LABORATORY SPECTRA OF POLYCYCLIC AROMATIC HYDROCARBON IONS AND THE INTERSTELLAR EXTINCTION CURVE

Laboratory spectra have been obtained in the ultraviolet-to-visible wavelength range (200-800 nm) of both the neutral and ionized states of five kinds of polycyclic aromatic hydrocarbon (PAH) species: coronene, benzo[ghi]perylene, perylene, benzo(e)pyrene, and pyrene. The PAHs investigated in the present study may be considered as representative of the family of interstellar molecules proposed to be responsible for the unidentified infrared (UIR) bands. When isolated as molecules in boron oxide glass and ionized with gamma radiation, these PAHs exhibit a decrease in the strength of their ultraviolet absorption bands. The preirradiation spectrum of the original neutral state can be recovered by warming the irradiated sample to 100-180-degrees-C, demonstrating this weakening of the ultraviolet bands is due to ionization. Heating causes the ejected electrons to diffuse back to the ions resulting in recombination. Our experimental result of the decrease in strength of the ultraviolet bands indicates that the ionized interstellar PAHs might not add structure to the interstellar extinction curve at wavelengths from that of the bluest diffuse interstellar band (DIB) at 4428 angstrom to the 2175 angstrom bump, which is consistent with observations.