Coupled plasmon and phonon dynamics in embedded Na clusters

We investigate, from a theoretical perspective, the coupled electronic and ionic/atomic dynamics of Na clusters embedded in Ar matrices. The system is described by time-dependent density-functional theory for cluster electrons and classical motion for Na⁺ ions as well as for Ar atoms. The interaction with the surrounding Ar atoms is modelled by polarization potentials plus core repulsion. We use this model to study coupled electronic and ionic/atomic motion in embedded clusters following a very short laser pulse. For excitations in the non-linear regime, we find clear signs for the coherent coupling of the Mie plasmon resonance with ionic vibrations (phonons). In addition, an incoherent line stretching is observed which can be traced back to the turning point of ionic vibrations. The coupling to the atomic motion of the surroundings leads to a slow and far reaching rearrangement of the matrix. PACS 36.40.Gk; 36.40.Vz; 31.15.EW     

Linear and non-linear response of embedded Na clusters

We investigate Na clusters embedded in Ar matrices. The surrounding Ar atoms are modeled in terms of their dynamical polarizability and the strong electron repulsion. The calibration of the model is discussed. First results for the non-linear optical response of the Na clusters are presented for the test case of Na₈ embedded in Ar ensembles of different sizes. It is shown that blue shift through core repulsion and red shift through dipole polarizability counterweight each other to the end that very little global shift is seen in the spectra. This feature persists to all excitation strengths considered. There are, however, detailed effects, such as for example the Landau fragmentation of the Mie plasmon peak. PACS 36.40.Gk; 36.40.Vz; 31.15.Ew     

On fermionic stability of Vlasov descriptions of finite Coulomb systems

We explore from a numerical point of view the validity of the Vlasov equation as a semi-classical approximation of time-dependent Hartree-Fock and time-dependent LDA theories, in terms of the survival of the Pauli principle. The fermionic properties are investigated by using a Na₉⁺ cluster as test case and solving the Vlasov equation with the test particle method. This allows to derive a time span for which the Vlasov equation provides an acceptable approximation.     

QCT-based vibrational collisional models applied to nonequilibrium nozzle flows

We investigate from a theoretical perspective structure and dynamics of Na clusters on a surface built from Ar layers grown on a metal support. The system is modeled by a hierarchical quantum-mechanical/molecular-mechanical (QM/MM) approach treating the cluster electrons with time-dependent density-functional theory, the Ar atoms classically, and the metal support as a continuous dielectric medium. Caution has been taken to describe properly the dynamical polarizability of the Ar substrate. We study the effect of the Ar substrate and particularly of the metal support on the cluster structure and dynamics. The binding of Na₆ and Na₈ to the Ar surface is found to by very weak and the effect of the dielectric response of the metal (DRM) turns out to be negligible. The global properties of the optical response of the Na clusters are slightly changed by the Ar substrate and the DRM while the detailed spectral fragmentation depends sensitively on any change of the environment. The deposition dynamics of small Na clusters is crucially influenced by the mechanical hardness of the metal support while the DRM makes little effect. We also study the dependence on the number of Ar layers. For the first few layers (from two to four), the deposition dynamics changes dramatically with the number of layers. The results stabilize from six layers on upwards.     

Effect of helium/argon gas ratio in a He-Ar-Cu<Superscript>+</Superscript> IR hollow-cathode discharge laser: modeling study and comparison with experiments

The He-Ar-Cu⁺ IR laser operates in a hollow-cathode discharge, typically in a mixture of helium with a few-% Ar. The population inversion of the Cu⁺ ion levels, responsible for laser action, is attributed to asymmetric charge transfer between He⁺ ions and sputtered Cu atoms. The Ar gas is added to promote sputtering of the Cu cathode. In this paper, a hybrid modeling network consisting of several different models for the various plasma species present in a He-Ar-Cu hollow-cathode discharge is applied to investigate the effect of Ar concentration in the gas mixture on the discharge behavior, and to find the optimum He/Ar gas ratio for laser operation. It is found that the densities of electrons, Ar⁺ ions, Ar_m ^* metastable atoms, sputtered Cu atoms and Cu⁺ ions increase upon the addition of more Ar gas, whereas the densities of He⁺ ions, He₂ ⁺ ions and He_m ^* metastable atoms drop considerably. The product of the calculated Cu atom and He⁺ ion densities, which determines the production rate of the upper laser levels, and hence probably also the laser output power, is found to reach a maximum around 1–5 % Ar addition. This calculation result is compared to experimental measurements, and reasonable agreement has been reached. Received: 14 October 2002 / Revised version: 28 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +32-3/820-23-76, E-mail: annemie.bogaerts@ua.ac.be     

Electron-stimulated desorption of sodium atoms initiated by their reverse motion

This paper reports on the first measurement of the yield and energy distributions of sodium atoms in electron-stimulated desorption at T = 160 K from sodium layers adsorbed on tungsten with a gold film atop. The Na atom yield has a resonant pattern with an appearance threshold of 30 eV, which can be attributed to exciton excitation in the Na 2p level. The Na yield is associated with the formation of a semiconducting Na _x Au _y film at T ∼ 300 K and sodium and gold coverages in excess of one monolayer. Sodium atoms are desorbed through Auger neutralization of Na²⁺ ions in their reverse motion toward the surface and is limited by the resonant ionization of Na atoms as they pass through the adsorbed layer of Na⁺ ions. The energy distributions of Na atoms are bell shaped with a maximum at about 0.56 eV.     

