The adsorption of helium atoms on coronene cations

TL;DR

This study experimentally investigates how helium atoms attach to coronene cations, revealing stable configurations and magic numbers that align with theoretical predictions, which could impact understanding of interstellar chemistry.

Contribution

First experimental observation of helium atom attachment to coronene cations, identifying stable 'magic number' complexes and confirming theoretical models.

Findings

Helium forms a stable layer of 38 atoms on coronene cations.

Magic numbers at 38, 44 helium atoms indicate enhanced stability.

Results align with path integral Monte Carlo predictions.

Abstract

We report the first experimental study of the attachment of multiple foreign atoms to a cationic polycyclic aromatic hydrocarbon (PAH). The chosen PAH was coronene, C$_{24}$H$_{12}$, which was added to liquid helium nanodroplets and then subjected to electron bombardment. Using mass spectrometry, coronene cations decorated with helium atoms were clearly seen and the spectrum shows peaks with anomalously high intensities ("magic number" peaks), which represent ion-helium complexes with added stability. The data suggest formation of a rigid helium layer consisting of 38 helium atoms that completely covers both faces of the coronene ion. Additional magic numbers can be seen for the further addition of 3 and 6 helium atoms, which are thought to attach to the edge of the coronene. The observation of magic numbers for the addition of 38 and 44 helium atoms is in good agreement with a recent path integral Monte Carlo prediction for helium atoms on neutral coronene. An understanding of how atoms and molecules attach to PAH ions is important for a number of reasons including the potential role such complexes might play in the chemistry of the interstellar medium.