am Dienstag, 23. Mai 2000, 16.00 – 18.00 Uhr
im Hörsaal A des Fachbereichs Physik der FU Berlin
Thma: CHARGE
EXCHANGE in ATOM-CLUSTER COLLISIONS
Abstract
Cluster
metallicity has been probed through collisional cluster-atom charge transfer
process. Absolute cross-section values have been measured in an extended range
of collisional velocity for reaction X+n + Y→ Xn+Y+
where Xn is a n-atom numbered cluster and Y an atomic target. Bare
lithium and sodium atom clusters have been used first in the n=1-50 size range,
colliding with cesium or potassium atomic vapour in the laboratory energy range
1 – 10 keV. In a second step, heterogeneous clusters X+n(X2O)p
and X+(XOD)p have been used as projectiles. In any case
non resonat velocity cross-section profiles are observed. The absolute values
of the maximum cross-section sm depend on the electronic structure of both the
projectile and the target. With bare clusters, sm and the corresponding velocity vm
decrease when n increases. Absolute values are in between those corresponding
to atom-atom and surface-atom charge exchange, showing the specific character
of clusters. With heterogeneous oxydized clusters, the behaviour of sm is
determined by the n/p ratio. The larger the ratio, the larger sm. When n/p >> 1, sm merges with the one obtained for the
corresponding bare cluster, exhibiting a non metal-metal evolution. With
hydroxide clusters sm increases as p increases in agreement with
ionic binding.