Excited-state photoelectron spectroscopy of excitons inC60and photopolymerizedC60films

Abstract

Laser-excited states in films of pristine ${\mathrm{C}}_{60}$ and photopolymerized ${\mathrm{C}}_{60}$ $(pp\ensuremath{-}{\mathrm{C}}_{60})$ prepared in ultrahigh vacuum have been characterized in situ with pump-probe photoelectron spectroscopy using both synchrotron radiation and picosecond laser sources. Photoelectron spectra of singlet ${(S}_{1})$ and triplet ${(T}_{1})$ excitons overlap because of vibrational broadening in the photoemission final state. The spectra have been individually isolated in $pp\ensuremath{-}{\mathrm{C}}_{60}$ with time-resolved methods and are split by 0.33 eV. Signals from pristine ${\mathrm{C}}_{60}$ are weaker but are spectroscopically similar. The origin for exciton transitions for both ${\mathrm{C}}_{60}$ and $pp\ensuremath{-}{\mathrm{C}}_{60}$ is found to be properly located near the maximum of the highest occupied molecular orbital. Increasing excitation density favors ${T}_{1}$ production over ${S}_{1}$ such that, at high exciton concentrations, ${T}_{1}$ states predominate, even at times much shorter than the unimolecular intersystem crossing time, measured to be \ensuremath{\sim}2.5 ns for $pp\ensuremath{-}{\mathrm{C}}_{60}$ at 81 K. A weaker photoemission band located \ensuremath{\sim}0.5 eV above ${S}_{1}$ is also observed and is attributed to charge carriers and/or charge-transfer excitons.