More whiffs of the aromatic universe

Authors Alessandra Candian, Junfeng Zhen, and Xander Tielens (“The aromatic universe,”Physics Today, November 2018, page 38 ) give us a glimpse of the remarkable progress made by astrophysicists and astronomers in identifying carbon-based molecules found in the interstellar medium. That progress includes the identification of a subset of the diffuse interstellar band, one of the most enduring astrophysical riddles.

In addition to the emission of IR vibrational radiation and absorption across the visible spectrum into the UV, polycyclic aromatic hydrocarbon (PAH) molecules can also emit radiation from thermally populated excited electronic states. That radiation can have an important stabilizing effect on the molecules. The effect, predicted by Abraham Nitzan and Joshua Jortner in 1979, has been observed in several molecules, including fullerenes in which the radiation was seen to be enhanced two orders of magnitude or more relative to purely vibrational cooling. ¹

Also, the hydrogen-containing PAH molecule anthracene was identified as a radiator, ² and thermally emitted photons have been measured ³ from pure carbon clusters as small as ${\text{C}}_4^{-}$ and ${\text{C}}_6^{-}$. The large disparity in cooling rates for ${\text{C}}_4^{-}$ and ${\text{C}}_6{\text{H}}^{-}$ also illustrates the extreme variation caused by seemingly very small differences in molecular composition. ⁴ At excitation energies in which radiation competes with electron emission from those species, the photon-emission rate constants are 8 × 10⁴ s⁻¹ for ${\text{C}}_4^{-}$, for example, ⁵ and the energies removed (1.34 eV and 2.71 eV for the two allowed transitions) are an order of magnitude higher than vibrational quanta. Such extreme molecule-specific radiative cooling translates into strongly varying molecular survival probabilities after photoexcitation and may have significant implications for the populations of fullerenes, PAH molecules, and other molecules in interstellar space.