Electronic Spectroscopy of Polycyclic Aromatic Hydrocarbons PAHs in the Gas Phase

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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 di?use
interstellar bands. However, obtaining optical
spectra of isolated molecules at low temperature
(as in di?use 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 B2uA1g electronic
char-acter, induced by vibrational modes of e2g
symmetry. The two strongest vibronic features lie
at 4335.2 and 4262.1 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.