Ionization dynamics of simple metal clusters in intense fields by the Thomas-Fermi-Vlasov method

The time-dependent response of simple metal clusters to femtosecond laser pulses is investigated using the semiclassical theory based on the Vlasov equation. Starting from a Thomas-Fermi ground state the dynamics are calculated by use of the pseudoparticle method. Systems studied here are sodium clusters containing up to 147 atoms. Both, the energy transfer to the cluster, which is largely affected by the plasmon enhanced absorption, and the following release of energy to the ions are examined in detail. During the laser excitation the feedback of the absorption to the development of the plasmon energy is controlled by competing mechanisms: ionization and cluster expansion. Characteristics of the Coulomb explosion are studied as function of photon energy and cluster size, particularly with regard to the dynamical influence of collective excitations of the electrons. We also predict features in the angular distribution of the ions that could be measured to test the calculated dynamics.Received: 9 December 2003, Published online: 16 March 2004PACS: 52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.) - 36.40.Vz Optical properties of clusters - 36.40.Gk Plasma and collective effects in clusters     

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.     

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     

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.     

Ultrafast nuclear dynamics and non-uniform Coulomb explosion of heteroclusters

This paper reports on some unique features of the ion spatial distribution, energetics and time-resolved dynamics in Coulomb explosion of multicharged light-heavy heteroclusters, consisting of light, low-charge and heavy, high-charge, ions, e.g. hydroiodic acid [image omitted] and its isotopic substituents [image omitted] and [image omitted]. In these clusters, extreme multielectron ionization in ultraintense laser fields (peak intensity I = 1015 - 1020 W cm-2) results in highly charged heavy ions, e.g. qI ? 7 at I = 6×1015 W cm-2 and qI = 25 at I = 1019 W cm-2. Molecular dynamics simulations based on the cluster vertical ionization (CVI) initial conditions, together with complete simulations involving both electron and nuclear dynamics of heteroclusters subjected to a Gaussian laser pulse, which were conducted for Coulomb explosion of [image omitted] and [image omitted] ionic clusters, reveal expanding, thin, two-dimensional spherical shells of the light D+ or H+ ions, with the monolayer expansion occurring on the femtosecond time scale. The expanding spherical nanoshells of light ions are analogous to a 'soap bubble', characterized by negative surface tension and driven by Coulomb pressure. The energetic data for the light ions reveal high energies with a narrow energy distribution, characterized by a lower energy cut-off, e.g. average energy Eav = 23 keV at width ΔE/Eav = 0.16, and a cut-off energy of EMIN = 19.2 keV for Coulomb explosion of [image omitted] clusters. These dynamic, structural and energetic data for exploding multicharged light-heavy heteroclusters arise from kinematic overrun effects of the light ions.     

Low-energy structure in the ionization of argon: Comparison of experiment with theory

The above-threshold ionization of argon in an intense 70-fs,400-nm linearly polarized laser pulse has been investigated by the velocity map imaging techniques,combined with an attosecond-resolution quantum wave packet dynamics method.There is a quantitative agreement in all dominant features between the experiment and the theory.Moreover,a peak-splitting phenomenon in the first energy peak has been observed at high pulse intensity.Further,through the theoretical analysis,an ac Stark splitting with evident resonant and nonresonant ionization pathways has been found to be the physical reason for the experimental observations.     

Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions also have a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and charge-carrier transport. PACS 52.38.Mf; 68.34.Tj; 68.35.Rh; 79.20.Ds     

超短强激光脉冲激励甲烷团簇的内电离机理

采用三维粒子动力学模拟方法研究了甲烷团簇在超短强激光脉冲激励下的爆炸动力学行为,重点讨论了几种典型的内电离机理对团簇爆炸过程中离子的价态和动能的影响.研究表明,在激光脉冲强度比较小的情况下,团簇中的原子主要是在光场作用下通过隧道电离的方式发生电离.当激光场进一步增强时,势垒压低电离是电离的主要方式.在相同的较高激光强度下,团簇更容易通过势垒压低电离达到高的电离价态.团簇发生电离后,其内部库仑电场的点火电离效应和内部滞留自由电子的碰撞电离效应也将增强团簇的再次电离过程. 关键词： 超短强激光脉冲 甲烷团簇 内电离     

Surface heating of deuterium clusters by the field of a superintense ultrashort laser pulse for implementing the nuclear reaction d+d → 3He+n

A new mechanism for heating the electron component of plasmas formed upon the application of a superintense ultrashort laser pulse to atomic clusters is proposed. Clusters considered here consist of deuterium atoms. Upon the emission of a large number of electrons, an irradiated cluster, which acquires a positive charge, explodes (Coulomb explosion). Deuterons that are ejected as the result of this possess high kinetic energies, so that collisions between them can result in ³He formation accompanied by neutron emission. The new mechanism of the heating of the electron plasma from clusters is based on the conjecture that, when an ionization electron is reflected from the inner surface of the cluster ion in the presence of a laser field, it predominantly absorbs (rather than emits) laser photons.     

