Using narrowband excitation to confirm that the S∗ state in carotenoids is not a vibrationally-excited ground state species

The hypothesis that S∗ is a vibrationally-excited ground-state population is tested and discarded for two carotenoid samples: β-carotene in solution and rhodopin glucoside embedded in the light harvesting 2 protein from Rhodopseudomonas acidophila. By demonstrating that the transient absorption signals measured in both systems that are induced by broadband (1000 cm−1) and narrowband (50 cm−1) excitation pulses are near identical and hence bandwidth independent, the impulsive stimulated Raman scattering mechanism proposed as the primary source for S∗ generation is discarded. To support this conclusion, previously published multi-pulse pump–dump–probe signals [17] are revisited to discard secondary mechanisms for S∗ formation.