Theoretical analysis of ultra-short pulsed laser ablation of SiO_2 material based on a Coulomb explosion model

Based on the kinetic theoretical Vlasov-Poisson equation, a surface Coulomb explosion model of SiO2 material induced by ultra-short pulsed laser radiation is established. The non-equilibrium free electron distribution resulting from the two mechanisms of multi-photon ionization and avalanche ionization is computed. A quantitative analysis is given to describe the Coulomb explosion induced by the self-consistent electric field, and the impact of the parameters of laser pulses on the surface ablation is also discussed. The results show that the electron relaxation time is not constant, but it is related to the microscopic state of the electrons, so the relaxation time approximation is not available on the femtosecond time scale. The ablation depths computed by the theoretical model are in good agreement with the experimental results in the range of pulse durations from 0 to 1 ps.

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters （（CD4）n） in a gas jet subjected to intense femtoseeond laser pulses （170 mJ, 70 fs） have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5 nm in radius and the laser intensity used was 3 × 10^17 W/cm^2.The measured maximum and average energies of deuterons produced in the laser-cluster interaction were 60 and 13.5 keV, respectively. Prom DD collisions of energetic deuterons, a yield of 2.5（±0.4） × 10^4 fusion neutrons of 2.45 MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5 × 10^5 per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.

Review of stopping power and Coulomb explosion for molecular ion in plasmas

We summarize our theoretical studies for stopping power of energetic heavy ion,diatomic molecular ions and small clusters penetrating through plasmas.As a relevant research field for the heavy ion inertial confinement fusion(HICF),we lay the emphasis on the dynamic po-larization and correlation effects of the constituent ion within the molecular ion and cluster for stopping power in order to disclose the role of the vicinage effect on the Coulomb explosion and energy deposition of molecules and clusters in plasma.On the other hand,as a promising scheme for ICF,both a strong laser field and an intense ion beam are used to irradiate a plasma target.So the influence of a strong laser field on stopping power is significant.We discussed a large range of laser and plasma parameters on the coulomb explosion and stopping power for correlated-ion cluster and C 60 cluster.Furthermore,in order to indicate the effects of different cluster types and sizes on the stopping power,a comparison is made for hydrogen and carbon clusters.In addition,the deflection of molecular axis for diatomic molecules during the Coulomb explosion is also given for the cases both in the presence of a laser field and laser free.Finally,a future experimental scheme is put forward to measure molecular ion stopping power in plasmas in Xi’an Jiaotong University of China.

Geometric structure of N_(2)O^(q+)(q=5,6)studied by Ne^(8+)ion-induced Coulomb explosion imaging

We report the study on the complete three-body Coulomb explosion(CE)of N_(2)O^(q+)(q=5,6)induced by 56-keV/u Ne8+ion collision with N2O gaseous molecule.Six CE channels for N_(2)O^(5+)and seven for N_(2)O^(6+)are identified by measuring three ionic fragments and the charge-changed projectile in quadruple coincidence.Correspondingly the kinetic energy release(KER)and momentum correlation angle(MCA)distributions of three ionic fragments for each of the CE channels are also deduced.Numerical computation is presented to reconstruct the geometric structure of N_(2)O^(q+0prior to dissociation based on the measured KER and MCA.The N–N and N–O bond lengths and the N–N–O bond angles of N_(2)O^(q+)for each of the channels are determined.

Triatomic wake effect and the determination of the molecular structure of HD_2^+ from the Coulomb explosion

A new theoretical model of the triatomic molecular wake effect is proposed and applied to molecular ions D^＋3 and HD^＋2 while passing through a solid. The wake effects resulting from the reactions of the two similar ions with thin carbon foil are also investigated by using the Coulomb explosion technique. The experimental results are in good agreement with theoretical estimates and the molecular structure of HD^＋2 is determined by using the model.

