Calculation of Adsorption Energies of Elements 112 and 114, and their Homologues Mercury and Lead on Gold (111) Surface

In the last decade the gas-phase thermography becomes the method of choice for studying the chemical behavior of super-heavy elements in nuclear-chemistry. In such experiments, however, the whole information about the adsorption process of a chemical element is reduced to a single number – the adsorption temperature. The adsorption on the other hand is a very complex process, which requires an adequate theoretical description for interpreting the experimental data. Within our theoretical approach, which is based on fullyrelativistic 4c-DFT [1], the substrate surface is represented by a cluster of atoms at fixed positions. The adsorption behaviour, i.e. binding energies and structures strongly depend on the size of the chosen cluster. Therefore a cluster size convergence studies are mandatory in order to be able to obtain accurate results. In this report we present our calculations of the adsorption energies of elements 112 and 114 and their homologues Hg and Pb on a gold (111) surface for different cluster sizes. For all four possible adsorption positions – top, bridge, hollow1, and hollow2 – we first performed calculations for the smallest possible cluster, which for the on top site might even be a single atom only, for example. In the next step we added all atoms from the next coordination shell to our cluster and repeated the calculations. We continued this procedure until convergency was achieved in both the adsorption energy and adsorption distances. The results of this study are summarized in Figure 1, where the potential energy curves for elements 112 and 114, and mercury are presented. (Pb is not shown in the picture, because the binding energies are too large). The convergence of both the bond distance and the adsorption energy as function of cluster size was achieved with the largest clusters containing 95 (top), 94 (bridge), 120 (hollow1), and 107 (hollow2) gold atoms. The results of the calculations show that the binding energies of each of the considered elements for the bridge and hollow2 positions are very similar. However, element 114 and Hg have a slight preference to adsorb in the bridge position, while element 112 and Pb in the hollow2 one. Except for Pb, all the other three elements are weakly bound to the gold surface which is due to the strong relativistic effects in the outermost shells of these elements and a closed shell effect in Hg and element 112. An interesting and important feature of the present calculations is that the binding energies change with both the adsorption position and cluster size in such a way that the adsorption strength order Eb(E112) < Eb(Hg) < Eb(E114)≪ Eb(Pb) remains the same. This confirms our former qualitative predictions [2], which were based on calculations for dimers only. A detailed discussion of the presented results can be found in [3].