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).

S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices.

The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.