Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Mono-Deuterated Species

TL;DR

This study uses quantum chemistry to analyze the vibrational spectra of mono-deuterated PAHs, suggesting they could account for missing interstellar deuterium and providing insights into D-enrichment in the ISM.

Contribution

It presents the first quantum-chemical calculations of C-D vibrational modes in PAHs, linking spectral features to interstellar D abundance and deuteration levels.

Findings

C-D stretch modes shift to longer wavelengths by a factor of ~1.2.

Mean intrinsic strengths of C-H and C-D stretches are 13.4 and 7.4 km/mol.

Estimated D-deuteration fraction in PAHs is approximately 2.4%.

Abstract

Deuterium (D) is one of the light elements created in the big bang. As the Galaxy evolves, the D/H abundance in the interstellar medium (ISM) decreases from its primordial value due to "astration". However, the observed gas-phase D/H abundances of some interstellar sightlines are substantially lower than the expected reduction by astration alone. The missing D could have been depleted onto polycyclic aromatic hydrocarbon (PAH) molecules which are ubiquitous and abundant in interstellar regions. To quantitatively explore the hypothesis of PAHs as a possible reservoir of interstellar D, we compute quantum-chemically the infrared vibrational spectra of mono-deuterated PAHs (and their cations) of various sizes. We find that, as expected, when H in PAHs is replaced by D, the C-H stretching and bending modes at 3.3, 8.6 and 11.3 $\mu$m respectively shift to longer wavelengths at $\sim$4.4, 11.4 and 15.4 $\mu$m by a factor of $\sim$$\sqrt{13/7}$, the difference in reduced mass between the C-H and C-D oscillators. We derive from the computed spectra the mean intrinsic strengths of the 3.3 $\mu$m C-H stretch and 4.4 $\mu$m C--D stretch to be $\langle A_{3.3} \rangle \sim 13.4$ km/mol and $\langle A_{4.4} \rangle \sim 7.4$ km/mol for neutral deuterated PAHs which would dominate the interstellar 3.3 and 4.4 $\mu$m emission. By comparing the computationally-derived mean ratio of $\langle A_{4.4}/A_{3.3} \rangle \sim 0.56$ for neutral PAHs with the mean ratio of the observed intensities of $\langle (I_{4.4}/I_{3.3})_{\rm obs} \rangle \sim 0.019$, we estimate the degree of deuteration (i.e., the fraction of peripheral atoms attached to C in the form of D) to be ~2.4\%, corresponding to a D-enrichment of a factor of ~1200 with respect to the interstellar D/H abundance.