# Ionization and fragmentation of cold clusters of PAH molecules: collisions with keV ions

A. I. S. Holm, H Zettergren, H. A. B. Johansson, F Seitz, S Rosén, H T

Schmidt, A Ławicki, Jimmy Rangama, Patrick Rousseau, M Capron, et al.

► To cite this version:

A. I. S. Holm, H Zettergren, H. A. B. Johansson, F Seitz, S Rosén, et al.. Ionization and fragmentation of cold clusters of PAH molecules: collisions with keV ions. XXVII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2011), Jul 2011, Belfast, United Kingdom. pp.102060, 10.1088/1742-6596/388/10/102060 . hal-01084771

HAL Id: hal-01084771

https://hal.science/hal-01084771v1

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# Ionization and fragmentation of cold clusters of PAH molecules: collisions with keV ions

To cite this article: A I S Holm *et al* 2012 *J. Phys.: Conf. Ser.* **388** 102060

View the [article online](#) for updates and enhancements.

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# Ionization and fragmentation of cold clusters of PAH molecules: collisions with keV ions

A. I. S. Holm*, H. Zettergren*<fnref n="1" />, H. A. B. Johansson*, F. Seitz*, S. Rosén*, H. T. Schmidt*, A. Ławicki, J. Rangama, P. Rousseau, M. Capron, R. Maisonny, L. Adoui, A. Méry, J.-Y. Chesnel, B. Manil†, B. A. Huber, H. Cederquist*

Université de Caen Basse-Normandie, CNRS, ENSICAEN, CEA, CIMAP, UMR 6252, F-14070 Caen 5, France

*Department of Physics, Stockholm University, S-106 91 Stockholm, Sweden

†Université Paris 13, Phys Lasers Lab, Institut Galilée, CNRS, LPL, UMR 7538, F-93430 Villetaneuse, France

**Synopsis** We report on collisions between atomic ions and pure, loosely bound, clusters of Polycyclic Aromatic Hydrocarbon (PAH) molecules. We find that charge and excitation energy is distributed on the cluster constituents before break-up which mostly leads to emissions of intact, excited, singly charged monomers. Surprisingly, collisions with highly charged ions lead to hotter monomers than collisions with ions in low charge states. Small PAH clusters fragment promptly when singly ionized while clusters of more than 13 or 5 molecules may remain intact for anthracene or coronene clusters, respectively.

We present results on collisions between atomic ions (keV energies and low and high charge states) and pure, internally cold, clusters of the Polycyclic Aromatic Hydrocarbon (PAH) molecules anthracene $C_{14}H_{10}$ [1], pyrene $C_{16}H_{10}$, fluoranthene $C_{16}H_{10}$, triphenylene $C_{18}H_{12}$, and coronene $C_{24}H_{12}$. These clusters are loosely bound and as the binding energy is roughly 0.1 eV per carbon atom ring, the binding energy for e.g. the anthracene dimer is about 0.3 eV (anthracene has three hexagonal rings). For all clusters we find that charge and excitation energy, which initially are induced locally in the clusters close to the ion trajectories, is distributed on the individual PAH molecules before the clusters break apart and that the most probable fragment then always is a singly charged, initially intact, PAH monomer. Anthracene clusters containing up to about 13 molecules are found to fragment promptly even when only singly ionized by $He^+$ or $Xe^{20+}$ ions while larger such clusters may remain intact on microsecond timescales. Clusters of larger PAH molecules (coronene $C_{24}H_{12}$ c.f. Fig. 1) may be singly ionized without fragmentation in collisions with $He^{2+}$ already when they contain six molecules as we measure kinetic energies of the corresponding cluster ions to be very low ($\le 1$ meV). For anthracene clusters, we surprisingly find that collisions with highly charged ions lead to emission of much hotter singly charged anthracene monomers than collisions with ions in low charge states. Highly charged ions are known to remove electrons from targets at much larger distances than ions in lower charge states and would then be expected to give less heating. However, as charge is distributed before fragmenta-

tion a fairly large fraction of the observed, emitted, singly charged monomers come from multiply charged clusters produced in somewhat closer collisions than those in which only one electron is removed. Furthermore, electrons may not only be removed from the outermost shells of the individual PAHs but also from non-valence shells yielding excitation energies of several eV. Finally, the Coulomb-driven fragmentation of multiply charged clusters may induce vibrations in the monomers [2]. Ion-induced molecular fusion processes within PAH clusters and possible magic PAH cluster sizes will be discussed.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Mass-to-charge spectra of coronene clusters, $[C_{24}H_{12}]_k$, in collision with $He^{2+}$. Labels denote the size of the measured cluster. The insets are zoom-ins of the *j/q* region from 4-25, where *j* is the cluster size after the reaction.</figcaption></figure>

## References

[1] A. I. S. Holm et al 2010 *Phys. Rev. Lett.* **105** 213401

[2] H. Zettergren et al 2007 *Phys. Rev. A* **75** 051201

<footnote n="1">¹E-mail: henning@fysik.su.se</footnote>