Overrun phenomenon and neutron yield in Coulomb explosion of deuterated alkane clusters driven by intense laser field

By using a simplified Coulomb explosion model, the laser-driven Coulomb explosion processes of three deuterated alkane clusters, i.e., deuterated methane（CD4）N, ethane（C2D6）N and propane（C3D8）N clusters are simulated numerically.The overrun phenomenon that the deuterons overtake the carbon ions inside the expanding clusters, as well as the dependence of the energetic deuterons and fusion neutron yield on cluster size, is discussed in detail. Researches show that the average kinetic energy of deuterons and neutron yield generated in the Coulomb explosion of（C2D6）N cluster are higher than those of（CD4）N cluster with the same size, in qualitative agreement with the reported conclusions from the experiments of（C2 H6）N and（CH4）N clusters. It is indicated that（C2D6）N clusters are superior to（CD4）N clusters as a target for the laser-induced nuclear fusion reaction to achieve a higher neutron yield. In addition, by comparing the relevant data of（C3D8）N cluster with those of（C2D6）N cluster with the same size, it is theoretically concluded that（C3D8）N clusters with a larger competitive parameter might be a potential candidate for improving neutron generation. This will provide a theoretical basis for target selection in developing experimental schemes on laser-driven nuclear fusion in the future.

Ultrafast Coulomb explosion imaging of molecules and molecular clusters

Taking an image of their structure and a movie of their dynamics of small quantum systems have always been a dream of physicists and chemists. Laser-induced Coulomb explosion imaging(CEI) provides a great opportunity to make this dream a reality for small molecules or their aggregation — clusters. The method is unique for identifying the atomic locations with angstrom spatial resolution and capturing the structural evolution with a femtosecond time scale, in particular for imaging transient state products. This review summarizes the determination of three-dimensional equilibrium geometry of molecules and molecular cluster system through the reconstruction from the fragments momenta, and also shows that the dissociation dynamics on the complex potential energy surface can be tracked in real-time with the ultrafast CEI(UCEI).Furthermore, the detailed measurement and analysis procedures of the CEI, theoretical methods, exemplary results, and future perspectives of the technique are described.

Coulomb explosion of CS2 molecule under an intense femtosecond laser field

We experimentally demonstrate the Coulomb explosion process of CS_2 molecule under a near-infrared（800 nm）intense femtosecond laser field by a DC-sliced ion imaging technique. We obtain the DC-sliced images of these fragment ions S~＋, S^（2＋）, CS~＋, and CS^（2＋）by breaking one C–S bond, and assign their Coulomb explosion channels by considering their kinetic energy release and angular distribution. We also numerically simulate the dissociation dynamics of parent ions CS_2^（k＋）（k = 2–4） by a Coulomb potential approximation, and obtain the time evolution of Coulomb energy and kinetic energy release, which indicates that the dissociation time of parent ions CS_2^（k＋） decreases with the increase of the charge number k.These experimental and theoretical results can serve as a useful benchmark for those researchers who work in the related area.

Laser-induced Alignment and Coulomb Explosion of CO2

Dynamic processes of CO2 are experimentally studied in intense femtosecond laser fields with laser intensity varying from 1×10^13 W/cm^2 to 6×10^14 W/cm^2. When the laser intensity is below the ionization threshold, a coherent rotational wave-packet is formed for CO2 at room temperature through nonadiabatic rotational excitation. The evolution of the wave-packet leads to transient alignment. The field-free alignment revives periodically after the laser pulse is over. The revival structure can be modified by a second laser pulse for the rotational wave-packet through precisely adjusting the time delays between the two laser pulses. When the laser intensity excesses the ionization threshold, ionization and Coulomb explosion occur. The atomic ions C^m＋ （re=1-3） and On＋ （n=1-3） observed in the experiment exhibit highly anisotropic angular distributions relative to the laser polarization. Using two linearly polarized laser pulses with crossed polarization, we conclude that the anisotropic angular distribution results from dynamic alignment, in which the rising edge of the laser pulse aligns the neutral CO2 along the laser polarization direction prior to ionization.

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.

A Coulomb explosion theoretical model of femtosecond laser ablation materials

In this paper,a kinetic theory of Vlasov equation is proposed to depict electron and ion's nonequilibrium transport processes in a femtosecond time scale.A Coulomb explosion model of femtosecond laser ablation of materials is proposed and numerically simulated.The mechanism of surface Coulomb explosion induced by self-consisted electric field and the impact of laser parameters on the ablation of materials are quantitatively analyzed.The ablation depths calculated by the model are in good agreement with the experimental results.It is shown that,the intensity of self-consisted electric field generated on the dielectric material's surface is much greater than that generated on the metal or the semiconductor material's surface,and Coulomb explosion ablation is more easily to occur on the dielectric material's surface.

