Intramolecular vibrational relaxation in jet-cooled phthalocyanine

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Abstract
We have observed the dispersed fluorescence spectrum of free base phthalocyanine cooled in supersonic free jet. The spectrum consists of a single strong feature and some weaker red shifted structure. The frequencies of the single strong feature and the red shifted structure change only slightly as a function of the exciting frequency. This indicates that the potential surface does not change greatly upon electronic excitation and that the strong fluorescence transitions are Δv = 0. For excitation at frequencies greater than 720 cm−1 above the origin, the features in the fluorescence spectrum are broadened although the absorption features remain narrow. This is taken to indicate a mixing of background vibrational levels leading to intramolecular vibrational relaxation that is fast compared to the fluorescence lifetime. The intramolecular vibrational relaxation in phthalocyanine is compared to that observed in a recent series of experiments on alkylbenzenes.