The electronic absorption spectrum of ferrocene has been investigated in the vapor, in liquid solutions, and in glassy matrices. Temperatures used ranged from 420° to 77°K. The absorption spectrum contains at least 11 distinct electronic absorption bands. Three of these appear to be triplet←singlet in nature, and to be made allowed by spin—vibronic perturbations (i.e., spin—orbit coupling of the triplet manifold with vibronically coupled singlets). It is shown that a spin—orbit coupling factor 300≤ζ≤400 cm−1 can account for the observed intercombination intensities. The singlet←singlet absorption spectrum is dominated by one allowed transition at ∼50 000 cm−1 with f∼0.69. It is possible that all other electronic transitions, the 53 000-cm−1 band excepted, are dipole forbidden and obtain intensity by first-order vibronic stealing from the ∼50 000-cm−1 band. The higher-energy absorption bands show vibrational structure, and this structure is analyzed herein; unfortunately, the resolution is restricted by ``molecular'' reasons associated with vibrational ``richness,'' hindered rotations, and vibrational ``hot bands.'' The higher-energy absorption bands are heavily localized on the aromatic rings in contrast to the three low-energy diffuse electronic systems in Regions IV, V, and VI which contain much d-orbital and intramolecular charge-transfer character. No phosphorescence emission of ferrocene has been observed here.

Armstrong, A. T., Smith, F., Elder, E., & McGlynn, S. P. (1967). Electronic Absorption Spectrum of Ferrocene. The Journal of Chemical Physics, 46(11), 4321–4328.