Excited-state photoelectron spectroscopy of excitons in C60 and photopolymerized C60 films -: art. no. 205415

Laser-excited states in films of pristine C-60 and photopolymerized C-60 (PP-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-C-60 with time-resolved methods and are split by 0.33 eV. Signals from pristine C-60 are weaker but are spectroscopically similar, The origin for exciton transitions for both C-60 and PP-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 similar to2.5 ns for PP-C-60 at 81 K. A weaker photoemission band located similar to0.5 eV above S-1 is also observed and is attributed to charge carriers and/or charge-transfer excitons.