Laboratory photo-chemistry of pyrene clusters: an efficient way to form large PAHs

TL;DR

This study demonstrates that pyrene clusters can efficiently form large PAHs through photodissociation processes, offering a new bottom-up pathway for PAH formation in the interstellar medium, supported by experimental and quantum chemical analyses.

Contribution

The paper introduces a novel laboratory method showing pyrene clusters can convert into large PAHs under radiation, revealing an efficient formation route in space.

Findings

Pyrene clusters convert into larger PAHs under strong radiation.

Quantum calculations support the formation pathways.

Provides a new bottom-up formation mechanism for large PAHs.

Abstract

In this work, we study the photodissociation processes of small PAH clusters (e.g., pyrene clusters). The experiments are carried out using a quadrupole ion trap in combination with time-of-flight (QIT-TOF) mass spectrometry. The results show that pyrene clusters are converted into larger PAHs under the influence of a strong radiation field. Specifically, pyrene dimer cations (e.g., [C$_{16}$H$_{10}$$-$C$_{16}$H$_{9}$]$^+$ or C$_{32}$H$_{19}$$^+$), will photo-dehydrogenate and photo-isomerize to fully aromatic cations (PAHs) (e.g., C$_{32}$H$_{16}$$^+$) with laser irradiation. The structure of new formed PAHs and the dissociation energy for these reaction pathways are investigated with quantum chemical calculations. These studies provide a novel efficient evolution routes for the formation of large PAHs in the interstellar medium (ISM) in a bottom-up process that will counteract the top-down conversion of large PAHs into rings and chains, and provide a reservoir of large PAHs that can be converted into C$_{60}$ and other fullerenes and large carbon cages.