P. Foggi et al., TRANSIENT ABSORPTION AND VIBRATIONAL-RELAXATION DYNAMICS OF THE LOWEST EXCITED SINGLET-STATE OF PYRENE IN SOLUTION, Journal of physical chemistry, 99(19), 1995, pp. 7439-7445
The spectrum of highly excited electronic states of pyrene in n-octane
solution and the relaxation processes in the S-1 singlet state are in
vestigated by femtosecond transient absorption spectroscopy. Three dif
ferent UV excitation wavelengths (360, 307, and 290 nm) are utilized t
o 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 bee
n 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 t
he absorption spectra on a larger time scale is attributed to the vibr
ational cooling in the S-1 state; its rate corresponds to an exponenti
al relaxation with time constant of about 4 ps. The evidence of the fo
rmation of pyrene excimer at long time delays in highly concentrated s
olutions is reported.