Cluster impact energy

For smaller clusters the primary route for dissociation is the heterogeneous dissociation of the molecule. Just as for isolated collisions in the gas-phase, the probability of dissociation of a diatomic molecule upon impact at the surface is enhanced by its vibrational excitation. As the small cluster impacts the surface, it can be that the halogen molecule reaches the surface immediately, so that it still has the same velocity as that of the cluster center of mass. If that velocity is above the threshold, it will dissociate with about a 40% probability, just as a bare molecule would. The other process that can happen is that the halogen molecule collides first with a cluster atom, an atom that has already hit the surface, and therefore lost a fraction of its translational energy to the surface. Such a collision does two things. It slows the halogen molecule and so reduces its probability to dissociate at the surface. At the same time, at the supersonic velocities of... 

The efficiency of cluster impact in driving four-center reactions is due to a matching between what the cluster can do to the reactants or products and the very selective energy requirements and the specific energy disposal in a concerted reactive collision, as discussed in details in Sec. 3.2. The cluster serves to provide both the steric and energetic conditions necessary for this reaction. In terms of the impact parameter of the relative motion of the two reactants, their confinement by the cluster keeps it low, so that they do not miss one another. This confinement within the cluster favoring low impact parameter collisions is a key ingredient in why such processes are so efficient. Furthermore, both the activation of the reactants before the reaction and the stabilization of the hot product after it, are due to the cluster atoms. 

Once again we see how cluster impact chemistry provides a new dynamical regime. The unique features of the scheme are the high material and energy density conditions which can be established very rapidly, leading to super heating on a time scale shorter than even typical intramolecular... 

Fig. 30. The hyperradius p of the cluster, defined in Eq. (10), in A vs. time, in ps, for the same cluster impacting a cold surface at two slightly different low supersonic velocities of 450 and 500 ms respectively. This figure illustrates how a small increment in the energy of impact changes the outcome dramatically. The cluster (which has identical initial conditions in both runs) evaporates just one monomer at the lower energy but fully shatters at the higher energy.

Pig. 31. The fraction of intact parents that recoil from the surface as a function of the impact energy. MD simulations for a cluster of 8 NH3 molecules interacting only by a central van der Waals potential and by a potential containing in addition a dipolar attraction. The extra attraction shifts the shattering transition to a higher energy. 

The point about the technique of cluster impact is that it enables one to heat the cluster on a time scale short compared to that needed for expansion and ipso facto for evaporation. In this way one can prepare superheated clusters with enough energy for breaking most or all intermolecular bonds so that the cluster shatters into its constituents. The sharp transition from evaporation regime to the shattering one, shown in this section, indicate a fast translational thermalization of the extreme disequilibrium formed immediately after the impact with the surface. The need for only two constraints in the maximum entropy formalism (conservation of matter and total energy) to predict the experimental results on the fragment size... 

The reason we employ two rather distinct methods of inquiry is that neither, by itself, is free of open methodological issues. The method of molecular dynamics has been extensively applied, inter alia, to cluster impact. However, there are two problems. One is that the results are only as reliable as the potential energy function that is used as input. For a problem containing many open shell reactive atoms, one does not have well tested semiempirical approximations for the potential. We used the many body potential which we used for the reactive system in our earlier studies on rare gas clusters containing several N2/O2 molecules (see Sec. 3.4). The other limitation of the MD simulation is that it fails to incorporate the possibility of electronic excitation. This will be discussed fmther below. The second method that we used is, in many ways, complementary to MD. It does not require the potential as an input and it can readily allow for electronically excited as well as for charged products. It seeks to compute that distribution of products which is of maximal entropy subject to the constraints on the system (conservation of chemical elements, charge and... 

Kim, C. K., Kubota, A., and Economou, D. J., Molecular-dynamics simulation of silicon surface smoothing by low-energy argon cluster impact. J. Appl Phys 86, 6758-6762 (1999). Kohn, W., and Sham, L. J., Self-consistent equations including exchange and correlation effects. 

From a case study [5], the different excitation mechanisms (electronic and vibrational) as well as related relaxation phenomena (phase transitions and fragmentation) in atom-cluster collisions will be presented in Sect. 3. We show the gradual change of these mechanisms as a function of the impact energy in a wide range for the collision system Nag+ + Na. 

We have studied the reaction dynamics of the collision Nag+ + Na for a fixed collision geometry (with impact parameter 6=0) but in a wide range of impact energies Ecm. In Fig. 1, the total kinetic energy loss (tkel) AE =Eem -Ecm(t -> +oo), with Ecm(t +oo) the final kinetic energy of the relative motion between cluster-projectile and atomic target in the centre-of-mass system is shown for Ecm = 0.2 eV... 1 MeV. 

The dynamics of the collision process and the subsequent cluster relaxation can be seen for characteristic impact energies in Fig. 2. It shows the time dependence of E(t)=Ecm-Ecm(t), with Ecm(t) the actual kinetic energy of the relative motion, and the displacement of the cluster atoms d(t). The quantity AE(t) gives insight into the collision and excitation dynamics (forces, interaction time, tkbl). The displacement, defined as... 

Figure 2 Difference of the impact energy and the actual kinetic energy AE(t) of relative motion (left column) and displacement d(t) of the cluster atoms (right) as a function of time t for selected impact energies Ecm and the same collision geometry as in Fig. 1, calculated with na-qmd (dark solid lines). In the left column, the corresponding adiabatic qmd calculations (dotted lines) are also shown for comparison.

Atom cluster collisions have been investigated by means of a self-consistent and simultaneous treatment of the atomic and electronic dof involved. Fbr the collision system Na9++ Na we have examined the excitation mechanisms as well as the relaxation scenarios which strongly depend on the impact energy. Three basically different fragmentation mechanisms— impulsive fragmentation, electronic fragmentation, and statistical decay—have been discussed. 

There are only a relatively small number of articles reporting on the use of specific sample preparation techniques in combination with cluster beams for sensitivity improvement, and even fewer with pronaising results. Several studies demonstrated that MetA-SIMS with cluster projectiles at the usual impact energies often lead to lower, instead of enhanced, molecular ion yields [333,341]. In particular, MD simulations showed that 10-15 keV Cw projectiles were inefficient at generating bond scissions and energetic subcascades in the organic sample when impinging on metallic clusters [336]. 

Cluster impact chemistry provides a new dynamical regime, hitherto not accessible to systematic exploration. High barrier chemical reactions can be studied under de facto isolated gas phase collsion conditions. On the other hand, ternary (and even higher order) encounters, which were of special interest to Bodenstein [24], can be explored. The unique features of the scheme are the high material and energy density conditions which can be established very rapidly, leading to super-heating (which has already been experimentally demonstrated [7]) on a time scale shorter than even typical intramolecular events. 

Multiscale Simulation and Its Application to High Energy Cluster Impacts. 

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