PHOTOEXCITATIONS IN PARA-HEXAPHENYL

We investigate the absorption and emission of photoexcitations in poly crystalline 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 spectru m can be assigned to at least three different species: (1) transitions between elicited states (S-1-->Sn) and (2) transitions between triple t states (T-1-->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 f or both the dynamics of the initial photoinduced absorption and for st imulated emission. The stimulated emission found in the transient opti cal modulation spectroscopy shows a very limited spectral overlap with the photoinduced absorption. In contrast to doping experiments we fin d no evidence for long living polaronic states upon photoexcitation. [ S0163-1829(37)09740-3].