Numerical study of the effect of gas flow in low pressure inductively coupled Ar/N<Subscript>2</Subscript> plasmas

The effect of gas flow in low pressure inductively coupled Ar/N₂ plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5–700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N₂ molecules and their seven different excited levels, N atoms, and Ar⁺, N⁺, N₂ ⁺, N₄ ⁺ ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.     

Surface analysis by ion-electron spectroscopy; Silicon target

The energy spectra of secondary elections emitted from a Si(111) surface due to bombardment by 6 keV He⁺ and O⁺₂ ions have been examined. The fine structure in the spectra is explained on the basis of a novel mechanism of creation of Auger electrons at the surface. There are two stages of interaction between incoming ions and the substrate via adsorbed atoms. In the first stage, due to a level promotion mechanism, vacancies in the adsorbed atoms are created. In the second stage, Auger neutralization processes accompanied by the emission of electrons from a solid with characteristic energies take place. These electrons provide a good indication of the degree of coverage of the silicon surface with contaminant atoms. The energy losses of escaping electrons are also discussed.     

Cold Ar 3 + generated by ionization of argon clusters in large helium clusters: Temperature dependence of the photofragmentation

Ionized argon clusters were generated by electron impact ionization of neutral argon clusters embedded in large neutral helium clusters. Photofragmentation spectroscopy of Ar ₃ ⁺ and Ar ₃ ⁺ He produced in this way demonstrates the strong influence of vibrational excitation on the photodissociation dynamics, and indicates the low internal energy of the latter cluster.     

Cluster modelling of O(1s) core excitons at the (100) surface of alkaline-earth oxides

Calculations have been performed on the O(1s) X-ray absorption spectra of the O^2- ion in the series of alkaline-earth oxides MgO, CaO, SrO. The static exchange approach has been used to calculate the near-edge X-ray absorption fine structure (NEXAFS) spectra for a range of cluster models (up to 50 atoms), representing both the (100) surface and the bulk of the different lattices. The clusters have been embedded in a proper representation of both the surrounding crystal ions and the Madelung potential generated by the infinite crystal. In the surface clusters discrete levels lying around 1.5 eV below the calculated ionization potential were found and characterized. This surface state has been studied further by performing complete active space self-consistent field (CASSCF) calculations in which the polarization of the crystal ions has been taken into account by means of a core polarization potential (CPP) method as well as the Mott-Littleton approach. After inclusion of this potential, the calculated ionization potentials show good agreement with XPS measurements.     

The violet emissions produced by laser excitation of Na vapor in the 570–595 nm region

We have observed atomic Na and molecular Na₂ emissions in the violet region when Na vapor in a heatpipe (～10³—10¹⁶ atoms/cm³) is irradiated with a pulsed dye laser with output wavelength in the 570–597 nm region. The Na atomic emissions probably result from recombination of Na⁺ + e^- and energy-pooling involving highly excited atoms and molecules, while the diffuse violet emission bands are probably produced through collisions among excited Na atoms and Na₂ molecules.     

The structures and stabilities of mixed inert gas cluster ions: NeHe+ n and ArHe+ n

DIM-type matrices for RgHe⁺ _n clusters have been established from the results of ab initio calculations on RgHe and RgHe⁺ (Rg = Ne and Ar). The method has been tested against ab initio calculations on linear and T shaped RgHe⁺ ₂ and found to be satisfactory. Rotational invariance has been established for larger clusters, and the geometries and energies of clusters up to n = 16 have been determined. Ne cluster ions are more stable than Ar cluster ions because of the greater contribution from charge transfer, and are structurally different. The relative stabilities of these cluster ions are consistent with the available experimental data.     

Hindered Coulomb explosion of embedded Na clusters — stopping,shape dynamics and energy transport

We investigate the dynamical evolution of a Na₈ cluster embedded in Ar matrices of various sizes from N=30 to 1048. The system is excited by an intense short laser pulse leading to high ionization stages.We analyze the subsequent highly non-linear motion of cluster and Ar environment in terms of trajectories, shapes, and energy flow. The most prominent effects are: temporary stabilization of high charge states for several ps, sudden stopping of the Coulomb explosion of the embedded Na₈ clusters associated with an extremely fast energy transfer to the Ar matrix, fast distribution of energy throughout the Ar layers by a sound wave. Other ionic-atomic transfer and relaxation processes proceed at slower scale of few ps. The electron cloud is almost thermally decoupled from ions and thermalizes far beyond the ps scale.     

