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 - Dedicated to Professor F. Doerr on the occasion of his 80th birthday

The 1st electronic singlet transition S0 -> S1 of the 10p-arom. compd. 1,6-methano[10]annulene (MA) cooled in a supersonic jet was studied up to an excess energy of 4000 cm-1. The strongest line at 25,154 cm-1 is assigned as the electronic origin. Anal. of the rotational envelope of this line proved that the transition dipole is parallel to the long axis of the mol. 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 a1 symmetry and 11 of a2 symmetry, based on the anal. of the rotational contours. The fundamental vibrational frequencies of the excited state are in very good agreement with ab initio calcns. Based on these calcns. 8 further lines which are not combination bands are tentatively assigned to double quantum transitions in b1 and b2 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 1st electronically excited singlet state