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Electroluminescence of phthalocyanine molecules was measured using the tip of a scanning tunneling microscope. The highly resolved fine structure of the spectra was theoretically modeled using Franck-Condon principle for libration states of the molecule that arise due to its interaction with the surface. The model highlights the impact of the asymmetry of the molecule-surface interaction potential on the shape of the spectra. It is one of the first such detailed analyses of a single-molecule spectrum.