8 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. 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 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 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 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 FIG. 9: Time evolution of z coordinates during collision of Na$_{6}$ with initial total kinetic energy E 0 kin = 8 . 16 eV (1.36 eV per Na atom) on an Ar(001) surface. Data taken from [47]. FIG. 8: Dipole polarization of the cluster electrons during collision of Na$_{6}$ on MgO(001). Upper panels: Dipole moments of the electron cloud of the Na$_{6}$ cluster in x - y - (horizontal) and z -direction as function of time. Lower panels: Time- evolution of the z componente of the center-of-mass of the Na$_{6}$ cluster; the line z = 0 corresponds to the center of mass of a fully relaxed configuration of Na$_{6}$ on MgO(001). Left panels: case of soft deposition at rather low initial kinetic energy (as indicated). Right panels: case of more energetic collision which leads to immediate reflection. FIG. 7: The trajectories of the Na$_{6}$ center of mass, projected on a plane parallel to the surface. The lateral drift sensitively depends on the site above which the cluster impinges (hollow, O or Mg site) and on its orientation (top ion above – up – and top ion below – down – the ring). C. MgO versus Ar Substrate -15 -10 -5  0  5  10  15 -15 -10 -5  0  5  10  15 y [a 0 ] x [a$_{0}$] top-down hollow top-up O top-up Mg -2  0  2  4  6  8  0  1  2  3 ionic c.m. in z (a 0 ) time (ps) -0.2  0  0.2  0.4  0.6  0.8  1 electronic dipole (a 0 ) E$_{kin,0}$=0.16 eV z y x  0  0.5 -2  0  2  4  6  8 time (ps) -0.2  0  0.2  0.4  0.6  0.8  1 E$_{kin,0}$=8.16 eV -30 -20 -10  0  10  20  30  40  50  0  0.5  1  1.5  2  2.5  3 ionic z-coordinate [a 0 ] time [ps] Na$_{6}$ at Ar, E $^{0}$$_{kin}$=8.16 eV