Pierre Labastie

A novel experimental method for the measurement of the caloric curves of clusters

A novel experimental scheme has been developed in order to measure the heat capacity of mass selected clusters. It is based on controlled sticking of atoms on clusters. This allows one to construct the caloric curve, thus determining the melting temperature and the latent heat of fusion in the case of first-order phase transitions. This method is model-free. It is transferable to many systems since the energy is brought to clusters through sticking collisions. As an example, it has been applied to Na(90) (+) and Na(140) (+). Our results are in good agreement with previous measurements.

Two-step melting of Na41+

The heat capacity of the mass selected Na(41) (+) cluster has been measured using a differential nanocalorimetry method. A two-peak structure appears in the heat capacity curve of Na(41) (+), whereas Schmidt and co-workers [M. Schmidt, J. Donges, Th. Hippler, and H. Haberland, Phys. Rev. Lett. 90, 103401 (2003)] observed, within their experimental accuracy, a smooth caloric curve. They concluded from the absence of any structure that there is a second order melting transition in Na(41) (+) with no particular feature such as premelting. The observed difference with the latter results is attributed to the better accuracy of our method owing to its differential character. The two structures in the heat capacity are ascribed to melting and premelting of Na(41) (+). The peak at lower temperature is likely due to an anti-Mackay to Mackay solid-solid transition.

Attachment cross-sections of protonated and deprotonated water clusters

Attachment cross-sections of water molecules onto size selected protonated (H(2)O)(n)H(+) and deprotonated (H(2)O)(n - 1)OH(-) water clusters have been measured in the size range n = 30-140 for 10 eV kinetic energy of the clusters in the laboratory frame. Within our experimental accuracy, the attachment cross-sections are found to have the same magnitude and size dependence for both species. It is shown that electrostatic interactions are likely to play a role even for the largest sizes investigated.

Heat capacities of mass selected deprotonated water clusters

Heat capacities of mass selected deprotonated water clusters (H(2)O)(n-1)OH(-) have been measured in the size range n = 48-118, as a function of temperature. We have found that they undergo a melting-like transition in the range 110-130 K. The transition temperature is size dependent with a strong correlation with the dissociation energy around the shell closure at n = 55.

Experimental nanocalorimetry of protonated and deprotonated water clusters

An experimental nanocalorimetric study of mass selected protonated (H2O)nH(+) and deprotonated (H2O)n-1OH(-) water clusters is reported in the size range n = 20-118. Water clusters heat capacities exhibit a change of slope at size dependent temperatures varying from 90 to 140 K, which is ascribed to phase or structural transition. For both anionic and cationic species, these transition temperatures strongly vary at small sizes, with higher amplitude for protonated than for deprotonated clusters, and change more smoothly above roughly n ≈ 35. There is a correlation between bonding energies and transition temperatures, which is split in two components for protonated clusters while only one component is observed for deprotonated clusters. These features are tentatively interpreted in terms of structural properties of water clusters.

Experimental determination of nucleation scaling law for small charged particles.

We investigated the nucleation process at the molecular level. Controlled sticking of individual atoms onto mass selected clusters over a wide mass range has been carried out for the first time. We measured the absolute unimolecular nucleation cross sections of cationic sodium clusters Na{n}{+} in the range n=25-200 at several collision energies. The widely used hard sphere approximation clearly fails for small sizes: not only should vapor-to-liquid nucleation theories be modified, but also, through the microreversibility principle, cluster decay rate statistical models.

Fragmentation cross sections of protonated water clusters

We have measured fragmentation cross sections of protonated water cluster cations (H(2)O)(n=30-50)H(+) by collision with water molecules. The clusters have well-defined sizes and internal energies. The collision energy has been varied from 0.5 to 300 eV. We also performed the same measurements on deuterated water clusters (D(2)O)(n=5-45)D(+) colliding with deuterated water molecules. The main fragmentation channel is shown to be a sequential thermal evaporation of single molecules following an initial transfer of relative kinetic energy into internal energy of the cluster. Unexpectedly, that initial transfer is very low on average, of the order of 1% of collision energy. We evaluate that for direct collisions (i.e., within the hard sphere radius), the probability for observing no fragmentation at all is more than 35%, independently of cluster size and collision energy, over our range of study. Such an effect is well known at higher energies, where it is attributed to electronic effects, but has been reported only in a theoretical study of the collision of helium atoms with sodium clusters in that energy range, where only vibrational excitation occurs.

Sticking and fragmentation cross-sections of water clusters

Absolute sticking and fragmentation cross-sections have been measured for the collision of mass selected water clusters with water molecules.