TRANSIENT ABSORPTION AND VIBRATIONAL-RELAXATION DYNAMICS OF THE LOWEST EXCITED SINGLET-STATE OF PYRENE IN SOLUTION

The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxation processes in the S-1 singlet state are investigated by femtosecond transient absorption spectroscopy. Three different UV excitation wavelengths (360, 307, and 290 nm) are utilized to excite the molecule at the edge of the S-1 <-- S-0 transition and to higher S-n <-- S-0 transitions. The buildup of S-1 population has been observed to be completed in the first 2 ps, with a time constant of 300-400 fs, depending on the excitation wavelength. The evolution of the absorption spectra on a larger time scale is attributed to the vibrational cooling in the S-1 state; its rate corresponds to an exponential relaxation with time constant of about 4 ps. The evidence of the formation of pyrene excimer at long time delays in highly concentrated solutions is reported.