Temperature dependence of the optical response of sodium cluster ionsNan+,with4<~n<~

Abstract

The temperature dependence of the optical response of small sodium cluster ions, Na n ϩ (nϭ4, . . . ,16), was measured. At low temperature ͑35 to 100 K͒ individual lines are seen, whose peak positions agree with ab initio calculations if the geometric positions of the atoms in the cluster are taken into account. The peaks broaden slowly with increasing temperature and coalesce, until at high temperature only one, two, or three broad peaks remain, whose positions are for nу7 in good agreement with the nearly free-electron model. Although the detailed shape of the optical spectra changes strongly with temperature, its moments are nearly temperature independent. No pronounced effect due to the solid to liquid phase transition has been observed for these small cluster sizes.

Orbital-free molecular dynamics study of melting inK20,K55Aguado1 2001Phys. Rev. B18151View full textAdd to dashboardCite

“…Our predictions of the temperatures at which homogeneous cluster melting occurs were in good agreement with the experiments of Haberland's group, 10,11 excepting the enhancement of the melting temperature around N=55, which was not reproduced. We also observed a number of interesting premelting effects, mostly the establishment of a surface melting stage at a temperature lower than the homogeneous melting temperature for Na 20 , Na 92 and Na 142 , and several isomerization transitions in Na 8 and Na 20 .…”

“…The experimental results of Haberland et al [10][11][12] suggest that both electronic and atomic shell effects determine the irregular size evolution of the melting temperatures of sodium clusters. Our orbital-free method does not account for quantum shell effects, and gives cluster energies that vary smoothly as a function of cluster size (that is, it does not reproduce the energy oscillations associated with electronic shell closures).…”

“…The melting-like transition in small clusters composed of a finite number of atoms spreads instead over a temperature interval that widens as the cluster size decreases. A number of thermal properties sensitive to the cluster size and essentially different from those found in bulk materials emerge in the transition region defined by that temperature interval, which has motivated a lot of theoretical [1][2][3][4][5][6][7][8] and experimental [9][10][11][12] investigations. The experiments of Schmidt et al 10 have shown strong nonmonotonic variations of the melting temperature of free sodium clusters with size, which can not be completely explained either by electronic or geometric shell closing arguments.…”

Stability and symmetry breaking in metal nanowires: The nanoscale free-electron model

“…It is well known in the physics of crystals and molecules that a Jahn-Teller deformation breaking the symmetry of the system can be energetically favorable. In metal clusters, Jahn-Teller deformations are also very common, 22,23 and most of the observed structures show a broken spherical symmetry. By analogy, it is natural to assume that for nanowires, too, a breaking of axial symmetry can be energetically favorable, and lead to more stable deformed geometries.…”

Photoabsorption Spectra of Nan+ Clusters: Thermal Line-Broadening Mechanisms

“…18 While in the text we focus our discussion on the larger cluster Na + 9 , we remark that isomerization broadening was found by us also for sodium Na + 7 , where at T =300 K we observed structural transitions between the D 5h pentagonal bipyramidal GS and a D 6h planar hexagonal isomer (lying 141 meV above the GS) resulting in a bimodal spectral maximum for ω∼ 2.4 eV, in addition to a broad absorption peak at 3.5 eV (see ref. 2 ). 19 The Na + 9 GS structure (a tricapped trigonal prism, α in Fig.…”