Matrix-Isolation in Cryogenic Water-Ices: Facile Generation, Storage, and Optical Spectroscopy of Aromatic Radical Cations

Abstract

Radical cations of naphthalene and 4-methylpyrene have been generated for the first time in high conversion efficiencies in cryogenic water-ices at 15 K through vacuum ultraviolet photolysis. With these radical cations as probes it is shown that cryogenic water-ices at temperatures below 50 K are of good optical quality and inert matrices to isolate and study the electronic spectroscopic properties of neutral and ionic species in the wavelength region 250−900 nm. The spectral energies of guest-species in the cryogenic water-ices are closely comparable with those observed using rare-gas matrices, indicating similar host−guest interactions in rare-gas matrices and water-ices below 50 K. The radical cations are converted to the corresponding alcohols at temperatures higher than 50 K due to reactions between the host and ionized guest species. Thus, cryogenic water-ices are inert matrices that resemble and complement the rare-gas matrices in many aspects. Efficient ionization of organic molecules, such as PAHs studied here, in water-rich ices indicates that ionization-mediated processes play an important role in the evolution of cosmic ices that are exposed to ionizing radiation.