TDDFT study of excitation of water molecules with short laser pulses

We explore the excitation of water molecules subject to short and intense laser pulses in the frame of time-dependent density function theory (TDDFT) at the level of the time-dependent local-density approximation (TDLDA), applied to valence electrons, coupled non-adiabatically to molecular dynamics (MD) of ions. We first study the optical absorption spectra of the water molecule as an observable in the "linear" domain and results are in good agreement with experiments. We then explore the influence of the laser frequency on the excitation. It is found that when the laser frequency is off-resonant or highly above the resonant region, the excitations are weak whereas for the resonant frequency case, the ionization is enhanced and bond lengths are enlarged. Furthermore, a direct coupling of ions with the laser pulse with the off-resonant frequency is found when investigating the OH bond lengths. We finally study the effect of laser intensity on the excitation of H 2 O and it is found that ionization increases when the laser intensity varies from low to high and we observe stable vibrations to Coulomb fragmentation when the ionization is up to typically two more charge units.     

飞秒强激光脉冲中氩团簇库仑爆炸特性研究

利用Bathe模型,理论模拟了氩团簇在飞秒强激光中(100 fs, 1016 W/cm2)的电离和爆炸过程.研究结果显示,在团簇尺寸较小时,离子平均能量与团簇初始半径平方成正比,爆炸机制为典型的库仑爆炸.随着团簇尺寸的增加,能量增加的速度趋缓并在一定团簇尺寸后趋于饱和.模拟结果与实验数据有较好的吻合.     

Femtosecond surface plasmon resonance dynamics and electron-electron interactions in silver nanoparticles

Femtosecond excitation and relaxation of nonequilibrium electrons are investigated in silver clusters using a two color pump-probe technique with resonant excitation of the surface plasmon resonance and off resonant probing. The excitation process is shown to be identical to that in metal films, and permits creation of a strongly athermal single electron excitation in a time scale shorter than the duration of the pulses (25-30 fs), in agreement with the free-electron absorption model. Following the time evolution of the nonequilibrium distribution yields information on the internal energy redistribution dynamics of the conduction electrons and of its modification by confinement in metal clusters. Received 1st December 2000     

团簇在超强超短脉冲激光场中的演化

 利用低温脉冲气阀产生氘团簇束,在SILEX-Ⅰ激光装置上开展实验,研究氘团簇在超强超短脉冲激光场中的演化过程,获得了数十keV的高能氘离子,这些氘离子的能谱分布与库仑爆炸模型计算结果一致。实验结果表明,在一定的激光功率密度条件下,团簇的平均尺度决定了释放出高能离子的能谱分布。激光辐照团簇后,通过阈上电离部分电子逃逸团簇,随着初始电离电子屏蔽作用加强,碰撞电离变成了团簇的主要电离机制。     

Probing time delay of strong-field resonant above-threshold ionization

The high-resolution three-dimensional photoelectron momentum distributions via above-threshold ionization(ATI)of Xe atoms are measured in an intense near circularly polarized laser field using velocity map imaging and tomography reconstruction. Compared to the linearly polarized laser field, the employed near circularly polarized laser field imposes a more strict selection rule for the transition via resonant excitation, and therefore we can selectively enhance the resonant ATI through certain atomic Rydberg states. Our results show the self-reference ionization delay, which is determined from the difference between the measured streaking angles for nonadiabatic ATI via the 4 f and 5 f Rydberg states, is 45.6 as. Our method provides an accessible route to highlight the role of resonant transition between selected states, which will pave the way for fully understanding the ionization dynamics toward manipulating electron motion as well as reaction in an ultrafast time scale.     

Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two‐photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q‐switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.     

Dopant-induced ignition of helium nanodroplets in intense few-cycle laser pulses

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle (~10 fs) laser pulses. We find that less than 10 dopant atoms "ignite" the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs.     

Dipole blockade at Förster resonances in high resolution laser excitation of Rydberg states of cesium atoms

High resolution laser excitation of np Rydberg states of cesium atoms shows a dipole blockade at F?rster resonances corresponding to the resonant dipole-dipole energy transfer of the np+np --> ns+(n+1)s reaction. The dipole-dipole interaction can be tuned on and off by the Stark effect, and such a process, observed for relatively low n(25-41), is promising for quantum gate devices. Both Penning ionization and saturation in the laser excitation can limit the range of observation of the dipole blockade.     

Ultrafast dynamics of all-trans--carotene explored by resonant and nonresonant photoexcitations

Excitation energy dependence of transmittance change has been investigated in -carotene. The signal induced by nonresonant excitation is ascribed to the ac Stark effect and the two-photon absorption of the excitation and probe pulses in three-level systems. The ultrafast response following resonant excitation is assigned to the two-photon absorption and the transient absorption of the photogenerated S(2) state with a lifetime of 150 fs. The long-debated S(2)-S(1) relaxation in beta-carotene can be explained by a two-state model (S(2), S(1)) without involving any intermediate states.     

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stückelberg-like oscillations of measurable quantities, such as the electron emission yield. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001