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While action spectroscopy of cold molecular ions is a well-established technique to provide vibrationally resolved absorption features, fluorescence experiments are still challenging. Here we report the fluorescence spectra of pyronin-Y and resorufin ions at 100 K using a newly constructed setup. Spectra narrow upon cooling, and the emission maxima blueshift. Temperature effects are attributed to the population of vibrational excited levels in S, and that frequencies are lower in S than in S. This picture is supported by calculated spectra based on a Franck-Condon model that not only predicts the observed change in maximum, but also assigns Franck-Condon active vibrations. In-plane vibrational modes that preserve the mirror plane present in both S and S of resorufin and pyronin Y account for most of the observed vibrational bands. Finally, at low temperatures, it is important to pick an excitation wavelength as far to the red as possible to not reheat the ions.
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