Sequential singlet internal conversion of 1Bu+ → 3Ag− → 1Bu− → 2Ag− → (1Ag− ground) in all-trans-spirilloxanthin revealed by two-dimensional sub-5-fs spectroscopy

The time-resolved absorption spectra of all-trans-spirilloxanthin (n=13) in tetrahydrofuran solution were recorded by sub-5-fs pump–probe spectroscopy in the 500–750 nm region. The difference absorption spectra and the lifetimes of the four singlet states involved were separately determined from the two-dimensional data (delay time vs. probe frequency) analyzed by the singular-value decomposition method followed by global fitting, using the title sequential model. The 1Bu+, 3Ag−, 1Bu−, and 2Ag− states were identified, and the lifetimes of the former three states were determined to be ≈10, 25 ± 2, and 140 ± 30 fs, respectively.