Dissociation of doubly charged alkali metal clusters

We have studied the competition experimentally observed between fission and neutral atom evaporation, as dissociation channels of excited doubly charged sodium clusters, using the Density Functional Theory and the jellium model. The fission barrier has been obtained from an Extended Thomas-Fermi calculation including density gradient corrections to the kinetic energy of the electronic cloud. We discuss the influence of the coefficient of the density gradient term on the barrier height.     

Twist mode in spherical alkali metal clusters

A remarkable orbital quadrupole magnetic resonance, so-called twist mode, is predicted in alkali metal clusters where it is represented by Ipi = 2(-) low-energy excitations of valence electrons with strong M2 transitions to the ground state. We treat the twist by both macroscopic and microscopic ways. In the latter case, the shell structure of clusters is fully exploited, which is crucial for the considered size region ( 8相似文献   

Conserving quasiparticle calculations for small metal clusters

A novel approach for GW-based calculations of quasiparticle properties for finite systems is presented, in which the screened interaction is obtained directly from a linear response calculation of the density-density correlation function. The conserving nature of our results is shown by explicit evaluation of the f-sum rule. As an application, energy renormalizations and level broadenings are calculated for the closed-shell Na₉ ⁺ and Na₂₁ ⁺ clusters, as well as for Na₄. Pronounced improvements of conserving approximations to RPA-level results are obtained.     

Static polarizability of excited and charged alkali metal clusters

The static polarizability of the excited and positively charged (from 1 to 5) sodium, lithium, and potassium clusters containing the “magic” number of valence electrons (from 8 to 198) is calculated by the density-functional method within the “jellium” model. The dependences of the polarizability on the state, size, charge, and composition of clusters are analyzed.     

Van der Waals coefficients for alkali metal clusters and their size dependence

In this paper we employ the hydrodynamic formulation of time-dependent density functional theory to obtain the van der Waals coefficientsC ₆ andC ₈ of alkali metal clusters of various sizes including very large clusters. Such calculations become computationally very demanding in the orbital-based Kohn-Sham formalism, but are quite simple in the hydrodynamic approach. We show that for interactions between the clusters of the same sizes,C ₆ andC ₈ scale as the sixth and the eighth power of the cluster radius, respectively, and approach their classically predicted values for the large size clusters.     

Towards single-particle spectroscopy of small metal clusters

We investigate the kinetic-energy spectra of electrons emitted from a metal cluster following laser excitation. This is done in the framework of a coupled ionic and electronic dynamics. Properly chosen laser parameters, leading to gentle excitations, yield kinetic-energy spectra which nicely resolve the multiphoton processes for each occupied state separately. This gives access to the single-particle energies in clusters, provided one works at sufficiently low temperatures and low electron flow.     

Photoionization and fragmentation of alkali metal clusters in supersonic molecular beams

A supersonic molecular beam, pulsed laser and time-of-flight mass spectrometer are arranged together in order to study photo- and auto-ionization, two-photon-ionization and fragmentation processes of alkali clusters as a function of the laser wavelength. In spite of an unfavourable duty cycle, however, the apparatus reaches a considerably higher sensitivity than cw experiments. Alkali clustersM _n (n21) have been observed and investigated. The well resolved TOF mass spectra are commonly accompanied by significant and broad signals of metastable ions, a phenomenon which cannot be observed by quadrupole mass spectrometry. ParticlesM _n (n4) have been investigated by twophoton ionization using two different laser wavelengths. Several new electronic transitions for Na₃ are found; commonly, however, the excitation and ionization channels are accompanied by strong fragmentation processes. The fragment patterns are very sensitively dependent upon the laser wavelength even when working near the ionization threshold. The results are a strong indication, that the peak intensities of cluster mass spectra cannot easily be related to the intensity distribution of the neutral cluster beam.     

Surface and bulk melting of small metal clusters

We present an analytical solution to the two-parabola Landau model, applied to melting of metal particles with sizes in the nanoscale range. The results provide an analytical understanding of the recently observed pseudo-crystalline phase of nanoscale Sn particles. Liquid skin formation as a precursor of melting is found to occur only for particles with radii greater than an explicitly given critical radius. The size dependences of the melting temperature, and of the latent heat, have been calculated, and a quantitative agreement is found with the experiment on tin particles.     

A model for alkali chemisorption

The linear response function in the density functional formalism, developed by Ying, Smith and Kohn, is applied to alkali chemisorption on metallic surfaces. The adion is represented by a pseudopotential perturbing the surface. Binding energies, induced dipole moments, and vibrational frequencies at the equilibrium position are first calculated for the alkalis at finite coverage, as well as, in the limit of zero coverage, on a jellium substrate. The theoretical results are in good agreement with available experimental results. The effect of electric fields on chemisorbed alkali ions is discussed with reference to interpretation of experimental results. The crystalline structure is then introduced in the form of a periodic pseudocharge density representing the ionic lattice. In this model, corrections to the quantities calculated in the jellium approximation are obtained for both low index and high index planes. Preferred adsorption sites are determined and found to be consistent with experimental findings.     

