of the self-trapped exciton and polaron are about 570± 120 fs and 54± 7 ps, but no clear lattice vibration was resolved. 5 A systematic investigation was done by Chergui's group on the ultrafast intramolecular relaxation, where they found that the transient absorption spectra of C 60 have a characteristic rise time of 200 fs.…”

Photoinduced Ultrafast Electron Transfer and Charge Transport in a PbI2/C60 Heterojunction

“…27 The photoinduced absorption (PIA) in the range of 520−700 nm is ascribed to the S 1 −S n transition. 27,50,51 The negative signal at around 500 nm has been explained as a superposition of PIA and GSB. 27 Let us now focus on the planar mixed heterojunction with a configuration of quartz/C 60 (10 nm)/C 60 :PbI 2 (5 nm, weight ratio = 1:5)/PbI 2 (30 nm), which is similar to the structure of our solar cell.…”