Fluorescence Excitation and Dispersed Fluorescence Spectra of the 1-Hydronaphthyl Radical (1-C₁₀H₉) in Solid para-Hydrogen

Matrix isolation spectroscopy with para-hydrogen (p-H₂) has previously been employed to record IR absorption spectra of hydrogenated and protonated polycyclic aromatic hydrocarbons (PAHs), prospective carriers of unidentified infrared and diffuse interstellar bands. Despite the promising prospects of p-H₂as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H₂matrix isolation spectroscopy has rarely been applied to study electronic transitions of guest molecules. Here, we present the dispersed fluorescence and fluorescence excitation spectrum of the 1-hydronaphthyl radical (1-C₁₀H₉) isolated in solid p-H₂. We observed a strong 00₀band associated with the electronic transition to the first excited electronic state at 18881 cm^-1, red-shifted by ?68 cm^-1relative to a value reported for jet-cooled 1-C₁₀H₉. From a comparison of our experimental results to simulated vibrationally resolved electronic absorption and emission spectra computed on the basis of (TD-)DFT geometry optimizations and scaled harmonic vibration calculations using the FCclasses code, we derived assignments for observed vibronic transitions. The dispersed fluorescence spectrum of 1-C₁₀H₉is new; it complements the infrared spectrum and identified many vibrational modes unidentifiable with infrared. The excitation spectrum covers a much wider spectral range than previous reports. We compare the excitation spectrum in solid p-H₂to the reported electronic absorption spectrum of jet-cooled gaseous 1-C₁₀H₉and that of 1-C₁₀H₉isolated in solid Ne to assess the influence of p-H₂as a matrix host on the electronic transition of 1-C₁₀H₉and discuss a potential contribution of 1-C₁₀H₉to the diffuse interstellar bands.