Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in short laser pulses of 100 nm wavelength were performed. For comparison, the cluster response to 800 nm laser pulses was investigated as well. The inner ionization dynamics of multi-electron atoms inside the cluster was treated explicitly. The simulation results underpin the belief that at XUV (extreme ultraviolet) wavelengths collisions play an important role in the energy absorption. The generation of the surprisingly high charge states of Xe atoms inside clusters, as they were observed in a free-electron laser experiment at DESY, Hamburg, Germany by Wabnitz et al. is due to the reduced ionization potential of atoms inside charged clusters, the ionization ignition mechanism, and collisions. PACS 36.40.Gk; 36.40.Vz; 31.15.Gy     

Ionization of Clusters in Intense Laser Pulses through Collective Electron Dynamics

The motion of electrons and ions in medium-sized rare gas clusters ( approximately 1000 atoms) exposed to intense laser pulses is studied microscopically by means of classical molecular dynamics using a hierarchical tree code. Pulse parameters for optimum ionization are found to be wavelength dependent. This resonant behavior is traced back to a collective electron oscillation inside the charged cluster. It is shown that this dynamics can be well described by a driven and damped harmonic oscillator allowing for a clear discrimination against other energy absorption mechanisms.     

Orbital magnetism in axially deformed sodium clusters

Low-energy orbital magnetic dipole excitations, known as the scissors mode (SM), are studied in alkali metal clusters. Subsequent dynamic and static effects are explored. The treatment is based on a self-consistent microscopic approach using the jellium approximation for the ionic background and the Kohn-Sham mean field for the electrons. The microscopic origin of SM and its main features (structure of the mode in light and medium clusters, separation into low- and high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole plasmons, contributions of shape isomers, etc.) are discussed. It is shown that the scissors M1 strength acquires large values with increasing cluster size. The mode is responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum shell effects induce a fragile interplay between Langevin diamagnetism and van Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic susceptibility of particular light clusters. Finally, several routes for observing the SM experimentally are discussed.Received: 20 December 2002, Published online: 29 July 2003PACS: 36.40.-c Atomic and molecular clusters - 36.40.Cg Electronic and magnetic properties of clusters - 36.40.Gk Plasma and collective effects in clusters - 36.40.Vz Optical properties of clusters     

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     

Velocity dependence of metal cluster deposition on an insulating surface

We present a detailed study of the deposition of small sodium clusters on a NaCl surface. To that end, we use a microscopic model based on the Time-Dependent Local Density Approximation (TDLDA) for electrons, coupled to classical Molecular Dynamics (MD) for ions. We discuss in particular the deposition mechanism for different initial cluster velocities and demonstrate that extremely slow velocities are necessary to ensure a cluster deposition without damaging the original cluster.Received: 13 March 2004, Published online: 11 May 2004PACS: 36.40.Cg Electronic and magnetic properties of clusters - 36.40.Sx Diffusion and dynamics of clusters - 68.49.Fg Cluster scattering from surfaces     

Dynamics simulation on interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.     

Laser modification of silver nanoclusters in SiO2 thin films

Thin films of silica containing silver nanoclusters have been deposited by magnetron co-sputtering followed by thermal annealing. Laser modification of the mean cluster size was performed using the fourth harmonic of a Nd:YAG laser with energies of between 35 and 125 mJ/cm². The mean size of the clusters was estimated from the shape of the plasmon resonance band in the optical absorption spectra with the help of a computer simulation based on the Mie theory in static approximation. It was found that laser treatment with fluences above a certain threshold leads to a reduction of the mean size of the clusters and this reduction is greater for greater fluences. After a long treatment with the same fluence the effect saturates. The final mean size of the clusters after saturation depends only on the laser fluence and not on the initial mean cluster size. When lower laser fluences were used it was possible after laser annealing to return the mean cluster size to its initial value by thermal annealing. In this way by using a combination of laser treatment and thermal annealing a predetermined mean cluster size could be achieved. The mechanism of laser-induced cluster-size modification is discussed. PACS 81.07.-b; 42.62.-b; 36.40.Qv     

