We report threshold collision induced dissociation experiments on protonated water clusters thermalized at low temperature for sizes n = 19–23. Fragmentation cross sections are recorded as a function of the collision energy and analyzed with a statistical model. This model allows us to account for dissociation cascades and provides values for the dissociation energies of each cluster. These values, averaging around 0.47 eV, are in good agreement with theoretical predictions at various levels of theory. Furthermore, the dissociation energies show a trend for the n = 21 magic and n = 22 anti-magic numbers relative to their neighbours, which is also in agreement with theory. These results provide further evidence to resolve the disagreement between previously published experimental values. A careful quantitative treatment of cascade dissociation in this model introduces interdependence between the dissociation energies of neighboring sizes, which reduces the number of free fitting parameters and improves both reliability and uncertainties on absolute dissociation energies deduced from experiments.

Continuer la lecture Partager

Entre 1849 et 1851, Hippolyte Fizeau a réalisé successivement trois expériences fondamentales relatives à la vitesse de la lumière. Ce sont toutes trois de véritables tours de force expérimentaux qui ont eu des conséquences majeures dans l’évolution de nos conceptions de la lumière et, comme l’a attesté Albert Einstein lui-même, dans l’avènement de la relativité restreinte. Nous présentons à la fois le contexte et les conséquences historiques ainsi que la réplique moderne que nous avons réalisée de l’expérience de 1851 dite « d’entraînement partiel de l’éther ».

Continuer la lecture Partager

PIRENEA and PIRENEA 2 are experimental setups dedicated to the study of fundamental molecular processes involving species of astrochemical interest. The coupling of a VUV source to PIRENEA has allowed us to study the fragmentation pathways and stability of polycyclic aromatic hydrocarbons (PAHs) containing aliphatic bonds under conditions relevant for astrophysical photodissociation regions. PIRE-NEA 2 will open the possibility to extend these studies to larger systems such as PAH clusters, and more generally to study gas-nanograin-photon interactions.

Continuer la lecture Partager

We report measurements of the attachment rates of water molecules onto massselected cationic pyrene clusters for size from n=4 to 13 pyrene units and for different collision energies. Comparison of the attachment rates with the collision rates measured in collision induced dissociation experiments provides access to the values of the sticking coefficient. The strong dependence of the attachment rates on size and collision energy is rationalized through a model in which we use a Langevin-type collision rate and adjust on experimental data the statistical dissociation rate of the water molecule from the cluster after attachment. This allows us to extrapolate our results to the conditions of isolation and long time scales encountered in astrophysical environments.

Continuer la lecture Partager