Concerns regarding recurrent fluorescence’s impact on smaller diffuse ISM aromatics

Abstract

Abstract Recent research implied that recurrent fluorescence (RF) could bolster smaller aromatics against fragmentation in the diffuse ISM, yet that hypothesis is contested by timescales for unrelenting dissociative recombination (electrons), and unceasing dissociating photons. Specifically, neutral cyanonaphthalene can sustain 13.6 eV photoionization, and the ensuing cation’s excess energy is channeled through intramolecular vibrational redistribution (IVR), with RF providing the radiative stabilization pathway (≃ 7 eV negligible survival limit). However, that cation endures dissociative recombination every $\tau \approx 0.5^{+5.6}_{-0.4}$ years, which deposits ≃ 8.6 eV and exceeds the limit. Moreover, that is paired with 7 − 13.6 eV photodestruction each $\tau \approx 16^{+223}_{-14}$ years (total visual dust extinction AV = 0 − 1, and $\tau \approx 4^{+7}_{-2}$ years for AV ≈ 0). Recent laboratory characterizations of RF were seminal, but the mechanism may not overturn models indicating a gap in smaller NC ≈ 7 − 11 diffuse ISM aromatics, nor support those molecules as viable diffuse interstellar band carriers.