BzAr

For BzAr , as for the smaller clusters we study the effect of changing the LJ parameters on the underlying PES. The lowest minimum found, type (7|5|7) (Fig. 3), is the same two-sided structure for both sets of LJ parameters. The Ar atoms do not completely surround the benzene molecule in this minimum, and there is partial `solvation'. We note that the (l|m|n) notation is not able to describe the geometries for BzAr as well as for the smaller clusters, but it can still be used to differentiate between one- and two-sided structures or structures involving bridging atoms. Also, unlike the descriptions of the smaller clusters, this notation no longer implies the existence of pseudo-planar arrangements of Ar atoms. Our lowest minimum has some residual double icosahedral structure for the Ar atoms, but much more icosahedral order is preserved in some higher energy minima (Fig 4). The double icosahedron is the global minimum for Ar and when we add a benzene molecule the effect on the Ar atoms is reduced compared to smaller Ar clusters. Reordering is localised to Ar atoms in the vicinity of the benzene ring.

  
Figure 3: Two views of the lowest energy BzAr minimum found in the present study. The overall symmetry is and there is clearly some residual icosahedral structure.

Even though we find the same lowest energy minimum (aside from detailed bond lengths etc.) for both sets of LJ parameters there are important differences in the corresponding PES's. For the OBJ parameters two-sided structures dominate the lowest-lying minima. However, for the SCM parameters, although the lowest minimum is two-sided, there exists an energy range of about 0.4millihartree (mh) above this structure in which we find only one-sided minima. Such differences in the PES's are likely to be of some importance when considering the dynamics of the system. We decided to use the SCM parameters for our Monte Carlo and Molecular Dynamics simulations as we prefer the larger Ar-Ar well depth and suspect that the smaller Ar-H well depth is more realistic. The experimental dissocation energy for BzAr is 1.55mh [44] which is closer to our SCM value of 1.72mh than our OBJ value of 1.92mh.

We start separate simulations from both the lowest-lying two-sided and one-sided minima located by systematic geometry optimisations from preliminary Monte Carlo simulations. These structures are shown in Fig. 3 and Fig. 4. Their binding energies are -39.04mh and -38.90mh respectively. Experimentally there appears to be no definitive conclusion regarding how these clusters form. There may be only one-sided structures initially, which then undergo solvation; alternatively both one-sided and two-sided structures may be formed so that both are observed spectroscopically. In fact, Adams and Stratt [8] have recently suggested that most BzAr_n clusters formed in a jet are trapped in local minima in which the benzene molecule is bound to the surface of the Ar cluster. This argument is based upon new calculations of the benzene spectral shift which include the collective dielectric response of the cluster. [8] The suggestion that most benzene molecules are trapped in surface states contradicts the original conclusions of Guillaume et al. [3] which, however, have been revised in more recent work (Guillaume et al. 1995).

  
Figure 4: The lowest energy one-sided minimum found for BzAr in this study. The residual AR double icosahedron can clearly be seen.