Fluorescence excitation and UV-UV double-resonance spectroscopy of the S0→S1(Lb) transition of 1,6-methano[10]annulene cooled in a supersonic jet

The first electronic singlet transition S-0 --> S-1 of the 10pi-aromatic compound 1,6-methano[10]annulene (MA) cooled in a supersonic jet has been studied up to an excess energy of 4000 cm(-1). The strongest line at 25 154 cm(-1) is assigned as the electronic origin. Analysis of the rotational envelope of this line proved that the transition dipole is parallel to the long axis of the molecule. Optical-optical double resonance was used to identify the lines which share the same ground state with the origin transition. These lines occur all at higher energies. A few weaker lines which are always present but do not lead to double resonances are tentatively attributed to a van-der-Waals dimer of MA. The rich vibrational structure is interpreted in terms of 13 fundamental vibrations of a(1) symmetry and 11 of a(2) symmetry, based on the analysis of the rotational contours. The fundamental vibrational frequencies of the excited state are in very good agreement with ab initio calculations. Based on these calculations 8 further lines which are not combination bands are tentatively assigned to double quantum transitions in b(1) and b(2) modes. These results strongly support the assignment of a delocalized structure without bond length alternation to the electronic ground state as well as to the first electronically excited singlet state.