Modeling the formation of alkali clusters attached to helium nanodroplets and the abundance of high-spin states

The doping process of helium nanodroplets with alkali atoms has been modeled in order to study deviations from the Poissonian statistics of measured pick-up statistics which are important for assigning cluster or complex sizes in many experimental studies. Several, formally unexplained findings are reproduced and their origin has been analyzed: derivations from the expected functional form of the initial incline, the suppression of the formation of lithium clusters, the influence of the functional form and width of droplet size distributions. Furthermore, the controversially discussed formation of high-spin alkali clusters on helium droplets has been calculated within the model. The selection of high-spin states comes out to depend strongly on the experimental conditions, and is in general not pronounced for cluster sizes ≥ 3. The enhancement factor of 50 of high-spin states reported in earlier experiments is reproduced when choosing the conditions of these experiments.     

Heteronuclear and homonuclear high-spin alkali trimers on helium nanodroplets

The electronic excitation spectra of all possible homo- and heteronuclear high-spin (quartet) trimers of K and Rb (KxRb(3-x), x=0...3) assembled on the surface of superfluid helium droplets, are measured in the spectral range from 10,600 to 17,400 cm(-1). A regular series of corresponding bands is observed, reflecting the similar electronic structure of all these trimers. For the assignment and separation of overlapping bands, we determine x directly, with mass-selected beam depletion, and indirectly with a V-type double-resonance scheme. The assignment is confirmed by high-level ab initio calculations of the electronic structure of the bare trimers. The level structure is rationalized in terms of harmonic-oscillator states of the three valence electrons in a quantum-dot-like confining potential. We predict that three should be a magic number for high-spin alkali clusters.     

Tunable dipolar magnetism in high-spin molecular clusters

We report on the Fe17 high-spin molecular cluster and show that this system is an exemplification of nanostructured dipolar magnetism. Each Fe17 molecule, with spin S=35/2 and axial anisotropy as small as D approximately -0.02 K, is the magnetic unit that can be chemically arranged in different packing crystals while preserving both the spin ground state and anisotropy. For every configuration, molecular spins are correlated only by dipolar interactions. The ensuing interplay between dipolar energy and anisotropy gives rise to macroscopic behaviors ranging from superparamagnetism to long-range magnetic order at temperatures below 1 K.     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.     

Formation and stability of large B 6 O clusters with icosahedral structure

In this paper, the formation and the stability of large B₆O icosahedral particles was discussed on the basis of elastic deformation theory. Our calculation illustrate the stability of macroscopic Mackay packing B₆O icosahedral particles at high pressure. The transition pressure from rhombohedral structure of B₆O particles to macroscopic B₆O icosahedral ones was calculated to be 6 GPa, which is in good agreement with the experimental data (4.0-5.52 GPa). The maximum diameter of B₆O icosahedral particles at low pressure is estimated to be 200-300 nm. Received 30 November 2000     

Formation of polaron clusters

The formation of spherical polaron clusters is studied within the Fröhlich polaron theory. In a dilute polaron gas, using the non-local statistical approach and the polaron pair interaction obtained within the Pekar strong coupling theory, the homogeneous phase results to be unstable toward the appearance of polaron clusters. The physical conditions of formation for the clusters are determined calculating the critical values of electron-phonon interaction for which bound states in the collective polaron potential develop. Finally the sequence in the filling of the states is found and the stability of the clusters is assessed.Received: 6 May 2004, Published online: 12 October 2004PACS: 71.38.-k Polarons and electron-phonon interactions     

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.     

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   

A pseudo-hamiltonian model for small alkali metal clusters

The dissociation energies and equilibrium geometries of the molecules Na₃, Na₃ ⁺, K₃ and K₃ ⁺ are calculated using a simple model hamiltonian. The model was previously developed by Roach and Baybutt to explain an anomalous feature in the bonding of alkali metal diatomics. For polyatomics it is convenient to introduce a small gaussian expansion of Slater orbitals, and this also allows the use of an improved method of evaluating integrals over the core volumes. Although Na₃ and K₃ are only predicted to be metastable (like H₃), their calculated ionization potentials are in quite good agreement with experiment. The failure of the model for the other alkali metals is discussed.     

Stability of two-component alkali clusters formed on helium nanodroplets

The stability of two-component clusters consisting of light (Na or K) and heavy (Rb or Cs) alkali atoms formed on helium nanodroplets is studied by femtosecond laser ionization in combination with mass spectrometry. Characteristic stability patterns reflecting electron shell-closures are observed in dependence of the total number of atoms contained in the mixed clusters. Faster decay of the stability of mixed clusters compared to the pure light ones as a function of size indicates a destabilizing effect of heavy alkali atoms on light alkali clusters, presumably due to second order spin-orbit interaction.     

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.     

Spectroscopy of alkali atoms and molecules attached to liquid He clusters

Large helium clusters, ranging in size from a few hundred to several thousand atoms, are produced in a nozzle expansion. Combining this source with a pick-up scattering cell in which the clusters can be seeded with chromophores allows us to probe the influence of the helium environment on the atoms and molecules attached to the clusters. Using an alkali as chromophore we recorded laser induced fluorescence spectra of Na atoms and molecules attached to helium clusters. Apart from the spectrum of the Na monomer, we have found spectroscopic bands which can unambiguous be assigned to two bound Na atoms. The first of this bands is due to 1^{1 +} (A) 1¹ _g ⁺ (X) excitations of the covalently bound singlet Na₂ molecule while the second is due to 1³ _g ⁺ 1^{3 +} excitations for the van der Waals bound triplet Na₂ dimer. Both bands have been vibrationally resolved. Furthermore we found very large fluorescence intensities in the region 605–635 nm which are likely due to the excitation of a species containing three Na atoms attached to a helium cluster.     

Formation of metal-encapsulating Si cage clusters

We report the formation of a series of metal-containing hydrogenated silicon clusters using an ion trap. Mass analyses reveal that many types of transition metal ions M(+) ( M = Hf, Ta, W, Re, Ir, etc.) react with silane (SiH4) to form dehydrogenated MSi( +)(n) cluster ions ( n = 14, 13, 12, 11, 9, respectively) as an end product, indicating that the metal atom is endohedral and stabilizes the Si polyhedral cage. This finding is confirmed by our ab initio calculation that WSi12 is a W-encapsulating Si12 cage cluster, and is very stable owing to both the electronic and the geometrical shell closures.