Experimental Indication of a Naphthalene-Base Molecular Aggregate for the Carrier of the 2175 Å Interstellar Extinction Feature

Experiments where the simple polycyclic aromatic hydrocarbon (PAH) naphthalene (C₁₀H₈) is subjected to the energetic environment of a plasma have resulted in the synthesis of a molecular aggregate that has ultraviolet spectral characteristics that suggest it provides insight into the nature of the carrier of the 2175 Å interstellar extinction feature and may be a laboratory analog. Ultraviolet, visible, infrared, and mass spectroscopy, along with gas chromatography, indicate that it is a molecular aggregate in which an aromatic double ring ("naphthalene") structural base serves as the electron "box" chromophore that gives rise to the envelope of the 2175 Å feature. This chromophore can also provide the peak of the feature or function as a mantle in concert with another peak provider such as graphite. The molecular base/chromophore manifests itself both as a structural component of an alkyl-aromatic polymer and as a substructure of hydrogenated PAH species. Its spectral and molecular characteristics are consistent with what is generally expected for a complex molecular aggregate that has a role as an interstellar constituent.