Direct Observation of S2–S1 Internal Conversion in Pyrene by Femtosecond Transient Absorption

UV pulses of 100 fs at 340 nm are utilised to excite pyrene to the vibrationally cold S 2 state. Population and depopulation processes occurring in the S 2 state are investigatedby transient absorption spectroscopy with a white continuum probe. In the first 400 fsafter excitation it is possible to measure the growth and disappearance of a band at585 nm which is assigned to the S 14 ←S 2 transition. As a consequence of the internalconversion process from S 2 to S 1 state, the disappearance of the 585 nm band and thegrowth of the transient spectrum due toS n ←S 1 transitions can be observed. Kineticmeasurements of the S 2 decay and S 1 growing signals provide an estimate of the rate ofinternal conversion which occurs with a time constant of 150±50 fs.