Excited-state photoelectron spectroscopy of excitons in C-60 and photopolymerized C-60 films - art. no. 205415

Laser-excited states in films of pristine C-60 and photopolymerized C-60 (P P-C-60) prepared in ultrahigh vacuum have been characterized in situ with p ump-probe photoelectron spectroscopy using both synchrotron radiation and p icosecond laser sources. Photoelectron spectra of singlet (S-1) and triplet (T-1) excitons overlap because of vibrational broadening in the photoemiss ion final state. The spectra have been individually isolated in pp-C-60 wit h time-resolved methods and are split by 0.33 eV. Signals from pristine C-6 0 are weaker but are spectroscopically similar, The origin for exciton tran sitions 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 de nsity favors T-1 production over S-1 such that, at high exciton concentrati ons, T-1 states predominate, even at times much shorter than the unimolecul ar intersystem crossing time, measured to be similar to2.5 ns for PP-C-60 a t 81 K. A weaker photoemission band located similar to0.5 eV above S-1 is a lso observed and is attributed to charge carriers and/or charge-transfer ex citons.