Growth of gas phase nanoparticles with an accretion mechanism

Nano-droplet growth in a supersaturated vapor has been investigated in a gas aggregation source using laser-ionization time-of-flight mass spectrometry. During its propagation into an atomic vapor, a small particle grows by sticking atoms on its surface. This accretion process has been highlighted through the clustering of homogeneous particles Mn and heterogeneous Mn(M2O) and Mn(MOH)2 particles in a metallic vapor and a helium buffer gas (M = Na or K). A modelization is introduced so as to connect the measured cluster mass distributions to the pertinent physical parameters. The mass distribution width is particularly sensitive to the efficiency of the first steps in the growth sequence. We used this property to compare the ability of this vapor-condensed matter phase transition to occur around various homogeneous and heterogeneous nucleation seeds.

Homogeneous and heterogeneous clustering in the accretion regime

Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of NanX clusters. Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.

Doubly charged clusters - neutral atom charge transfer: the role of the Coulombic repulsion

We have studied experimentally the collisional charge transfer between a neutral atom and a multicharged metal-atom cluster. The charge transfer cross section measured for Na$_{31}^{++} +$ Cs is in the range of 400 Å2. The time-of-flight mass analysis of the singly charged collision products demonstrates that an energy of about 0.5 eV is deposited in the cluster fragment during the charge transfer collision. This effect can be interpreted as a charge transfer to an excited state of the metal cluster. The measured cross section for Na$_{31}^{++} +$ Cs is larger than the one for Na$_{31}^{+} +$ Cs collisions. This difference between these two systems is due to the existence, for the first one, of a Coulombic repulsion term in the collision output channel

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M$_n^+$ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1-10 keV. For each cluster size, the experimental values of the charge transfer cross-section $\sigma(v)$ are fitted with an universal parametric curve with two independent parameters $\sigma_m$ and vm, the maximum cross-section and the corresponding velocity. For small size clusters ($n \leq 15$), the $\sigma (v)$ characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the $\sigma_m$ and vm parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures

Ionization vs.vs. neutralization in alkali-atom clusters

Alkali clusters are considered as prototypes of metallic clusters since their electrons are delocalized from the small sizes. The two techniques applied here i.e. photoionization and its reverse neutralization by charge exchange probe the delocalization of their valence electrons. These methods extending over other elements should produce a pertinent analyse of the binding evolution with cluster size.Les agrégats d'alcalins, prototypes des agrégats métalliques, s'apparentent dès les plus petites tailles à des systèmes à électrons délocalisés. Les deux techniques discutées ici, la photoionisation et sa réciproque la neutralisation par échange de charge, sondent le caractère délocalisé des électrons de valence. L'extension de ces méthodes à d'autres éléments devrait permettre une analyse pertinente de leur liaison en fonction de leur taille

Charge Transfer and Dissociation in Collisions of Metal Clusters with Atoms

We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li- 31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer