Benchmarking Astrochemistry Paradigms: Relative Absence of $\rm{C_6H_5CN^+}$ in the Diffuse ISM

Abstract

The detectability of $\rm{C_6H_5CN^+}$ (benzonitrile cation) in the diffuse ISM is re-evaluated. A holistic evidentiary framework suggests $\rm{C_6H_5CN^+}$ is relatively absent in the diffuse ISM owing to the following concurrently: a marginal intramolecular vibrational energy redistribution (IVR) favoring fragmentation, recurrent fluorescence being an improbable mechanism in this case to prevent dissociation, mismatches between observed DIBs and experimental results, and the hitherto absence of DIBs matching any similarly sized cations. The putative gap in bottom-up synthesis is reaffirmed (diffuse ISM), and although DIB sources are largely unknown, within a broader approach the lines can help benchmark astrochemistry paradigms. The results relied on new advantageous Daly et al. experimental spectra, an expanded observational DIB analysis (APO catalog), and complementary $ω$B97X-D/cc-pVTZ computations.

Figures (1)

• Figure 1: Mismatches exist between observed DIBs (the expansive APO catalog), and $\omega$B97X-D/cc-pVTZ and experimental spectra. Experimental $\rm{C_6H_5CN^+}-{\rm{He}}$ data (blue line, merely broadly illustrative) were inferred from a diagram in da24. APO DIBs shown exhibit $r\gtrsim0.8$ relative to 5600.72 Å (EW pairs, $n\ge8$), and $|\Delta r| \lesssim 0.3$ between EW$-$E(B-V). Ultimately, solely two DIBs perhaps align with no less than ten experimental lines, which when paired with benzonitrile cation's IVR al96mo13 and lack of recurrent fluorescence (§ \ref{['sec-RF']}), suggest in tandem with other evidence that the molecule is comparatively absent from the diffuse ISM.