S(1)<--S(0) transition of 2,3-benzofluorene at low temperatures in the gas phase.

The S(1)((1)A('))<--S(0)((1)A(')) absorption spectrum of jet-cooled 2,3-benzofluorene (Bzf) has been measured by cavity ring-down spectroscopy. The potential energy surfaces of the S(n=0,1,2) states of Bzf have been investigated with calculations based on the time-dependent density functional theory (TD-DFT). At the B3LYP/TZ level of theory, TD-DFT does not deliver a realistic difference between the excited S(1) and S(2) potential energy surfaces, a problem which can be avoided by introducing a reference geometry where this difference coincides with the observation. In this geometry, an expression for the Herzberg-Teller corrected intensities of the vibronic bands is proposed, allowing a straightforward assignment of the observed a(') modes below 900 cm(-1), including realistic calculated intensities. For vibronic bands at higher energies, the agreement between calculated and observed modes is deteriorated by substantial Dushinsky rotations and nonparabolicities of the potential energy surface S(1).