Size dependence of the maximum energy of protons from Coulomb explosion of methane clusters under intense femtosecond laser irradiation

An investigation of the cluster size dependence of the maximum energy of protons ejected from explosion of methane clusters in an intense femtosecond laser field has been conducted on the basis of the cluster size estimation by Rayleigh scattering measurements. The interaction of a 2 × 1016-W/cm2 intense laser pulse (790 nm, 60 fs) with the methane clusters revealed that the clusters were Coulomb exploded and the maximum energy (Emax) of the protons produced was linearly proportional to the square of the cluster radius (r2c). In a cluster size range, with the methane cluster radii up to about 3 nm, the established relation of Emax and rc2 was found to be Emax (keV) = 3.3 + 0.75r2c (nm2), in good agreement with the simulation results. This demonstrated that Coulomb explosion of ionic clusters (C+4H4+)n took place following the cluster vertical ionization in the laser-cluster interaction.     

Design and performance analysis of a compact magnetic proton recoil spectrometer for DT neutrons

A magnetic proton recoil (MPR) spectrometer is a novel instrument with superior performance, including high energy resolution, high count rate and good signal-to-noise ratio (SNR) for measurements of neutron spectra from inertial confinement fusion (ICF) experiments and high power Tokomaks. In this work, the design of a compact MPR spectrometer (cMPR) was evaluated for deuteron-tritium (DT) neutron spectroscopy. The characteristics of the spectrometer were analyzed using 2-D beam transport simulations, 3-D particle transport calculations and Monte-Carlo simulations. Based on the theoretical results, an instrument design that satisfies special experimental requirements is proposed. The energy resolution and efficiency of the spectrometer are also evaluated. The results indicate that the proposed cMPR spectrometer would achieve a detection efficiency and energy resolution of approximately 10^-8 and 4%, respectively, for DT neutrons.     

MEASUREMENT OF IONIC ENERGY FROM FEMTOSECOND LASER-HEATED ARGON CLUSTERS

The high backing pressure argon gases adiabatically expand into vacuum through a pulsed gas jet to nucleate into large clusters. The clusters were heated by a 45fs, 2.3×10¹⁶ Wcm^-2 Ti: sapphire laser. The high energy of the ions produced in the cluster explosion was measured using time-of-flight spectrometry. The maximum and average kinetic energy of the ions were 0.2MeV and ～12.5keV, respectively, indicating that the femtosecond laser interactions with argon clusters are more energetic than interactions with atoms and molecules.     

ELECTRONIC STRUCTURE STUDIES OF THE C60 FILM CONDENSED ON 2H-MoS2(0001) SURFACE

The electronic structure and vibrational spectrum of the C₆₀ film condensed on a 2H- MoS₂(0001) surface have been investigated by X-ray photoelectron spectroscopy (XPS), ul-traviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) and infrared high-resolution electron-energy-loss spectroscopy (HREELS). AES analysis showed that at low energy side of the main transition, C₆₀ contains a total of three peaks just like that of graphite. However, the energy position of the KLL main Auger transition of C₆₀ looks like that of diamond, indicating that the hybridization of the carbon atoms in C₆₀ is not strictly in sp²- bonded state but that the curvature of the molecular surface introduces some sp²pz- bonded character into the molecular orbitals. XPS showed that the C 1s binding energy in C₆₀ was 285.0eV, and its main line was very symmetric and offered no indication of more than a single carbon species. In UPS measurement the valence band spectrum of C₆₀ within 10eV below the Fermi level (E_F) shows a very distinct five-band structure that character-izes the electronic structure of the C₆₀ molecule. HREEL results showed that the spectrum obtained from the C₆₀ film has very rich vibrational structure. At least, four distinct main loss peaks can be identified below 200 meV. The most intense loss was recorded at 66 meV, and relatively less intense losses were recorded at 95, 164 and 197meV at a primary energy of electron beam E_P = 2.0eV. The other energy-loss peaks at 46, 136, 157 and 186meV in HREEL spectrum are rather weak. These results have been compared to infrared spectrum data of the crystalline solid C₆₀ taken from recent literatures.     

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.     

Quasiclassical calculation of the chemical reaction Ba＋C3HTBr → BaBr＋C3H7

<正>The quasi-classical trajectory（QCT） method based on the extended London-Eyring-Polanyi-Sato potential energy surface is used to investigate the product vibrational distribution,angular distribution and angle-resolved kinetic distribution of the reaction Ba+C₃H₇Br→BaBr+C₃H₇ at 2.58 kcal/mol.The calculated results show that the product BaBr vibrational distribution is quite hot,the vibrational population peaks are located at v = 12,and the angular product distribution tends to backward scattering.The calculated angle-resolved kinetic distribution shows that the kinetic distribution is obviously related to angle.The QCT results are always qualitatively acceptable and sometimes even quantitatively.     

