M. B. Torres

Linear response to spin-dependent and spin-independent fields of alkali metal clusters

We have obtained the dipole response functions of alkali-metal clusters to spin-dependent and spin-independent fields within the framework of the time-dependent local-spin-density approximation. As test cases we have studied the response of spherical jellium clusters with spin-saturated configurations (Na9+, Na8 and Na20) and with fully spin-polarized configurations (the spherical isomer of Na6+ and Na13). For a spin-dependent excitation we have obtained in all these clusters a strongly collective spin mode of surface type lying at lower energies than the unperturbed particle-hole excitations. This mode uncouples cleanly from the electric dipole mode (Mie plasmon) in the case of spin-saturated clusters, but if the ground state breaks spin symmetry by a finite spin polarization the two modes turn out to be intertwined in the responses to spin-dependent as well as spin-independent fields.

DENSITY FUNCTIONAL STUDIES OF NOBLE METAL CLUSTERS. ADSORPTION OF O2 AND CO ON GOLD AND SILVER CLUSTERS

We study structural and chemisorption properties of pure and doped noble metal clusters by means of first-principles density functional calculations, based on norm-conserving pseudo-potentials and numerical atomic basis sets. First, we show that, together with relativistic effects, the level of theory, that is, the use of GGA or LDA exchange-correlation functionals, is of paramount importance to determine the onset of three dimensional structures in Au clusters, whereas for Ag or Cu clusters it is not so critical. Second, within the GGA framework, we find cage-like stable structures for neutral Au18 ,A u 20 ,A u 32 ,A u 50 ,a nd Au 162. However, after addition or substraction of an electron only Au − and Au + remain cage- like. On the other hand, only Au20 results cage-like within LDA, whereas Ag20 and Cu20 clusters adopt compact amorphous-like Cs structures within both, LDA and GGA frameworks. Third, we investigate the element- and size-dependent electron stability of gold clusters cation doped with a tran- sition metal impurity, AunTM + (TM = Sc, Ti, V, Cr, Mn, Fe; n ≤ 9), and we obtain a clear explanation of the cluster abundance peaks observed recently in photo-fragmentation experiments. Fourth, we study the size

Density functional study of photoabsorption in metallic clusters using an exchange-correlation potential with correct long-range behaviour

The atomic exchange-correlation (xc) potential with the correct -1/r asymptotic behaviour constructed by Parr and Ghosh (Parr R G and Ghosh S K 1995 Phys. Rev. A 51 3564) is adapted here to study, within time density functional theory, the linear response to external fields of (i) neutral and charged sodium clusters, and (ii) doped clusters of the type NanPb (n = 4, 6, 16). The resulting photoabsorption cross sections are compared to experimental results, when available, and to results from previous calculations using local and non-local xc functionals. The calculated static polarizabilities and plasmon frequencies are closer to the experimental values than previous results.

The effect of surface diffusibility on the collective modes of metal clusters

We calculate the eigenmode spectrum for collective multipole vibrations of the electrons of alkali clusters including coupling between surface and volume plasmons. We formulate the equations of motion for the collective variables (density and velocity potential) starting from the hydrodynamic approximation. We investigate the effect of the diffusibility of the valence electrons on their collective modes, considering an equilibrium density of the valence electrons with a smooth surface profile. This effect was not considered in previous work carried out by the first author, who assumed the equilibrium density of the valence electrons to be a constant that has the bulk value. The results indicate that, within the hydrodynamic model, surface spill-out effects lead to a tendency for a red-shift from the Mie frequency and a mixing of resonance modes when the size of the cluster decreases. The eigenmodes fulfil the linear energy-weighted sum rule, the inverse energy-weighted sum rule and orthogonality relations.