Dynamics in a cluster under the influence of intense femtosecond hard X-ray pulses

In this paper we examine the behavior of small cluster of atoms in a short (10-50 fs) very intense hard X-ray (10 keV) pulse. We use numerical modeling based on the non-relativistic classical equation of motion. Quantum processes are taken into account by the respective cross-sections. We show that there is a Coulomb explosion, which has a different dynamics than one finds in classical laser driven cluster explosions. We discuss the consequences of our results to single molecule imaging by the free electron laser pulses.Received: 22 September 2003, Published online: 9 March 2004PACS: 61.80.-x Physical radiation effects, radiation damage - 36.40.-c Atomic and molecular clusters - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Inverse-bremsstrahlung absorption of an intense laser field in cluster plasma

Inverse-bremsstrahlung absorption of an intense laser field in cluster plasma is considered in the Born approximation with allowance made for electron interaction with the entire subsystem of clustered ions. The electromagnetic power absorbed in plasma is calculated for linearly and circularly polarized laser radiations. It is shown that plasma “clustering” can give rise to much more effective absorption of electromagnetic energy as a result of collisions. The collective effects (the action of the overall field of clustered ions on an electron) dominate over the elementary processes (electron scattering by individual ions in the cluster) in the course of inverse bremsstrahlung.     

Interaction of intense laser pulses with hydrogen atomic clusters

The interaction between intense femtosecond laser pulses and hydrogen atomic clusters is studied by a simplified Coulomb explosion model. The dependences of average proton kinetic energy on cluster size, pulse duration, laser intensity and wavelength are studied respectively. The calculated results indicate that the irradiation of a femtosecond laser of longer wavelength on hydrogen atomic clusters may be a simple, economical way to produce highly kinetic hydrogen ions. The phenomenon suggests that the irradiation of femtosecond laser of longer wavelength on deuterium atomic clusters may be easier than that of shorter wavelength to drive nuclear fusion reactions. The product of the laser intensity and the squared laser wavelength needed to make proton energy saturated as a function of the squared cluster radius is also investigated. The proton energy distribution calculated is also shown and compared with the experimental data. Our results are in agreement with the experimental results fairly well.     

Nonlinear resonance absorption in the laser-cluster interaction

Rare-gas or metal clusters are known to absorb laser energy very efficiently. Upon cluster expansion, the Mie plasma frequency may become equal to the laser frequency. This linear resonance has been well studied both experimentally and theoretically employing pump probe schemes. In this work, we focus on the few-cycle regime or the early stage of the cluster dynamics, where linear resonance is not met but, nevertheless, efficient absorption of laser energy persists. By retrieving time-dependent oscillator frequencies from particle-in-cell simulation results, we show that nonlinear resonance is the dominant mechanism behind outer ionization and energy absorption in near infrared laser-driven clusters.     

Collective electronic excitations in clusters in the vicinity of metal surfaces

We study the collective excitations of metal clusters approaching a metal surface. Using a simple model for the frequency-dependent dielectric constant ε(ω) and the multiple scattering method, we numerically investigate the shift in the plasmon resonance due to the coupling of the collective modes of the sphere with those of its mirror image. Results of the model calculation are verified by means of ab initio theory. As a prototype system we study Na₉ ⁺ cluster on the Cu(100) surface. The representation of the solid surface by a cluster of several, typically 54 substrate atoms, is used in combination with a high level configuration interaction (CI) calculation. PACS 31.15.Dv; 36.40.Gk; 73.20.-f     

Sidebands in the light absorption of driven metallic nanoparticles

The dynamics of the surface plasmon in laser-driven metallic nanoparticles is described by means of a master-equation formalism. Within the Markov approximation, the dynamics is studied for different regimes ranging from weak excitation in photoabsorption experiments to strong excitation in pump-probe spectroscopy. It is shown that two collective levels are sufficient to describe the dynamics of the surface plasmon. On this basis, we predict the appearance of sidebands in the absorption spectrum of the probe laser field in pump-probe experiments.     