Average kinetic energy of ions ejected from pure Coulomb explosions of methane clusters

We use an electrostatic model to study the average kinetic energy of ions ejected from the pure Coulomb explosions of methane clusters(CA_4)_n(light atom A=H and D).It is found that the ratio of the average kinetic energy of the ions to their initial average electrostatic potential energy is irrelevant to the cluster size.This finding implies that as long as the ratio is given,the average kinetic energies of the ions can be simply estimated from their initial average electrostatic potential energies,rather than from the time- consuming simulations.The ratios for the different charge states of carbon ions are presented.     

快速C60离子团在固体中的库仑爆炸过程Ⅰ——球壳层模型

研究了快速C₆₀离子团在固体中穿行时的库仑爆炸过程.假定离子团中离子之间位置矢量的取向是随机的,并且采用球壳模型描述C₆₀离子团的结构.借助于线性介电响应理论和等离子-极点近似介电函数,推导出C₆₀离子团自能的解析表达式.通过数值求解描述离子团半径变化的运动方程,可以发现自能中的“尾效应”可以降低C₆₀离子团的库仑爆炸速度,甚至可以稳定C₆₀离子团的结构. 关键词： 离子团 库仑爆炸 球壳模型     

张量力在16O+16O熔合动力学中的效应（英文）

利用时间相关Hartree-Fock 理论和完整Skyrme 有效相互作用研究了16O+16O 碰撞在库仑位垒附近的熔合动力学。数值计算是在没有任何对称性约束的三维笛卡尔基下完成。将时间相关Hartree-Fock 理论和冻结密度近似下的能量密度泛函方法给出的库仑位垒与实验结果进行了比较,发现同位旋标量的张量项能降低自旋饱和体系16O+16O的库仑位垒,而库仑位垒高度随着同位旋矢量的张量项的耦合常数减小而降低。并计算了包含和不包含张量力的16O+16O熔合截面,发现张量力对16O+16O碰撞在库仑位垒附近的熔合动力学影响较小。The fusion dynamics of 16O+16O around Coulomb barrier has been studied in the timedependent Hartree-Fock (TDHF) theory with the full Skyrme effective interaction. The calculations have been carried out in three-dimensional Cartesian basis without any symmetry restrictions. We have included the full tensor force and all the time-odd terms in Skyrme energy density functional (EDF). The Coulomb barrier obtained from the dynamical TDHF calculations and EDF with frozen density approximation has been compared with the available experimental data. The isoscalar tensor terms and the rearrangement of other terms are found to decrease the barrier height in the spin-saturated system 16O+16O, while the energy of Coulomb barrier tends to decrease as the isovector coupling constant decreases. The fusion cross section for 16O+16O collision has been calculated with and without the tensor force. We found that the tensor force has minor effect on the fusion dynamics of 16O+16O at the energies around Coulomb barrier.     

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.     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Coulomb explosion of large polyatomic molecules assisted by nonadiabatic charge localization

The electron-nuclear dynamics of the Coulomb explosion of a large polyatomic molecule, anthracene, is probed using kinetic energy distributions of produced H+ ions. The kinetic energy release of ejected protons exceeds 30 eV for anthracene exposed to 10(14) W cm(-2), 800 nm pulses of 60 fs duration. We propose a strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the charge localization lasts many laser periods and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of the H+ kinetic energy on the laser intensity. Dissociative ionization of a polyatomic molecule enabled by long-lived charge localization is a new type of electron-nuclear dynamics and is essential for understanding the pathways of molecular or ionic fragmentation in strong fields.     

Pulsed Gas Jet for Large Size Atomic Cluster Production

A pulsed gas jet of large size noble gas atomic clusters as targets for high intensity femtosecond laser pulses is reported. The jet can work for gas stagnation pressure in excess of 40 atmospheres and with a repetition rate of 10 Hz to fit 10 Hz table-top terrawatt femtosecond Ti∶Sapphire laser. The scaling law indicates that the monomer argon clusters produced in the jet can be as large as 22,000 atoms/cluster at room temperature. Preliminary experiments for argon ionic kinetic energy spectrum indicated that the argon clusters are produced in the jet.     

Pulsed Gas Jet for Large Size Atomic Cluster Production

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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.     

C60团簇与等离子体相互作用的分子动力学模拟

用分子动力学模拟方法研究了C60团簇与稠密等离子体的相互作用。在线性化的伏拉索夫-泊松理论框架下,借助于经典的等离子体介电函数,推导了团簇的阻止本领的一般表达式。用数值方法求解了团中离子的运动方程,研究了团簇的库仑爆炸过程,讨论了入射速度、等离子体密度和电子温度对阻止本领和库仑爆炸的影响。结果发现尾流效应降低了C60团簇离子的库仑爆炸速度,甚至压缩了团簇的结构,在高速、低密、高温的情况下库仑爆炸进行得更快。     

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.     

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.