Rare gas atom clusters

FJ clusters (in FJ units, or as a model for specified rare-gas atom clusters) continue to be used as a benchmark system for verification and tuning in method development. With the work of Romero et al. [52], there are now proposed global minimum structures and energies available on the internet [53], up to n=309. This considerably extends the Cambridge cluster database [54], but the main body of data comes from EA work that used the known FJ lattices (icosahedral, decahedral, and face-centered cubic) as the input. This is obviously dangerous,... 

Further calculations on rare gas atom clusters, which are believed to model many metal atom clusters, have shown that tetrahedral groupings of atoms are usually preferred over octahedral groupings. 

In addition to the dependence of the intennolecular potential energy surface on monomer vibrational level, the red-shifting of the monomer absorption as a fiinction of the number of rare gas atoms in the cluster has been studied. The band origin for the Vppp = 1 -t— 0 vibration in a series of clusters Ar -HF, with 0 < n < 5, was measured and compared to the HF vibrational frequency in an Ar matrix (n = oo). The monomer vibrational frequency Vp p red shifts monotonically, but highly nonlinearly, towards the matrix value as sequential Ar atoms are added. Indeed, roughly 50% of the shift is already accounted for by n = 3. 

Studies of larger species are more complex and the difficulty in the evaluation of their potential surfaces increases with their size. Up to now accurate potentials have been obtained by inversion of spectroscopic data or through high level ab initio calculations " for several triatomic vdW systems. Thus, the interactions for such clusters are available with satisfactory accuracy, which permits the testing of various models of nonadditivity for their ability to reproduce a number of experimental observations. These facts made complexes composed of two rare-gas atoms and a dihalogen molecule especially attractive targets for the study of nonadditive forces. The first attempt to extract information on nonadditive interactions from... 

Interatomic Coulombic decay (ICD) is an electronic decay process that is particularly important for those inner-shell or inner-subshell vacancies that are not energetic enough to give rise to Auger decay. Typical examples include inner-valence-ionized states of rare gas atoms. In isolated systems, such vacancy states are bound to decay radiatively on the nanosecond timescale. A rather different scenario is realized whenever such a low-energy inner-shell-ionized species is let to interact with an environment, for example, in a cluster. In such a case, the existence of the doubly ionized states with positive charges residing on two different cluster units leads to an interatomic (or intermolecular) decay process in which the recombination part of the two-electron transition takes part on one unit, whereas the ionization occurs on another one. ICD [73-75] is mediated by electronic correlation between two atoms (or molecules). In clusters of various sizes and compositions, ICD occurs on the timescale from hundreds of femtoseconds [18] down to several femtoseconds [76-79]. 

The molecular beam deflection method is shown schematically in Figure 3-17 (Buck et al. 1985). It is based on momentum transfer between clusters entrained in a molecular beam and rare gas atoms which are the constituents of a second molecular beam at 90° to the cluster beam. Collisions between the rare gas atoms and the clusters under single-collision conditions deflect a small percentage of the clusters from their original path. The maximum deflection angle depends on the mass of the cluster. For example, binary clusters may be deflected into a broad range of angles with a well defined upper limit set by the momentum conservation... 

Evidence that clustering of rare gas atoms occurs around ions comes from (a) ion mobility measurements, and (b) volume changes occurring on electron attachment to solutes. The mobility of positive ions in xenon decreases with increasing pressure and at pressures near 100 bar is 1.3 X 10 cm /Vs [see Fig. 3(a)] near room temperature. An estimate of the size of the cluster moving with the ion may be obtained from such data using the Stokes equation. 

An important implication is that collisional activation within the cluster can be described as a sequence of binary events. In other words, it is typically one rare gas atom at a time that undergoes a close in collision with a reactant, as illustrated in Fig. 5. 

We count the N2 + O2—>N + N+ 0 + 0 channel as a reactive one because all the trajectories we examined yielded a four atom final state only when it was preceded by the formation of NO, however briefly. In addition, this channei has a dynamical energy threshold significantly higher than the endoergicity. The total yield is shown vs. the impact velocity for N2 + O2 molecules embedded in a cluster of 125 rare gas atoms, One can replot this figure in terms of a reduced variable, see Fig. 13. 

Diatomic molecules embedded in rare gas cluster can dissociate in one of two ways a heterogeneous dissociation of the molecule on the surface and a homogeneous mechanism where dissociation takes place inside the cluster without the molecule reaching the surface. The sudden collision regime insures that an effective vibrational excitation of the diatomic molecule will occur due to a collision between the diatomic molecule and the rare gas atoms of the cluster. Yet, the size of the cluster and the identity of the rare gas atoms influence the yield of dissociation. 

Beyond the regime (say fewer than 13 rare gas atoms), where the yield increases with cluster size, there is a decline in the yield, a decline which is more noticeable, the lower is the velocity of impact. 

Fig. 18. Bond distances (see legend) vs. time, in fs, for the old and new bonds in the N2 + O2 —> 2N0 reaction in a 125 atom cluster. The figure illustrates the higher efficiency of the heavier rare gas atoms in providing a more rigid cage for the outcome of the first bimolecular collision. The inset shows the hyperspherical radius [see Eq. (10)] p vs. time in fs. The hyperspherical radius is a measure of how near the four atoms that take part in the reaction are to one another. Top panel A Xei25 cluster at an impact velocity of 7 km/s. Note how the atoms are almost as compressed in their second as in the first bimolecular collisions. (The times of these collisions are indicated by arrows.) Bottom panel A Nei25 cluster at an impact velocity of 12 km/s. The second collision is not very eflfective in bringing the four atoms together. In both panels only one stable NO molecule is formed.

Due to their long range attraction the reactants cluster together and the rare gas atoms surround the reactive molecules. The three roles of the cluster are as in the previous cases, to make sure that the reactions occur through the bulk of the cluster and not only in the layer nearest to the surface, to activate the reactants and to stabilize the products. 

Fig. 26. The vibrational energy, in units of De, of CI2, embedded in a cluster of 125 Ar atoms vs. time, as a probe for a shock front. Examination of the trajectories shows that the molecule is moving towards and has not yet reached the surface. It is excited by collisions with rru e gas atoms which were at the front of the cluster and have already reflected from the rigid surface, back into the cluster. Upper panel Low velocity of impact. A layer of rare gas atoms requires 100 fs to move a distance of one

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