Dans le sillage de la physico-chimie sous rayonnements : vers l’institut des sciences moléculaires d’Orsay

Cet article évoque le cheminement qui a conduit une partie de la communauté scientifique d’Orsay, dès 1960, de la chimie sous rayonnements à la physique des collisions atomiques et moléculaires et à la photophysique, pour aboutir à la création, en 2010, de l’institut des sciences moléculaires d’Orsay, aujourd’hui organisé selon trois grands pôles : la physique moléculaire, les nanosciences, la physique pour la biologie. Si la personnalité des pionniers a joué un rôle essentiel dans l’établissement des concepts, les avancées techniques et les découvertes fortuites ont eu leur importance.This article evokes the pathway which, since 1960, has led a part of the scientific community at Orsay, from radiation chemistry to atomic and molecular collision physics and photophysics, resulting, by intensifying exchanges, in the creation of the Institute for Molecular Sciences at Orsay, presently organized along three main centers of expertise: molecular physics, nanosciences, physics for biology. If the pioneers’ personal qualities have played a major role in the establishment of concepts, technical advances and fortuitous discoveries have made important contributions

Fully versus constrained statistical fragmentation of carbon clusters and their heteronuclear derivatives

International audienceThe Microcanonical Metropolis Monte Carlo (MMMC) method has been shown to describe reasonably well fragmentation of clusters composed of identical atomic species. However, this is not so clear in the case of heteronuclear clusters as some regions of phase space might be inaccessible due to the different mobility of the different atomic species, the existence of large isomerization barriers, or the quite different chemical nature of the possible intermediate species. In this paper, we introduce a constrained statistical model that extends the range of applicability of the MMMC method to such mixed clusters. The method is applied to describe fragmentation of isolated clusters with high, moderate, and no heteronuclear character, namely, CnHm, CnN, and Cn clusters for which experimental fragmentation branching ratios are available in the literature. We show that the constrained statistical model describes fairly well fragmentation of CnHm clusters in contrast with the poor description provided by the fully statistical model. The latter model, however, works pretty well for both Cn and CnN clusters, thus showing that the ultimate reason for this discrepancy is the inability of the MMMC method to selectively explore the whole phase space. This conclusion has driven us to predict the fragmentation patterns of the C4N cluster for which experiments are not yet availabl

Charge and energy sharing in the fragmentation of astrophysically relevant carbon clusters

International audienceThe breakup of a molecule following a fast collision with an atom in gas phase can be understood as resulting from two steps. In the first step, the atom transfers energy to the molecule, which is thus electronically and vibrationally excited. In the second step, the molecule decays leading to different fragments, while the initial charge, energy, and angular momentum are conserved. Here, we demonstrate that, by maximizing the entropy of the system under these conservation laws, it is possible to reproduce the fragmentation yields resulting from collision experiments. In particular, our model is applied to investigate fragmentation of excited neutral and singly charged carbon clusters and mono-hydrogenated carbon clusters. These species are commonly exposed to energetic ionizing radiation in the interstellar medium, so understanding the key aspects of their fragmentation, in particular the way energy and charge are shared in the process, can be relevant to get a deeper insight on the evolution of carbonaceous species in the universe

Source, piège à ions, laser, détection en coïncidence ; nouvel outil expérimental

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Ion pair dissociation of highly excited carbon clusters and carbon-based molecules

info:eu-repo/semantics/nonPublishe

Ion-pair dissociation of highly excited carbon clusters, size and charge effects

info:eu-repo/semantics/nonPublishe

Ion pair dissociation of highly excited carbon clusters

info:eu-repo/semantics/nonPublishe

Ion-pair dissociation of highly excited carbon clusters: Size and charge effects

International audienceWe present measurements of ion-pair dissociation (IPD) of highly excited neutral and ionized carbon clusters Cn=2 -5 (q =0 -3 )+. The tool for producing these species was a high-velocity collision between Cn+ projectiles (v =2.25 a.u.) and helium atoms. The setup allowed us to detect in coincidence anionic and cationic fragments, event by event, leading to a direct and unambiguous identification of the IPD process. Compared with dissociation without anion emission, we found typical 10-4 IPD rates, not depending much on the size and charge of the (n ,q ) species. Exceptions were observed for C2+ and, to a lesser extent, C43 + whose IPDs were notably lower. We tentatively interpret IPDs of C2+ and C3+ by using a statistical approach based on the counting of final states allowed by energetic criteria. The model is able to furnish the right order of magnitude for the experimental IPD rates and to provide a qualitative explanation of the lower IPD rate observed in C2+