Raman Spectroscopic Study of Pyrene in Cosmic Dust Analogues: Evolution from the Gas to the Solid Phase

Raman spectroscopy was employed to characterize pyrene (C16H10), the smallest peri-condensed polycyclic aromatic hydrocarbon (PAH), in microcrystals and in pyrene-derived nanograins. Laboratory spectra in the 200–2000 cm–1 Raman shift region were obtained with a 532 nm laser. The spectrum of microcrystalline pyrene consists of narrow bands (FWHM ∼7 cm–1), in good agreement with previous laboratory measurements of single and powder pyrene crystals, and was compared to density functional theory-computed Raman scattering activities of gas phase pyrene monomer and pyrene tetramer clusters, showing that packing in crystals induces shifts in band positions to higher wavenumbers. On the other hand, the spectrum of pyrene-derived nanograins exhibits a broad graphitic G band at 1587 cm–1 (FWHM ∼56 cm–1) and multiple disorder D bands, the largest one at 1388 cm–1 (FWHM ∼101 cm–1), attributed to defects in the carbonaceous structure. When compared to the D-to-G band strength ratios from the insoluble organic matter (IOM) found in various carbonaceous chondrites, the values found for the pyrene nanograins match those from the highly disordered Allende meteoritic IOM best.

Hali tarjima qilinmagan