Université Paul Sabatier - Bat. 3R4 - 118 route de Narbonne 31062 Toulouse Cedex 09, France

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The Cluster Group is currently studying nucleation and fragmentation processes
at a molecular level. Our purpose is to learn how a polyatomic system grows by successive attachment of atomic (or molecular) subunits (unimolecular nucleation).
Our original experimental setup allows us to perform collisions under very well controlled conditions (cluster temperature, collision energy, cluster size) between atoms or molecules and size selected clusters.

Three different kind of experiment can be achieved :

1. at very low collision energy (few 1/10 eV) absolute attachment cross-sections of an atom or a molecule onto a size selected cluster are measured.

2. for moderate collision energies (few tens eV) collision induced fragmentation is observed.

3. by varying the initial temperature of the clusters, it is possible to measure caloric curves (heat capacity as a function of temperature) of mass selected clusters or to study their thermal stability.

The three types of experiments above mentioned are currently used to study different properties of water clusters :

1. Attachment experiments :

The attachment experiments of water molecules onto water clusters revealed an unexpected behaviour of the attachment cross-section : they are found to be lower than expected due to a dynamical effect that prevents from an efficient absorption of collision energy by the cluster when the collision duration is too short [“Sticking properties of water clusters”, S. Zamith, P. Feiden, P. Labastie, and J-M L’Hermite, Phys. Rev. Lett. 104, 103401 (2010)].

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Figure 1. Attachment cross-section of water molecule onto protonated water clusters as a function of size for different collision energies. (In the figure the energies are the one in the lab frame. the collision energy is approximately this value divided by the size). The attachment cross-section is always lower thant the geometric cross-section, represented here by the dashed line. The full line are the results of a dynamical model [S. Zamith et al, PRL,2010].

2. Collision induced dissociation :

Polycyclic aromatic hydrocarbon (PAH) clusters are believed to be present in the interstellar medium and to be a potential source of complex molecules. In the laboratory, we measured the energies of dissociation of pyrene clusters (C16H10) by collision-induced dissociation experiments. In these experiments, mass selected pyrene clusters collide with rare gas atoms. At low collision energy, we do not observe any fragmentation. It is only from a certain collision energy that fragmentation is observable. We thus measure the fragmentation cross section as a function of the collision energy. This curve has a threshold of onset that can be related to the energy of dissociation. The figure below shows such an example of measurements. These results were the subject of a publication :["Threshold collision induced dissociation of pyrene cluster cations", S. Zamith, J.-M. L’Hermite, L. Dontot, L. Zheng, M. Rapacioli, F. Spiegelman and C. Joblin, J. Chem. Phys. 153, 054311 (2020)]

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Fragmentation cross-section as a function of the collision energy obtained for a cluster of 4 pyrene molecules colliding with argon.

3. Heat capacities :

The heat capacities of protonated and deprotonated water clusters have been measured, thanks to a novel technique developed in our group [“A novel experimental method for the measurement of the caloric curves of clusters”, F. Chirot, P. Feiden, S. Zamith, P. Labastie, and J-M L’Hermite, J. Chem. Phys. 129, 164514 (2008)].

The existence of a phase transition (solid-liquid or structural solid-solid transition) has been revealed for deprotonated water clusters, with a size dependence of the transition temperature. The transition temperatures are correlated with the cohesive energies [“Heat capacities of mass selected deprotonated water clusters”, S. Zamith, P. Labastie, and J-M L’Hermite, J. Chem. Phys. 138, 034304 (2013)]. In particular for the magic number at n=55, the temperature of transition is higher than for the neighboring sizes.

For protonated water clusters, the transition temperature is also correlated with the clusters cohesive energy. Furthemore our results indicate that around n=35 there is a global structural transition, probably from clathrate type structure to amorphous structures.

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Caloric curves of protonated wtaer clusters for different sizes.