The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18

TL;DR

This paper reports the first optical spectrum of a large, isolated polycyclic aromatic hydrocarbon, hexa-peri-hexabenzocoronene, observed in conditions similar to interstellar space, providing insights into its abundance and potential role in interstellar phenomena.

Contribution

It presents the first optical spectrum of a large PAH under interstellar-like conditions, enabling testing of PAHs as carriers of diffuse interstellar bands.

Findings

Detected vibronic bands in the 4080-4530 Å range.

Estimated upper limit on interstellar abundance of the molecule.

Identified potential for PAHs to explain diffuse interstellar bands.

Abstract

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to have an oscillator strength of f=1.4x10^-3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12 cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.