Picosecond acoustics in polythiophene thin films.

The picosecond correlation technique has been used to study the generation and detection of ultrasonic strain waves in polythiophene thin films, in the range of 0.3-19 6Hz. We show that the strain generation is mainly due to thermal expansion following energy relaxation associated with exciton formation and that the spectral dependence of the strain response is proportional to ja(tv)/&tv, the first partial derivative of a(tv). We found a linear dispersion relation for the LAphonon branch in the measured frequency range, with a room-temperature sound velocity of 28 A/ps; it stiffens by 10% at 80 K.