Polycyclic aromatic hydrocarbons (PAHs) are compounds containing carbon and hydrogen atoms of fused benzene rings in linear, angular or cluster arrangements. These compounds have a large (negative) resonance energy, resulting in a thermodynamically stable structures. PAHs may also contain alkyl- and nitro-substituent groups. The complexity in these molecules can be generated by substituting the aromatic carbon atom with nitrogen, oxygen or sulfur, giving rise to heterocyclic PAHs.