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     

Mechanism of negative ion emission from surfaces of ferroelectrics

Experimental studies of the mechanism of negative ion and cluster ion emission from surfaces of ferroelectrics are described. The emission was produced by negative voltage pulses with the amplitude of about 400 V, with a rapid rise-time (below 10 ns) and a slow decay‐time (several μs). The pulses were applied between the back side of the ferroelectric sample and the metal tip touching the front emitting side. The surface of the ferroelectrics could be cleaned in situ by 1 keV Ar⁺ bombardment. The morphologic changes around the tip were observed with an atomic force microscope (AFM). Mostly negative ions and cluster ions were emitted and studied in our experiments. Positive ions were detected with much lower probability and are produced by an entirely different microscopic process than negative ions. Masses as well as energies of emitted ions were measured with a time-of-flight (TOF) spectrometer and compared with available spontaneous desorption (SD) spectra and Cs‐SIMS spectra in order to clarify the mechanism of the emission. The trajectory of ions emitted from the sample was studied by computer simulation. The conclusion of these studies is that the negative ion emission is caused by the Coulomb explosion of a polarization cloud rapidly formed at the front edge of the pulse. The explosion takes place in the vicinity of the tip-sample contact at distances several tens of μm from the contact where the stabilizing effect of the positively charged tip is already small.     

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.     

Distribution and evolution of electrons in a cluster plasma created by a laser pulse

We analyze the properties and the character of the evolution of an electron subsystem of a large cluster (with a number of atoms n～10⁴?10⁶) interacting with a short laser pulse of high intensity (10¹⁷?10¹⁹ W/cm²). As a result of ionization in a strong laser field, cluster atoms are converted into multicharged ions, part of the electrons being formed leaves the cluster, and the other electrons move in a self-consistent field of the charged cluster and the laser wave. It is shown that electron-electron collisions are inessential both during the cluster irradiation by the laser pulse and in the course of cluster expansion; the electron distribution in the cluster therefore does not transform into the Maxwell distribution even during cluster expansion. During cluster expansion, the Coulomb field of a cluster charge acts on cluster ions more strongly than the pressure resulting from electron-ion collisions. In addition, bound electrons remain inside the cluster in the course of its expansion, and cluster expansion therefore does not lead to additional cluster ionization.     

Peculiarities of pulsed ion implantation from a laser plasma containing multiply charged ions

A mathematical model describing the dynamics of a pulsed laser plasma with multiply charged ions, as well as the formation of the accelerated ion flow in an external magnetic field, is developed. Experimental studies and mathematical simulation by the particle-in-cell method are used to determine the role of multiply charged ions in the process of ion implantation into a silicon substrate from the pulsed plasma containing singly and doubly charged titanium ions. The plasma spreads between parallel-plate electrodes (Ti target and Si substrate) along the normal to the surface of the target. Ions are accelerated by high-voltage negative pulses applied to the substrate. It is found that doubly charged ions effectively participate in the implantation process when an external electric field is applied very soon after the laser action on the target. The application of a high-voltage pulse with an amplitude of 50 kV 0.5 μs after a laser pulse leads to ion implantation with an energy close to 100 keV. With increasing delay in the application of the high-voltage pulse, the upper boundary of the energy spectrum of implanted ions is displaced towards lower energies. Comparison of the depth profiles of titanium distribution in silicon calculated from the results of simulation are compared with the experimental profiles shows that the model developed here correctly describes the formation of the high-energy component of the ion flow, which is responsible for defect formation and doping of deep layers of the substrate.     

Rescattering of electrons at laser-cluster interactions

The goal of this work is to derive the angular distributions of electrons irradiated at the outer ionization of large atomic clusters from Xe atoms by relativistic laser pulses taking into account rescattering processes. Both the magnetic field of the laser pulse and the Coulomb field of the ionized cluster significantly influence the rescattering of ejected electrons. The multiply inner ionization of atoms occurs at the leading edge of the laser pulse. The atomic ions with charge multiplicities up to Z = 26 are subsequently produced (each atomic ion with the next charge multiplicity appears in 3–5 fs) when the laser intensity increases. The measurements of the angular distributions of electrons allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.     

Charge Composition of Ions in a Cluster Plasma Formed under the Action of a High-Power Laser Pulse

A model describing the ionization of atoms and ions in a cluster under irradiation by a short laser pulse has been constructed. It is shown that the electron-impact ionization weakly affects the final charge composition, and the main mechanism of ion formation in the cluster is the over-barrier ionization by an electric field. The electric field acting on atoms and ions is the result of combined action of the external laser field and the intrinsic electric field of the cluster. The key parameters of the cluster beam and the laser pulse, which determine the properties of the charge composition of the cluster plasma, have been established.     

