Photodissociation action spectroscopy is often used as a proxy for measuring gas-phase absorption spectra of ions in a mass spectrometer. Although the potential discrepancy between linear optical and photodissociation spectra is generally acknowledged, direct experimental comparisons are lacking. In this work, we use a quadrupole ion trap that has been modified to enable both photodissociation and laser-induced fluorescence to assess how closely the visible photodissociation action spectrum of a fluorescent dye reflects its fluorescence excitation spectrum. Our results show the photodissociation action spectrum of gaseous rhodamine 110 is both substantially narrower and slightly red-shifted (∼120 cm(-1)) compared to its fluorescence excitation spectrum. Power dependence measurements reveal that the photodissociation of rhodamine 110 requires, on average, the absorption of three photons whereas fluorescence is a single-photon process. These differing power dependences are the key to interpreting the differences in the measured spectra. The experimental results provide much-needed quantification and insight into the differences between action spectra and linear optical spectra, and emphasize the utility of fluorescence excitation spectra to provide a more reliable benchmark for comparison with theory.
In nature, the finely tuned photophysical properties of chlorophyll a (Chla) are vital to the capture and transfer of sunlight during photosynthesis. In order to better understand how these properties are influenced by the molecular environment, we have examined the intrinsic spectroscopy of Chla in vacuo. Visible photodissociation action spectra (an indirect measure of absorption) of gaseous protonated Chla and Chla complexed with metal cations are reported. These show that spectral features within the Soret band (∼350-445 nm) have markedly different intensities depending on the identity of the cation. In contrast, fluorescence emission spectra of metalated Chla complexes show only small dependences on the identity of the metal ion, with emission maxima shifting from 661 to 654 nm. Remarkably, replacing the metal ion with a proton turns off the fluorescence of this key pigment. Density functional theory geometry-optimized structures indicate that the most favorable site of protonation differs from that of metal cationization, and may help explain the surprising on/off behavior of Chla's intrinsic fluorescence.
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