Molecular rearrangement and cluster formation in the secondary ion mass spectra (SIMS) of fluoride salts

Secondary ion clusters with mass greater than 700 amu, e.g., K(KF)₁₂⁺ and up to 27 atoms, e.g., Na(NaF)₁₃⁺, have been observed in the static SIMS spectra of MF (M = Li, Na, K), NaBF₄, and KPF₆. The long series of detected cluster ions of the type M(MF)_n⁺ indicates that there is a high degree of stability associated with these clusters. The observation of such clusters in the NaBF₄ and KPF₆ spectra suggest that there is significant molecular rearrangement occurring in the secondary ion emission process from such salts. The secondary ion Intensities provide a crude fit to the Saha-Eggert equation, yielding an electron temperature of ~12,000 K. The data are consistent with the plasma model of surface ionization in which rearrangement and cluster formation occur in the plasma.     

Manipulation of optical properties of Ag/Cu alloy nanowire arrays embedded in anodic alumina membranes

Arrays of Ag/Cu alloy nanowires embedded in anodic alumina membranes (AAMs) were synthesized by directly electrodepositing from a mixing electrolyte solution containing Ag⁺ and Cu²⁺ ions. Manipulations of optical properties of the resulting samples were successfully achieved by tuning the molar ratio of Ag⁺ and Cu²⁺ ions in the starting materials. When the ratio is less than 2:20, two surface plasma resonance (SPR) peaks corresponding to Ag and Cu appear, respectively. After annealing treatment, the SPR peak corresponding to Cu disappears, and that of Ag presents a red shift. Furthermore, this red shift can be up to 85 nm when the molar ratio of Ag⁺ and Cu²⁺ reduce to 1:20, which is attributed to the transferable electrons from Cu atoms.     

Production of Na Auger electrons by Na+ bombardment of Ag surface

In this paper it has been shown that for the La₂₃MM emission of Na Auger electrons in the case of low enregy Na⁺ ion interaction with the Ag surface contaminated with Na, symmetric collisions (Na → Na) are responsible.     

Influence of ion–ion correlation on Na+ transport in Na2Ni2TeO6: molecular dynamics study

Molecular dynamics (MD) study of Na⁺ transport in Na₂Ni₂TeO₆ is performed systematically with varying strength of Na⁺–Na⁺ short range repulsions to understand the physical principle governing ion transport mechanism. Na⁺ diffusion is enhanced by nearly an order of magnitude with reduced Na⁺–Na⁺ short range repulsion within the studied range. A similar behavior is also observed in other systems, e.g., AgI and Na₂Zn₂TeO₆, where mobile ions are located closely. The Na⁺ ion occupancy in Na₂Ni₂TeO₆ shows a significant shift from Na1 to Na2 sites gaining some degree of correlation. The study also emphasizes how mobile ion size influences the ionic diffusion. The fresh insight such as microscopic migration pathways, energy barriers, and jumping mechanism of Na⁺ are derived from the study.     

MCsn cluster formation on organic surfaces: A novel way to depth-profile organics?

In this work, we investigated the bombardment of polymer surfaces (PC, PMMA) with very low energy (250 eV) Cs⁺ ions.In the positive mode, we observed many cations combining a molecular fragment of the polymer (M) with two Cs atoms, similar to the well-known MCs₂⁺ clusters commonly observed in inorganic depth profiling with Cs. For example, Cs₂COOH⁺ or Cs₂C₆H₅O⁺ were measured on PC and Cs₂CH₃O⁺ or Cs₂C₄H₅O₂⁺ were measured on PMMA. In fact, most of the polymer characteristic fragments measured in negative spectra also appear in the positive spectra, combined with two Cs atoms. It is remarkable that stable negative ions tend to combine not with one Cs, but with two Cs atoms to create the MCs₂⁺ cluster.This effect is, to some extent, similar to the well-known cationisation of polymers (with Ag or Au) although here few clusters with only one Cs atom are detected (MCs⁺ clusters), like in classical cationisation. The most abundant clusters are the MCs₂⁺ clusters, but heavier clusters (MCs₃⁺, MCs₄⁺ and above) are also measured with high yields in the spectrum.Surprisingly, some of those MCs₂⁺ clusters were still detected even after a very long sputtering fluence (above 10¹⁷ ions/cm²), proving that some molecular depth profiling is also possible in this “Cs₂-cationisation” mode. In other words, this work could open the way to an extension of the MCs_n⁺ cluster analysis, commonly used in inorganic depth profiling, to the in-depth molecular analysis of organic layers.