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-H2 as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H2 matrix 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 000 band associated with the electronic transition to the first excited electronic state at 18881 cm-1, red-shifted by similar to 68 cm-1 relative 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-C10H9 is 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-H2 to the reported electronic absorption spectrum of jet-cooled gaseous 1-C10H9 and that of 1-C10H9 isolated in solid Ne to assess the influence of p-H2 as a matrix host on the electronic transition of 1-C10H9 and discuss a potential contribution of 1-C10H9 to the diffuse interstellar bands.

Automated summary

Matrix isolation spectroscopy with para-hydrogen (p-H2) was used to study the electronic transitions of the 1-hydronaphthyl radical (1-C10H9) isolated in solid p-H2. The dispersed fluorescence and fluorescence excitation spectra were observed, and vibronic transitions were assigned through comparison with simulated spectra. The fluorescence spectrum complemented the infrared spectrum and provided additional vibrational mode information, while the excitation spectrum covered a wider spectral range. Comparison with spectra from different matrices helped evaluate the influence of p-H2 as a matrix host on the electronic transition of 1-C10H9 and discuss its potential contribution to diffuse interstellar bands.