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<img src='content_image/440088.jpg'>

repulsive sites (Mg, hollow) while only moderate lateral drift appears for impact on the attractive O site. 

One can learn more about the electronic charge distri- bution in the cluster during a collision by a direct multi- pole analysis of this distribution. We discuss here briefly the lowest non-trivial moments, the dipoles. Figure 8 

<img src='content_image/440089.jpg'>

shows the time evolution of dipole polarization for two different scenarios, deposition vs. reflection. The figure is augmented by the time-evolution of the center-of-mass as global indicator of the dynamical situation (lower pan- els). The two scenarios differ by the initial kinetic ener- gies, the lower value related to a more or less soft depo- sition, while the higher initial velocity leads to immedi- ate reflection of the cluster, as discussed above. In the 

slow deposition process (left panels), there is a strong in- crease of the z -polarization at the time of closest impact. This polarization remains during the further evolution at about 10% of the Wigner-Seitz radius, hence repre- sents a considerable internal polarization. Also, some x - y polarization builds up during the ongoing deposition oscillations. In contrast, in the case of a reflection (right panels), one sees a large instantaneous polarization at the time of closest approach, but only a very small remain- ing effect when the cluster has departed from the surface. The very short interaction time limits the internal exci- tation. 

In previous works [33, 47], the deposition of Na$_{6}$ on a cold, condensed argon substrate Ar(001) was investi- gated. Like MgO, solid Ar has a large band gap. But apart from this insulating nature, the two materials have much different properties. The attractive interaction be- tween MgO and Na is much stronger than between Ar and Na, due to the larger polarizability of the oxygen ion. On the other hand, frozen Ar material is a very soft solid due to the weak Ar-Ar binding. The melting point of Ar is 83 . 78 K [59], much lower than that of MgO around 3073 K [60]. The softness of the Ar material thus changes the energy balance to the extent that the Ar substrate takes up most of the impinging energy, leaving rather little internal excitation for the cluster itself. Thus Ar substrates are very efficient soft stopper materials. In the former analysis [33, 47], we had run a similar series of impact energies as above and we also found soft deposi- tion for energies up to at least E 0 $_{kin}$/N$_{ion}$ = 0 . 272 eV. An attempt to reach a reflection regime by further increasing the impact energy then led to a significant destruction of the substrate. Figure 9 illustrates that violent collision of 

<img src='content_image/440090.jpg'>

Na$_{6}$ on an Ar(001) surface. The cluster is indeed finally reflected, but the process evolves much different from the case of the collision with MgO shown in figure 6. The 

## C. MgO versus Ar Substrate 