纳秒激光电离分子团簇产生高价离子实验研究

 利用飞行时间质谱仪，研究了功率密度为10⁹~10¹¹ W/cm²，波长为532 nm 的纳秒激光对苯、呋喃、甲醇及碘甲烷分子团簇的激光电离过程。实验观察到了高平动能的高价离子C^q+(q≤3)，O^q+(q≤3)和I^q+(q≤4)，该过程经历了以“初始的多光子电离引发-逆轫致吸收加热-电子碰撞电离模式”为主的激光团簇作用过程，后期经历了团簇的库仑爆炸过程。实验发现：即使激光能量变化一个量级以上时，主要高价离子的种类及占全部离子产物的比率也没有明显的变化，但是高价离子的初始平动能随激光强度的增大而增加；分子中含有较多个外壳层电子的氧、碘原子更容易电离产生高价离子，而碳离子的价态和强度相对较低。     

Recombination processes in atomic clusters irradiated by superintense femtosecond laser pulses

Various mechanisms of recombination of electrons with multiply charged atomic ions in atomic clusters irradiated by superintense femtosecond laser pulses are discussed. All of the recombination mechanisms are shown to take a time considerably longer than the laser pulse duration and, hence, they can develop only in a homogeneous, fairly rarefied cluster plasma after pulse termination. All autoionization states of multiply charged ions in a dense cluster plasma have been found to be destroyed by the Holtsmark electric field.     

Mechanisms of ultra-short laser pulse ablation from ionic crystals

In ultra-short laser pulse ablation from dielectric crystals two different ablation regimes are observed: 1) At laser intensities well below the single shot damage threshold, Coulomb explosion upon multiphoton surface ionization is the dominant process, yielding electrons and fast positive ions. 2) At higher laser intensities, ablation exhibits signs of hyper-thermal emission (phase explosion) as a result of rapid hot electron thermalization. This regime is characterized by massive etching, mostly of neutral particles. The transition from regime one to two is associated with the appearance of an additional, slower group of positive ions, saturation in the positive ion yield, and the occurrence of negative ions. PACS 61.80.Ba; 61.82.Ms; 79.20.Ds     

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)³He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD₂ foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and ¹¹B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

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.     

电离效率对激光电离团簇的高价离子产物的影响

纳秒激光与团簇相互作用产生高价离子逐渐成为分子物理界的热点之一, 为了深入研究团簇电离的本质, 本文以分子结构相似、元素组成相同的苯、环己烯和环己烷的分子团簇为对象, 利用飞行时间质谱研究了其与5 ns的532 nm激光相互作用时电离产物的价态和强度分布. 结果表明: 这三种化合物多光子电离效率苯>环己烯>环己烷, 但其高价离子的价态和比值苯是最低的, 环己烷的碳离子最高价态为4价, C³⁺和C²⁺的比值为1.1; 环己烯电离产物C³⁺和C²⁺ 的比值降低为0.6; 苯团簇的最高价态只有3价, C³⁺和C²⁺的比值约为0.4. 引起这种现象的原因可以归结于高的多光子电离效率会导致团簇多位点的电离, 引起团簇在电子加热到发生碰撞电离之前发生解离, 减小了团簇的尺寸, 进而减少了离子发生碰撞电离产生高价离子的反应时间, 最终阻碍了高价态离子的产生.     

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.     

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.     

Energetic-ion generation by the combination of laser pressure and Coulomb explosion

A scheme of generating energetic ions by the interaction of an ultrahigh-intensity laser pulse and a thin solid foil is studied. The combination of the effects of radiation pressure and Coulomb explosion makes the ion acceleration more effective. The maximum ion velocity variation with time is predicted theoretically while the temporal evolution of the electrostatic field due to the Coulomb explosion is taken into consideration. Two-dimensional particle-in-cell simulations are done to verify the theory.     

Highly charged ions from laser-cluster interactions: local-field-enhanced impact ionization and frustrated electron-ion recombination

Our molecular dynamics analysis of Xe_{147-5083} clusters identifies two mechanisms that contribute to the yet unexplained observation of extremely highly charged ions in intense laser cluster experiments. First, electron impact ionization is enhanced by the local cluster electric field, increasing the highest charge states by up to 40%; a corresponding theoretical method is developed. Second, electron-ion recombination after the laser pulse is frustrated by acceleration electric fields typically used in ion detectors. This increases the highest charge states by up to 90%, as compared to the usual assumption of total recombination of all cluster-bound electrons. Both effects together augment the highest charge states by up to 120%, in reasonable agreement with experiments.     

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.     

The dissociative ionization and Coulomb explosion of ethane by a femtosecond laser field

Femtosecond laser-induced dissociation and Coulomb explosion of polyatomic molecule C_2H_6 were systematically investigated using a time-of-flight mass spectrometer and a chirped pulse amplifier laser. With the laser intensity varying from 2.4×10^{15}W/cm^{2} to 1.2×10^{16}W/cm^2, strong molecular ions C_2H_n^+ (n=0－6) and atomic ions C^{m+} (m=1－3) signals were observed. The double-peak structure of atomic ions indicated the occurrence of Coulomb explosion. Compared with the nearly isotropic distribution of C^{+}, highly charged ions C^{m+} (m=2－3) exhibited a sharply anisotropic angular distribution, which was attributed to the geometric alignment.