Vibrational spectra of four polycyclic aromatic hydrocarbons under high pressure: implications for stabilities of PAHs during accretion

Infrared and Raman spectra of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, and pyrene have been examined up to 10-55 GPa at 300 K, to probe structural changes in these materials under high-pressures, and to relate these to shock measurements on these materials. The goal is to develop an understanding of how such hydrocarbons might be processed during planetary accretion. A range of phase transitions in PAHs are observed and, in accord with previous investigations, these typically initiate at relatively low pressures (0.3-4.0 GPa): the lower-pressure transitions are likely associated with inter-molecular changes such as changes in symmetry and/or molecular orientation, charge transfer processes, or changes in pi electron density, and are often sluggish. Higher-pressure (7-10 GPa) phase transitions in PAHs are likely associated with profound structural changes like dimerization, which are not always reversible. Laser-induced luminescence is encountered at pressures well below those at which PAHs amorphize, and a strong pressure-induced Fermi resonance is identified between the highest-lying inter-molecular modes and lowest-lying intra-molecular modes in each PAH examined. It is the increased strength of inter-molecular interactions under pressure that likely generates increasing overlap of pi orbitals and leads to cross-linking (dimerization) of the molecules and the destruction of their planar symmetry. The first step in the amorphization of these compounds is likely dimerization, and amorphization occurs when long-range order is lost and a greater diversity of local structural environments is introduced into these materials, such as carbons being shared between rings, embayed structures, sp, sp(2), and sp(3) hybridized carbon atoms, a broad range of C-H bonding environments, and fewer residual resonance-stabilized C-C units. Our results are consistent with pressure producing amorphous, hydrogenated carbon material from PAH precursors: hence, impact phenomena, coupled with post-shock hydrogen loss, could provide an alternate pathway to produce amorphous carbon assemblages of the type observed within a range of meteorites. Additionally, smaller PAHs tend to be most stable under compression; as these are the most volatile of the PAHs, the combination of shock during accretion, coupled with trends in volatility, may limit the presence of PAHs within objects formed in the early solar system.