Scattering of a proton with the Li4 cluster: non-adiabatic molecular dynamics description based on time-dependent density-functional theory

We have employed non-adiabatic molecular dynamics based on time-dependent density-functional theory to characterize the scattering behaviour of a proton with the Li$_4$ cluster. This technique assumes a classical approximation for the nuclei, effectively coupled to the quantum electronic system. This time-dependent theoretical framework accounts, by construction, for possible charge transfer and ionization processes, as well as electronic excitations, which may play a role in the non-adiabatic regime. We have varied the incidence angles in order to analyze the possible reaction patterns. The initial proton kinetic energy of 10 eV is sufficiently high to induce non-adiabatic effects. For all the incidence angles considered the proton is scattered away, except in one interesting case in which one of the Lithium atoms captures it, forming a LiH molecule. This theoretical formalism proves to be a powerful, effective and predictive tool for the analysis of non-adiabatic processes at the nanoscale.Comment: 18 pages, 4 figure

Modeling Na clusters in Ar matrices

We present a microscopic model for Na clusters embedded in raregas matrices. The valence electrons of the Na cluster are described by time-dependent density-functional theory at the level of the local-density approximation (LDA). Particular attention is paid to the semi-classical picture in terms of Vlasov-LDA. The Na ions and Argon atoms are handled as classical particles whereby the Ar atoms carry two degrees of freedom, position and dipole polarization. The interaction between Na ions and electrons is mediated through local pseudo-potentials. The coupling to the Ar atoms is described by (long-range) polarization potentials and (short-range) repulsive cores. The ingredients are taken from elsewhere developed standards. A final fine-tuning is performed using the NaAr molecule as benchmark. The model is then applied to embedded systems Na8ArN. By close comparison with quantum-mechanical results, we explore the capability of the Vlasov-LDA to describe such embedded clusters. We show that one can obtain a reasonable description by appropriate adjustments in the fine-tuning phase of the model.Comment: 17 pages, 7 figures, submitted to Annalen der Physi

Two-Photon Excitation of Low-Lying Electronic Quadrupole States in Atomic Clusters

A simple scheme of population and detection of low-lying electronic quadrupole modes in free small deformed metal clusters is proposed. The scheme is analyzed in terms of the TDLDA (time-dependent local density approximation) calculations. As test case, the deformed cluster $Na^+_{11}$ is considered. Long-living quadrupole oscillations are generated via resonant two-photon (two-dipole) excitation and then detected through the appearance of satellites in the photoelectron spectra generated by a probe pulse. Femtosecond pump and probe pulses with intensities $I = 2\cdot 10^{10} - 2\cdot 10^{11} W/cm^2$ and pulse duration $T = 200 - 500$ fs are found to be optimal. The modes of interest are dominated by a single electron-hole pair and so their energies, being combined with the photoelectron data for hole states, allow to gather new information about mean-field spectra of valence electrons in the HOMO-LUMO region. Besides, the scheme allows to estimate the lifetime of electron-hole pairs and hence the relaxation time of electronic energy into ionic heat.Comment: 4 pages, 4 figure

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

We investigate the dynamical evolution of a Na$_8$ 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 Na$_8$ 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.Comment: 12 pages, 10 figures, accepted in Euro. Phys. J.

Directed transport born from chaos in asymmetric antidot structures

It is shown that a polarized microwave radiation creates directed transport in an asymmetric antidot superlattice in a two dimensional electron gas. A numerical method is developed that allows to establish the dependence of this ratchet effect on several parameters relevant for real experimental studies. It is applied to the concrete case of a semidisk Galton board where the electron dynamics is chaotic in the absence of microwave driving. The obtained results show that high currents can be reached at a relatively low microwave power. This effect opens new possibilities for microwave control of transport in asymmetric superlattices.Comment: 8 pages, 10 figure

Electric multipole plasmons in deformed sodium clusters

The random-phase-approximation (RPA) method with separable residual forces (SRPA) is proposed for the description of multipole electric oscillations of valence electrons in deformed alkali metal clusters. Both the deformed mean field and residual interaction are derived self-consistently from the Kohn-Sham functional. SRPA drastically simplifies the computational effort which is urgent if not decisive for deformed systems. The method is applied to the description of dipole, quadrupole and octupole plasmons in deformed sodium clusters of a moderate size. We demonstrate that, in clusters with the size N>50, Landau damping successfully competes with deformation splitting and even becomes decisive in forming the width and gross structure of the dipole plasmon. Besides, the plasmon is generated by excitations from both ground state and shape isomers. In such clusters familiar experimental estimates for deformation splitting of dipole plasmon are useless.Comment: 27 pages, 10 figure

Hartree-Fock dynamics in highly excited quantum dots

Time-dependent Hartree-Fock theory is used to describe density oscillations of symmetry-unrestricted two-dimensional nanostructures. In the small amplitude limit the results reproduce those obtained within a perturbative approach such as the linearized time-dependent Hartree-Fock one. The nonlinear regime is explored by studying large amplitude oscillations in a non-parabolic potential, which are shown to introduce a strong coupling with internal degrees of freedom. This excitation of internal modes, mainly of monopole and quadrupole character, results in sizeable modifications of the dipole absorption.Comment: 4 pages, 4 embedded figure