High-Resolution Spectra of Carbon Nanoparticles: Laboratory Simulation of the Infrared Emission Features Associated with Polycyclic Aromatic Hydrocarbons

Using surface-enhanced Raman spectroscopy we have obtained high-resolution spectra of individual molecular components containing 10²-10³ carbon atoms at the surface of carbon nanoparticles. A number of well-defined spectral features occur in the 6.2, 7.6, 7.8, and 8.6 μm wavelength range that replicate those found in the infrared emission spectrum of interstellar and circumstellar sources. This suggests that the molecular components present in laboratory samples are of similar composition to those occurring in space and provides additional insight into the chemical species responsible for infrared emission at these wavelengths. In particular, we find that laboratory spectra are produced by a relatively small number of molecular groups containing C=C bonds. These include large polycyclic aromatic hydrocarbons and polyacetylenic chains. Spectral features characteristic of type A, AB, B, and C infrared emission sources appear in different samples, indicating that the observed wavelength variations in astronomical sources can be attributed to changes in molecular size and composition. We also find that the infrared emission feature detected at ≈8.6 μm in type A and B sources may arise from polyacetylenic species. Spectral profiles of laboratory bands are in good agreement with those of observed astronomical features. The broad background underlying the astronomical bands over the 6-9 μm (1660-1100 cm^-1) range is also reproduced in laboratory spectra.