Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 {\AA} UV bump

TL;DR

This study investigates the UV absorption spectra of medium-sized polycyclic aromatic hydrocarbons (PAHs) to understand their potential role as carriers of the interstellar 2175 Å UV bump, combining experimental and computational approaches.

Contribution

It provides new experimental spectra of medium-sized PAHs and suggests larger PAHs may be responsible for the interstellar UV bump, advancing understanding of interstellar dust composition.

Findings

Large PAHs (~50-60 C atoms) could cause the 2175 Å UV bump.

UV spectra of medium-sized PAHs were measured at low and room temperatures.

Mixtures of PAHs with different sizes were produced and analyzed.

Abstract

Mixtures of polycylic aromatic hydrocarbons (PAHs) have been produced by means of laser pyrolysis. The main fraction of the extracted PAHs were primarily medium-sized, up to a maximum size of 38 carbon atoms per molecule. The use of different extraction solvents and subsequent chromatographic fractionation provided mixtures of different size distributions. UV-VIS absorption spectra have been measured at low temperature by matrix isolation spectroscopy and at room temperature with PAHs as film-like deposits on transparent substrates. In accordance with semi-empirical calculations, our findings suggest that large PAHs with sizes around 50 to 60 carbon atoms per molecule could be responsible for the interstellar UV bump at 217.5 nm.