Ion spectrum under excitation of a cluster beam by a laser pulse

A model describing the decay dynamics of large charged clusters formed during the interaction of a short laser pulse with a cluster beam is constructed. It is shown that the dynamics of the evolution of the cluster plasma and the energy spectrum of ions are affected by the presence of the decay products from neighboring clusters and the electron background due to ionization. The parameters of the plasma being formed are determined as functions of the cluster beam and laser pulse parameters.     

Plasmon-enhanced electron acceleration in intense laser metal-cluster interactions

We have measured the energy and angular-resolved electron emission from medium-sized silver clusters (N approximately 500-2000) exposed to dual laser pulses of moderate intensity (I approximately (10(13-14) W/cm2). When the second pulse excites the plasmon resonantly, we observe enhanced emission along the laser polarization axis. The asymmetry of the electron spectrum is strongly increasing with electron energy. Semiclassical simulations reveal the following mechanism: Electrons bound in highly excited states can leave, return to, and traverse the cluster. Those electrons that return at zero plasmon deflection and traverse the cluster during a favorable plasmon half-cycle can experience maximum acceleration by the evolving polarization field. As a result of these constraints energetic electrons are emitted in direction of the laser polarization axis in subcycle bursts.     

Distributions of ions in a cluster plasma created by a laser pulse

Energy and charge distributions of ions are calculated for a cluster beam irradiated by a high-power ultrashort laser pulse. It is shown that the self-consistent field of a cluster ionized by the laser beam strongly affects the characteristics of the ion distributions obtained after the cluster explodes. The mean concentration of atoms bound into clusters in a beam, the cluster size distribution, and the focal-spot diameter are found to have a weak effect on both energy and charge distributions of the ions, whereas the energy spectrum of the produced ions is determined by the mean cluster size.     

Size dependence of the plasmon peak position in electron stimulated photon emission spectra of Ag clusters supported on amorphous carbon film

The plasmon energy of Ag clusters produced on an amorphous carbon substrate by gas-aggregation technique has been measured. It has been determined from the plasmon peak position in the light emission spectrum obtained during bombardment of Ag clusters by low-energy electrons. For Ag cluster films with maximum of the cluster size distribution at 30, 8 and 2.5 nm, the plasmon energy comprised 3.76, 4.13 and 4.28 eV (the wavelength was 330, 300 and 290 nm), respectively. The blue shift of the plasmon energy is probably related to the effect of confounding of collective and single-particle excitations.     

Angular distribution of ejected electrons at the laser-cluster interactions

The histograms of deflection angles of electrons ejected from Xe clusters irradiated by femtosecond super-intense laser pulses are presented. The dependence of the angular distribution on the peak laser intensity, the pulse duration, and the cluster position is considered. A clear relationship between the final electron energy and the deflection angle is shown. The deflection angles are calculated by solving the relativistic equation of motion taking into account the Lorentz force and the Coulomb field of the ionized cluster. The ions in the cluster undergo sequential multiple ionization up to charge multiplicity Z = 26. The measurements of the electron angular distributions allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.     

Three-dimensional simulation on explosions of hydrogen atomic clusters irradiated by an intense femtosecond laser pulse

Using classic particle dynamics simulations, the interaction process between an intense femtosecond laser pulse and icosahedral hydrogen atomic clusters H_{13}, H_{55} and H_{147} has been studied. It is revealed that with increasing number of atoms in the cluster, the kinetic energy of ions generated in the Coulomb explosion of the ionized hydrogen clusters increases. The expansion process of the clusters after laser irradiation has also been examined, showing that the expansion scale decreases with increasing cluster size.     

强激光场中氘团簇的离子能谱与团簇尺度

在SILEX-Ⅰ装置上进行了超短超强激光与氘团簇相互作用的实验。采用法拉第筒测量了激光与氘团簇靶相互作用后发射出的氘离子能谱,并根据氘离子能谱计算出氘团簇的尺寸和分布。由于激光预脉冲的影响,得到了团簇的尺度比瑞利散射测量的结果小。这些结果对团簇源制备系统的改进和优化激光打靶有着重要的参考意义,而且可以为团簇尺度的测量提供一种新的方法。