The S₀ → S₄ transition of jet-cooled fluoranthene: Vibronic coupling of S₄ with S₃

The S₁ → S₀ fluorescence excitation spectrum of fluoranthene in a continuous supersonic jet was measured in the region of the S₀ → S₄ absorption band. The vibrational structure of S₄ was studied up to excess energies of ≈ 1600 cm^-1 above the S_{4, 0} origin at ≈ 35664 cm^-1. In total 19 vibronic transitions were observed. The origin of the S₀ → S₄ absorption band consists of at least two transitions, S_{0, 0} → S_{4, 0′} (Γ_{4, 0′} ≈ 89 cm^-1) and S_{0, 0} → S_{4, 0″} (Γ_{4, 0″} ≈ 79 cm^-1), whose contributions to the 0, 0 band can be described by Lorentzian line profiles. The lower limit of the lifetime of S_{4, 0} was estimated to be ≳ 60 fs. In the overlapping region of the S_{4, 0} absorption band and the {S_{3, w}} manifold, fluoranthene exhibits an intermediate-level structure; the vibronic coupling S₄ ⇔ S₃ can be classified as a strong intermediate case.