As told by , PAHs are a class of aromatic organic molecules that are ubiquitous on Earth, in the solar system, and in the interstellar medium. PAHs are playing "important roles in the evolution of the cosmic environments" . Pyrene (C16H10), is the "smallest pericondensed polycyclic aromatic hydrocarbon (PAH)". Gavilan and coworkers used Raman spectroscopy to characterize pyrene in microcrystals and in pyrene-derived nanograins. The researchers observed a spectrum of microcrystalline pyrene which consisted of narrow bands, in agreement with measurements of single and powder pyrene crystals. The Gu and coworkers explain that the spectrum can be divided into five regions: (1) 1650-1500 cm -1 , characterized by the "C-C stretching mode mixed with some CH in-plane bending vibrations"; (2) 1500-1350 cm -1 , a region which is containing the bands that are due to the "strong C-C stretching coupled with weak ring-breathing vibrations"; (3) 1300-1000 cm -1 , it is the region where the bands are produced mainly by "CH in-plane bending vibrations"; (4) 1000-600 cm -1 , in this case the bands are coming from the "CH out-of-plane bends". Finally , it is the region consisting of the spectrum below 550 cm -1 , it "may be due to out-of-plane skeletal deformation vibrations" .