Experimental near-infrared spectroscopy of polycyclic aromatic hydrocarbons between 0.7 and 2.5 μm

The near-infrared (NIR) spectra and absolute band strengths of 27 polycyclic aromatic hydrocarbon (PAH) cations and anions ranging in size from C14H10 to C50H22 are reported. The spectra of all the ionized PAHs we have studied to date have strong, broad absorption bands in the NIR. This work shows that ionized PAHs have significant absorption bands at wavelengths longer than predicted by the current astronomical models that consider PAHs in their treatment of the radiation balance of the interstellar medium. Two implications are (1) that ionized/open-shell interstellar PAHs should add weak, broadband structure to the NIR portion of the interstellar extinction curve and (2) that UV-poor radiation fields can pump the PAH emission bands, provided ionized/open-shell PAHs are present.