Resonant Photoelectron Spectroscopy and Total Photodetachment Yield Spectroscopy of the Fluoranthene Radical Anion

Polycyclic aromatic hydrocarbons are believed to compose ~10% of the carbon in the diffuse interstellar medium and are promising candidates for carriers of diffuse interstellar bands.^[1] Given their abundance, this class of molecules and their corresponding molecular anions likely play an important role in the chemistry of circumstellar and interstellar space.^[2] Here, we probe the energies and autodetachment dynamics of the fluoranthene radical anion resonances using high-resolution cryogenic photoelectron spectroscopy, and by measuring total photodetachment yield as a function of wavelength. Several anionic resonances were observed above the neutral threshold in the energy range 6000 – 34000 cm^-1, two of which reveal some vibronic structure. Resonant photoelectron spectroscopy in the visible region suggests fast internal conversion to lower-lying resonances followed by autodetachment to the ground state neutral. We also see evidence for autodetachment into the excited triplet state of neutral fluoranthene in the ultraviolet region. Little to no internal conversion to the ground state anion was observed, as evidenced by a lack of thermionic emission in resonant photoelectron spectra. This suggests that electron capture into the anion resonances of fluoranthene is unlikely to lead to formation of the ground state anion.

[1] A. G. G. M. Tielens, Rev. Mod. Phys. 2013, 85, 1021–1081.

[2] T. J. Millar, C. Walsh, T. A. Field, Chem. Rev. 2017, 117, 1765–1795.