A three-level model for alkali metal vapor lasers. Part II: broadband optical pumping

The effects of pump laser spectral bandwidth on the performance of longitudinally pumped diode-pumped alkali lasers is explored by extending the analytic, three-level model using longitudinally averaged number densities. By assuming a statistical distribution between the upper two levels, the limiting solution for the quasi-two level system is achieved. A second limiting solution is identified for strongly bleached conditions where the atom recycle rate, limited by spin–orbit relaxation, fully specifies the output power. Performance in the intermediate regime depends significantly on the pump bandwidth relative to the atomic absorption line width and requires numerical simulation. The ratio of populations for the two excited, ²P_3/2,1/2 states completes an analytic solution and depends primarily on pump laser bandwidth, threshold, and alkali concentration. Absorption well into the wings on the atomic profile can be utilized by increasing alkali concentration, but imposes increased pump intensity threshold.     

Dynamic polarization in the strong-field ionization of small metal clusters

We report on the strong field ionization of small transition metal clusters (nickel, Ni(n) n=1-36) within the quasistatic regime at an infrared wavelength of 1.5 microm and at intensities up to 2 x 10(14) W/cm(2). From ion yields in a constant axial intensity beam, we obtained saturation intensities for the individual Ni(n) clusters. As compared to quasistatic, single active electron calculations, a dramatic suppression of ionization was observed. Dynamic polarization in the laser field likely leads to strong multielectron screening of the "active" electron. Representing the metal clusters as classical conducting spheres, we obtained, via a barrier suppression calculation, the classical ionization rates. Agreement was obtained for larger clusters with n>10 when the dynamic polarization was taken into account, emphasizing the multielectron nature of the ionization suppression.     

Lineshape studies of the X-ray photoemission of small metal clusters

It is shown that the lineshape asymmetry of the X-ray photoelectron spectra of the Pt(4f) and Pd(3d) core-levels in small metal clusters decreases with the size of the cluster. Such changes are related to the changes of the density of states of the valence electrons and the effective core-hole potential at the Fermi level. The decrease of the asymmetry leads to a shift of the line position toward a higher binding energy (about half of the observed shift) as governed by the first moment rule.     

Collectivity in the optical response. of small metal clusters

The question of whether the linear absorption spectra of metal clusters can be interpreted as density oscillations (collective “plasmons”) or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters with only a few electrons. We calculate the photo-absorption spectra of Na₂ and Na₅ ⁺ comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The changes in the electronic structure associated with particular excitations are visualized in “snapshots” via transition densities. Our analysis shows that even for the smallest clusters, the observed excitations can be interpreted as intuitively understandable density oscillations. For Na₅ ⁺, the importance of self-interaction corrections to the adiabatic local density approximation is demonstrated. Received: 1 July 2001 / Published online: 10 October 2001     

Magic numbers in small clusters made up of two kinds of alkali atoms

The magic numbers in small clusters made up of two kinds of alkali atoms are theoretically studied and new experiments are proposed. These clusters exhibit two series of almost identical magic numbers, one for each kind of atoms, which are further identical to those for protons and neutrons in nuclear physics (i.e.,2,8,20,50,82).     

Density function study transition metal chromium-doped alkali clusters: the finding of magnetic superatom

The structures, stabilities and magnetic properties of CrX_n (X = Na, Rb and Cs; n up to 9) clusters are studied using density functional theory to search for the stable magnetic superatoms. The geometrical optimisations indicate the ground-state structures of CrX_n evolve toward a close packed structure with an interior Cr atom surrounded by X atoms as the cluster size increase. Their stabilities are analysed by the relative energy, gain in energy (ΔE(n)) and the highest unoccupied molecular orbital and lowest unoccupied molecular orbital gaps. Furthermore, the magnetic moments of CrX_n clusters show an odd–even oscillation. Here, we mainly focus on the CrX₇ (X = Na, Rb and Cs) clusters due to the same valence count as the known stable magnetic superatoms VNa₈, VCs₈ and TiNa₉. Although these clusters all have a filled electronic configuration 1S²1P⁶ and large magnetic moment 5 μ_B, our studies indicate that only CrNa₇ is highly stable compared to its nearest neighbours, while CrRb₇ and CrCs₇ clusters are less stable. This suggests that Cr-doped Na₇ is most appropriate for filled electronic configuration and CrNa₇ is shown to be a stable magnetic superatom. More interesting, we find CrRb₈ and CrCs₈ with the filled electronic configuration 1S²1P⁶ have higher stability and large magnetic moment 6 μ_B in their respective series.     

Control of wavepacket dynamics in mixed alkali metal clusters by optimally shaped fs pulses

We have performed adaptive feedback optimization of phase-shaped femtosecond laser pulses to control the wavepacket dynamics of small mixed alkali-metal clusters. An optimization algorithm based on Evolutionary Strategies was used to maximize the ion intensities. The optimized pulses for NaK and Na₂K converged to pulse trains consisting of numerous peaks. The timing of the elements of the pulse trains corresponds to integer and half integer numbers of the vibrational periods of the molecules, reflecting the wavepacket dynamics in their excited states. Received 4 December 2001     

Self-diffusion of small clusters on fcc metal (111) surfaces

We use ab initio density-functional theory supplemented with the embedded-atom method to study the self-diffusion of small clusters on the (111) surface of eight fcc metals. A zigzag motion is found to be important in the dimer and tetramer diffusions. The dimer diffuses by a zigzag and concerted motion. The trimer diffuses by a concerted three-atom motion. The tetramer diffuses through a zigzag motion where only two atoms move simultaneously in each step. Thus, instead of increasing, the migration energy is lowered (or stays constant) for the tetramer as compared to that for the trimer. This novel break of the upwards trend in migration energy is predicted to be a general phenomenon.