Abstract
Vibrational relaxation and internal conversion of carotenoids (Cars) are closely related to singlet energy transfer from Cars to Bacteriachlorophyll (BChl). Here, subpicosecond time-resolved stimulated-emission spectra of all-trans-neurosporene, spheroidene and lycopene in n-hexane were recorded upon excitation at different vibronic levels of 1Bu+ (v=0, 1, 2) in order to investigate the effect of vibrational relaxation. From time-resolved spectra, we found stimulated-emission spectral patterns do not change with time evolution, however, the patterns at the same delay time are completely different when excited to different vibronic levels. Comparing the observed spectra at the beginning of different excitations with simulated emission spectra based on Frank-Condon principle, we found characteristic emission pattern from the 1Bu+(v=0), 1Bu+(v=1) and 1Bu+(v=2) levels. The results indicate vibrational relaxation is much slower than internal conversion from the 1Bu+ state, which suggest energy transfer rate and efficiency between Car and BChl (Chl) may be excitation energy-dependent in the antenna complexes.
