ENERGETICS AND STRUCTURES OF NEUTRAL AND CHARGED SI-N (N-LESS-THAN-OR-EQUAL-TO-10) AND SODIUM-DOPED SINNA CLUSTERS

Energetics and structures of neutral and charged Si-n (n less than or equal to 10) and sodium-doped SinNa clusters have been investigated us ing local spin density functional electronic structure calculations an d structural optimizations, with and without exchange-correlation grad ient corrections. For the Si-n clusters, the monomer separation energi es show local maxima for n=4, 7, and 10. The vertical and adiabatic io nization potentials are smaller than the values for the Si atom and ex hibit odd-even oscillations with values in agreement with experiments, and the adiabatic electron affinities show local minima for n=4, 7, a nd 10, with the value for the heptamer being the smallest, in agreemen t with the experimentally measured pattern. Binding of Na to Si-n is c haracterized by charge transfer from the sodium resulting in the devel opment of significant dipole moments for the SinNa clusters. The bindi ng energy of Na to Si-n oscillates as a function of n, with local maxi ma for n=2, 5, and 9, and local minima for n=4, 7, and 10, with the va lue for n=7 being the smallest. A similar trend is found for the verti cal and adiabatic ionization potentials of the doped clusters, correla ting with the electron affinity trend exhibited by the Si-n clusters, and in agreement with recent measurements. In the optimal adsorption g eometry of H2O on the Si7Na cluster, the oxygen is bonded to the Na, w ith a hydration energy significantly higher than that of an isolated s odium atom. The vertical and adiabatic ionization potentials of NaH2O are lower than those of Si7NaH2O, and the values for the latter are lo wer, by approximate to 0.2 eV, than those of the unhydrated Si7Na clus ter.