Coulomb expansion of deuterated methane clusters irradiated by an ultrashort intense laser pulse

The simulations of three-dimensional particle dynamics show that when irradiated by an ultrashort intense laser pulse, the deuterated methane cluster expands and the majority of deuterons overrun the more slowly expanding carbon ions, resulting in the creation of two separated subelusters. The enhanced deuteron kinetic energy and a narrow peak around the energy maximum in the deuteron energy distribution make a considerable contribution to the efficiency of nuclear fusion compared with the ease of homonuelear deuterium clusters. With the intense laser irradiation, the nuclear fusion yield increases with the increase of the cluster size, so that deuterated heteronuelear clusters with larger sizes are required to achieve a greater neutron yield.

Cluster-assisted generation of multicharged ions in nanosecond laser ionization of carbon bisulfide clusters at 1064nm

The photoionization of seeded carbon bisulfide molecular beam by a 1064nm nanosecond Nd-YAG laser with intensities varying from 0.8 × 10^11 to 5.6 × 10^11 W/cm^2 have been studied by time-of-flight mass spectrometry. Multiply charged ions of S^q＋ （q = 2 6） and C^q＋ （q = 2-4） with kinetic energy of hundreds of electron volts have been observed, and there are strong experimental evidences indicating that those multicharged ions originate from the ionization of CS2 neat clusters in the beam. An electron reeolliding ionization model is proposed to explain the appearance of those multiply charged atomic ions under such low laser intensities.

Influence of Incident Velocity on the Penetration for C20 Clusters Moving through Oxides

A theoretical model is established to simulate the penetration process of C20 clusters in oxides （Al2O3, SiO2） at different incident velocities. The induced spatial potential by the incident clusters is described by the dielectric response formalism, in which the Mermin-type dielectric function is adopted to provide a realistic evaluation of the electronic properties of the oxides. The charge distribution of individual ions is derived by using the Brandt-Kitagawa effective charge model, also under the consideration of the asymmetric influence from the wake potential. The stopping power of the clusters and the Coulomb explosion processes are derived by solving the motion equation of the individual ions, when taking into account the multiple scattering effect simulated by using the Monte Carlo method. It is found that the dynamical interaction potential between ions leads to a spatial asymmetry to the cluster structure and the charge distribution for high velocity clusters, and will not be in effect as the incident velocities decrease.

Vicinage Effects for a Nitrogen Molecular Cluster in Plasmas

The vicinage effects are studied for a fast nitrogen diatomic molecular cluster in a high-density plasma target.A variety of plasma parameters are discussed with regard to stopping power ratio,molecular axis deflection and Coulomb explosion.Emphasis is placed on the vicinage effects on Coulomb explosion and stopping power for a nitrogen cluster in plasmas.The results indicate that vicinage effects influence the correlation between ions in the cluster,and the Coulomb explosion will proceed faster with higher projectile speed,lower plasma density and higher plasma temperature.Comparing hydrogen and nitrogen molecular ions for Coulomb explosion and deflection angle under the same set of parameters,one can find that the nitrogen ion has faster Coulomb explosion and stronger deflection of molecular axis due to the contribution of charge.In the initial stage of the Coulomb explosion the stopping power ratio has a higher value due to enhanced vicinage effects while in the later stage the stopping power ratio approaches one,indicating that the vicinage effects disappear and the ions in the cluster simply behave as independent atomic ions in the plasma.

Dynamics simulation on the 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.

Growth of deuterium clusters in a gas jet and ion energy spectrum of clusters in ultra-short laser field

Large deuterium clusters are generated using a cryogenic pulse valve with a cone nozzle （21 mm long, 4° open angle）. Rayleigh scattering experiment is carried out to obtain the scaling relation between scattering signal SR and backing pressure P0. A method using the Coulomb explosion model is proposed to verify that the clusters continue to grow after their leaving the nozzle. Our experiments suggest a tentatively optimized position for laser cluster interaction.

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.