Femtosecond differential transmission spectroscopy of alpha-sexithienyl single crystals at low temperature

Spectral relaxation of photogenerated excitations and stimulated emission i s investigated in an alpha -sexithienyl single crystal. We have performed f emtosecond differential transmission spectroscopy, probing the spectral reg ion from 1.85 to 2.6 eV, and time-resolved photoluminescence measurements a t low temperature. Stimulated emission with amplification of the probe ligh t which results in a positive differential transmission signal is observed at two spectral positions for polarization parallel to the molecular c-axis . The observed stimulated emission decays with a time constant of 85 ps. Di fferent kinetics of the rise of the stimulated emission and the superimpose d excited-state absorption has been observed for excitation with 3.1 eV. Th is difference vanishes for excitation with 2.3 eV. Stimulated emission proc eeds also via trap luminescence.