An optical spectrum of a large isolated gas-phase PAH cation: C78H26+

TL;DR

This study presents the first gas-phase optical spectrum of a large PAH cation, C78H26+, revealing key absorption features and structural insights relevant to interstellar medium chemistry and diffuse interstellar bands.

Contribution

It provides experimental and theoretical spectra of a large PAH cation, demonstrating its stability, nonplanar geometry, and potential role in DIBs identification.

Findings

Main absorption peaks at 431 nm and 516 nm match theoretical predictions.

The PAH cation has a nonplanar C2 symmetry, unlike smaller PAHs.

Structural features may influence interstellar chemistry and DIBs.

Abstract

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26 +- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26 + fragments and ultimately theformation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.