Ionization energies and stabilities of Na n ,n<25: shell structure from measurements on cold clusters

Abstract

The stability patterns found in alkali-atomic clusters and their explanation in terms of electronic and structural factors have been controversial for some time. Generation of very cold Na, clusters in a novel source and use of a special photoion normalization method resolve the remaining questions by allowing precise determination of photoionization thresholds. This is demonstrated here for several sizes in the 7<n<26 range, where in two crucial cases the interpretations of earlier ionization threshold measurements on oven-beam clusters [M. Kappes, M. Sch/ir, P. Radi, and E. Sehumacher, J. Chem. Phys. 84, 1863 (1986)] disagreed with the explanation of the observed stability pattern. Combining the new values with the charged-cluster fragmentation energies of Br6chignac et al. [C. Br6chignac, P. Cahuzac, J. Leygnier, and J. Weiner, J. Chem. Phys. 90, 1492 (1989)] yields neutral cluster fragmentation energies that successfully account for the famous "magic-number" ledges [W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou, and M.L. Cohen, Phys. Rev. Lett. 52, 2141 (1984)]. Our measurements offer decisive support for the applicability of the spherical/spheroidal electronic shell-model to smaller Na, clusters, even in their low-temperature form.

Ionization potential measurements of hydrogenated lithium clusters

“…Figure 3a shows the evolution of Li n and LinH clusters from n = 2 to n = 11, and Fig. 3b confirmed by previous results obtained for Na n [6,7], K n [8,9] and Cs n [10] clusters. Finally, the odd-even alternation corresponds to a quantum electron-pairing effect.…”

Finite-size effects and the stabilized spin-polarized jellium model for metal clusters

“…Relaxing the shape of the jellium background as well as its density distribution while keeping charge neutrality at every point in space, called the ultimate jellium model (UJM), was introduced by Koskinen et al 20 Since the bulk density in the UJM (r s = 4.18) is close to that of sodium, its results can be compared with experiments on Na metal clusters. 21,22 However, since the jellium density and the electron density in the UJM are locally equal everywhere, the UJM cannot describe the ionized clusters.…”

Exchange effects on electronic states in jellium clusters

“…The actual calculation for the WKB equivalent potential and the suppression factor is done for the ls and lp states in the N = 8 case. The non-local potentials coming from exchange of electrons are not of the separable form as in (12). We therefore need to calculate the equivalent local potential starting from the general (11).…”

“…In Table l we show calculated eigenenergies and total energies with the Hartree, Hartree-Fock and KohnSham methods together with the experimental ionization potentials (IP) [1][2][3][4][5][6][7][8][9][10][11][12]. The states are denoted by the number of nodes plus one and the angular momentum symbol.…”