The Influence of Aliphatic Components on the Aromatic Emission Characteristics of Polycyclic Aromatic Hydrocarbons

TL;DR

This study examines how aliphatic side chains on PAHs influence their infrared emission features, revealing that molecular structure can affect diagnostic ratios used in astrophysics.

Contribution

It provides a systematic analysis of aliphatic effects on PAH emission properties using theoretical data, highlighting potential misinterpretations in astrophysical diagnostics.

Findings

Aliphatic functionalization modifies key emission band ratios.

Small PAHs show significant shifts in emission ratios, larger PAHs less affected.

Classic diagnostic grids remain valid but may require calibration for aliphatic-rich PAHs.

Abstract

Intensity ratios of aromatic emission features are widely used to diagnose the size and ionization state of polycyclic aromatic hydrocarbons (PAHs) in astronomical environments. However, PAHs are known to typically carry aliphatic side chains, a structural feature that may compromise the reliability of traditional diagnostic methods. This study systematically investigates the effects of aliphatic components on the aromatic emission properties of PAHs. Based on theoretical data from the NASA Ames PAH IR Spectroscopic Database, we compare the emission behavior of purely aromatic PAHs with those containing aliphatic substituents, revealing that aliphatic functionalization may modify the intensity ratio of the 11.2 $\mu$m band relative to the 7.7 $\mu$m and 3.3 $\mu$m bands. This potentially leads to misidentification of their ionization state if molecular structural effects are neglected. Further analysis indicates that the impact of aliphatic components on diagnostic band ratios strongly depends on PAH size: small PAHs exhibit significant emission ratio shifts, deviating from traditional size/ionization trends, while larger PAHs are minimally affected. Despite these shifts, the classic $(I_{11.2/7.7})$ versus $(I_{11.2/3.3})$ diagnostic grid remains largely applicable to mixed aromatic-aliphatic PAHs, although some systematic calibration may be needed. Our findings emphasize the necessity for caution when interpreting PAH band ratios in aliphatic-rich environments, as variations in PAH molecular composition may distort inferences about physical conditions.