Spectrum Links Hydrocarbon To Space

The first example of a high-resolution infrared spectrum of protonated naphthalene has been recorded in a laboratory, and it is helping astronomers make sense of a confusing collection of molecular signatures of organic compounds observed in interstellar space (Astrophys. J. 2009, 702, 301). The mid-IR range in space is clogged by a dense forest of spectral lines, dubbed the unidentified IR (UIR) bands. Astronomers have suspected that the vibrations of large molecules such as polycyclic aromatic hydrocarbons (PAHs) are responsible for many of these bands, “but there was never any IR spectroscopy on these species to test the idea,” explains University of Georgia’s Michael A. Duncan, who led the study. Duncan’s group measured the spectrum of a cold supersonic beam of protonated naphthalene—one of the smallest PAHs—noting lines at 7.7 and 8.6 μm and, in particular, a strong spectral line at 6.2 μm, matching those seen in space. Astronomers have observed the latter line in space for many years, but no one had ever tied it to a particular molecule. Although protonated naphthalene’s lab spectrum contains other lines, the new work suggests that larger protonated PAHs are responsible for most of the bands in the UIR region.