1. Electronic Spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs) in the Gas Phase
Thomas D. Varberg, Department of Chemistry, Macalester College, St. Paul, MN 55105
Organic material such as polycyclic aromatic hydrocarbons (PAHs) is held responsible for infrared emission features in carbon rich astronomical objects and has been suggested to account for as much as 20% of interstellar carbon. In addition, PAHs are considered to be leading candidates as carriers of the diffuse interstellar bands. However, obtaining optical spectra of isolated molecules at low temperature (as in diffuse clouds) remains a challenge of modern laboratory astrophysics. In collaboration with Dr. Tim Schmidt and Prof. Scott Kable of the School of Chemistry, University of Sydney, Australia, we measured the electronic spectrum of the polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC) by two-color resonant-enhanced multi-photon ionization spectroscopy. This is the largest neutral gas-phase PAH spectrum measured to date.
Quantum mechanical considerations reveal that the highest occupied and lowest unoccupied molecular orbitals of HBC are the same as those of benzene, which has the same symmetry (D6h point group). As such, the symmetries of the lowest transitions of HBC are the same as benzene. The lowest energy transition is thus of B2u–A1g electronic char-acter, induced by vibrational modes of e2g symmetry. The two strongest vibronic features lie at and Å. While these features do not match any reported diffuse interstellar bands, this work demonstrates the feasibility of getting large PAHs into the gas phase by laser ablation, with sufficient densities to allow the measurement of their electronic spectra by photo-ionization.