The fluorescence excitation spectrum of free base phthalocyanine cooled in a supersonic free jet

The fluorescence excitation spectrum of free base phthalocyanine was observed in a supersonic free jet. The spectrum was very intense, and the cooling provided by the supersonic expansion produced a very well-resolved spectrum showing extensive vibrational structure. This is the first time that vibrational structure has been resolved in the electronic spectrum of the isolated molecule. The separation between the origins of the first two excited singlet states was similar to that observed in the static gas, but in the cold spectrum both origins were at several hundred cm−1 higher energy than the band maxima observed in the static gas. The region near the origin of the first singlet was clean and well-resolved, while that near the origin of the second singlet was congested due to mixing with excited vibrational levels of the first singlet. No van der Waals molecules were observed.