Excited-state photoelectron spectroscopy of excitons inC60and photopolymerizedC60

Abstract

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 ϳ2.5 ns for pp-C 60 at 81 K. A weaker photoemission band located ϳ0.5 eV above S 1 is also observed and is attributed to charge carriers and/or charge-transfer excitons.

Intramolecular vibronic dynamics in molecular solids:C60

“…More recent experimental work has shown a significant role of such vibrational coupling for the HOMO in two dimensions 11 and for exciton dynamics in three dimensions. 19 These observations are difficult to reconcile with previous claims, suggesting that a new approach yielding more direct information on the vibrational coupling in such solids is needed.…”

“…8,[10][11][12] In C 60 multilayers, solid-state band structure in the HOMO has not been observed, leading to contrasting viewpoints in the interpretation of experimental results on the strength of vibrational coupling. 11,[13][14][15][16][17][18][19] Some workers have claimed that vibrational coupling is much smaller in the solid than for the isolated molecule. 17,18 Theoretical work, which did not consider vibrations, 14 shows that orientational disorder and the relatively large unit cell could explain some aspects of the experimental observations.…”

Determination of Band Curvatures by Angle-Resolved Two-Photon Photoemission in Thin Films of C60 on Ag(111)

“…A number of photoemission studies have previously been performed on thin films of C 60 − and alkali-doped C 60 . − Of these studies, those particularly relevant to the current manuscript are those by Ton-That et al, Yang et al, and Brouet et al Each of these papers examines angle-resolved photoemission spectra of C 60 on Ag(111), but unlike the current paper, they are primarily concerned with photoemission only from a C 60 monolayer, and furthermore, as they all utilize one-photon techniques, only characterize initially occupied states. It has only been relatively recently that the technique of angle- and time-resolved two-photon photoemission (2PPE) has been applied to study C 60 films. − The advantage of a two-photon technique over other photoemission techniques (such as ultraviolet photoelectron spectroscopy or inverse photoelectron spectroscopy) lies in the fact that it can simultaneously probe both initially occupied as well as initially unoccupied electronic band structure, and that it can do so with an ultrafast temporal resolution.…”

Unoccupied States in C60 Thin Films Probed by Two-Photon Photoemission

“…There have been several two-photon photoemission studies of C 60 thin films. Quast et al studied a chemisorbed monolayer of C 60 on Ni(110) by a combination of picosecond lasers and synchrotron radiation . These authors attributed a transiently populated feature at ∼1.8 eV above E Fermi to the LUMO + 1 level, formed by photoexcitation from filled states near or below the Fermi level.…”