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Photon echo evperlments are performed on delocabzed electromc evcltatlons of optxal dlmer states The delGuhzed dlmer states are found to have very long coherence times. close to the coherent hnxt The dlmer dephasmg IS quabtatlvely slmdar to dephasmg of pentacene monomers.

Optical dephasing of pentacene dimers in p-terphenyl is studied. Dimer states R3 and R4 exhibit an exponential temperature activation of T2' which is inconsistent with a mechanism involving scattering between delocalized dimer states.Other dephasing mechanisms are discussed. An upper limit is placed on the relaxation rate from the upper dimer state, R1, to R3.

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