Photoexcitations inpara-hexaphenyl

We investigate the absorption and emission of photoexcitations in polycrystalline hexaphenyl films. Transient and quasi-steady-state optical modulation spectroscopy is used to identify the photoexcited species and their time evolution. The observed photoinduced absorption spectrum can be assigned to at least three different species: (1) transitions between excited states ${(S}_1\to S_n)$ and (2) transitions between triplet states ${(T}_1\to T_n)$ which are formed by intersystem crossing from the singlet to the triplet regime in the 100 ps time domain. In addition we assign one peak to the (3) formation of aggregates in the oligomer films. The depopulation of the excited singlet states is responsible for both the dynamics of the initial photoinduced absorption and for stimulated emission. The stimulated emission found in the transient optical modulation spectroscopy shows a very limited spectral overlap with the photoinduced absorption. In contrast to doping experiments we find no evidence for long living polaronic states upon photoexcitation.