More whiffs of the aromatic universe

Candian, Zhen, and Tielens reply: Klavs Hansen and Piero Ferrari insightfully point out the importance of electronic fluorescence from thermally populated electronic states. Direct electronic fluorescence is a well-known process that occurs when excitation is temporarily trapped in, for example, the S₁ state during the internal conversion cascade whenever relaxation to the ground state is hampered by a large energy gap. Because internal conversion is a reversible process, an excited electronic state can be revisited and electronic fluorescence can also compete with vibrational fluorescence later on during the relaxation process. Delayed electronic fluorescence emission was measured in the 1960s for small polycyclic aromatic hydrocarbons (PAHs), including coronene ¹ (C₂₄H₁₂). As Hansen and Ferrari point out, it was also measured more recently in elegant thermo-ionic studies on small carbon-chain and fullerene anions.

Delayed fluorescence is controlled by the energy gap, which sets the fractional population of electronic states, and by competition between radiative and nonradiative processes, which depopulates electronic states. Despite the potential importance of delayed fluorescence, systematic studies of it are lacking, and it has received scant attention in astronomy. As Hansen and Ferrari emphasize, molecular stability in the interstellar medium may be closely tied to delayed fluorescence. Electronic fluorescence may also provide a means to characterize and identify specific molecules present in space. The narrow bands in the visible spectrum of the stellar outflow, the Red Rectangle, are generally ascribed to electronic fluorescence of large molecules, illustrating that further study of the processes involved might be beneficial.

Alan Tokunaga and Roger Knacke return to arguments first raised against the interstellar PAH hypothesis in the late 1980s. Studies driven by those arguments greatly elucidated the characteristics of the aromatic infrared band (AIB) carriers and helped to solidify the presence and importance of PAHs in space. ² The AIBs are carried by 50 carbon-atom PAH species. Some very weak AIBs may be due to functional groups—for example, methyl or quinone groups—attached to an aromatic skeleton, but their fractional coverage is small compared with aromatic H. Although some of the debate may seem semantic, we emphasize that 50 C-atom species behave, emit, and evolve like molecules. A treatment based on solid-state physics is sometimes convenient but obfuscates the underlying molecular physics. Three issues raised by Tokunaga and Knacke—the AIB profile, the underlying continuum, and the dearth of electronic absorption features—are a result of thinking about the PAHs through a solid-state physics lens rather than from a molecular perspective.

Anharmonic behavior is a key aspect of vibrational spectroscopy. Recently, anharmonic density functional theory calculations of moderately sized PAHs have come within reach, and resulting spectra agree well with laboratory experiments. ^{3 , 4} Calculations following the energy cascade of highly excited PAHs are in good agreement with AIB positions and provide a natural explanation for the observed, red-shaded AIB profiles. ⁵ Anharmonic interactions may also lead to a vibrational quasi continuum. ⁶ Alternatively, the delayed electronic fluorescence process pointed out by Hansen and Ferrari may result in a near-IR continuum.

Finally, the rapid (10–100 fs) nonradiative decay channels provided by conical intersections of highly excited electronic states ⁷ broaden UV absorption bands, and astronomical instruments are not well suited to detect resulting weak and broad features. The nano-grain approach misses those molecular-physics aspects and cannot explain the observations.

After the discovery of the first diatomic molecules some 100 years ago, astrophysicist Arthur Eddington lamented that “atoms are physics, but molecules are chemistry.” Ever since, astrophysicists have regretted that sometimes simple physical formulas have to give way to complex chemical solutions in a molecular universe. To us, though, interstellar molecules provide a tool to probe macroscopic aspects of the universe, whereas the harsh environment of space offers unique insight in microscopic processes controlling excitation and relaxation of isolated molecules.