Ultrafast optical probes of excited states in conducting polymers

Summary form only given. The question whether the electronic states of 7/spl pi/-conjugated polymers can be described by a band model or exciton model, has been extensively debated over the last couple of years. We present an overview of relevant ultrafast optical data, such as transient photoluminescence, gain, and photomodulation, obtained from several luminescent (such as poly-(arylene vinylene) and polythiophene derivatives) and nonluminescent (such as poly(diethynyl-silane) and poly(thienylene vinylene)) conducting polymers, and we critically examine their interpretation in the context of both models. For laser excitation photon-energy close to the HOMO-LUMD energy-gap we observed in all cases the dominant role of excitons in the ultrafast response. The electronic dynamics is faster for polymers with E(2A/sub g/) < E(IB/sub u/), but the relaxed 2A/sub g/ state plays an equally important role in other polymers. We also report a new ultrafast optical technique to directly probe excited electronic states in conducting polymers, using picosecond strain waves in the 20 GHz range. We demonstrate its potential to detect various excitonic states with both A/sub g/ and B/sub u/ character.