环境气体种类对激光烧蚀粒子速度劈裂的影响

采用蒙特-卡罗(Monte Carlo)方法, 模拟了激光烧蚀粒子输运动力学过程, 在环境气体压强为100 Pa的情况下, 研究了环境气体种类(He, Ne, Ar和假想气体等)对烧蚀粒子速度劈裂的影响. 研究结果表明, 在四种环境气体中传输的烧蚀粒子均出现了速度劈裂现象, 形成速度劈裂所需时间按He, Ne, 假想气体和Ar的次序减小. 还研究了环境气体分子的质量和半径对烧蚀粒子速度劈裂的影响, 形成速度劈裂所需时间随环境气体分子半径(或质量)增大而减小. 在假想气体中, 两速度峰强度相等时的强度最小. 结合欠阻尼振荡模型和惯性流体模型, 对劈裂的形成时间进行了解释. 所得结论可为进一步定量研究纳米晶粒生长机理提供基础.     

Plume splitting in pico-second laser-material interaction under the influence of shock wave

In this work, molecular dynamics simulations are conducted to study the physics of plume splitting in pico-second laser material interaction in background gas. The velocity distribution shows a clear split into two distinctive components. Detailed atom trajectory track reveals the behavior of atoms within the peaks and uncovers the mechanisms of peak formation. The observed plume velocity splitting emerges from two distinguished parts of the plume. The front peak of the plume is from the faster moving atoms and smaller particles during laser-material ablation. This region experiences strong constraint from the ambient gas and has substantial velocity attenuation. The second (rear) peak of the plume velocity originates from the larger and slower clusters in laser-material ablation. These larger clusters/particles experience very little constraint from the background, but are affected by the relaxation dynamics of plume and appear almost as a standing wave during the evolution. Density splitting only appears at the beginning of laser-material ablation and quickly disappears due to spread-out of the slower moving clusters. It is found that higher ambient pressure and stronger laser fluence favor earlier plume splitting.     

Dynamics evolution of shock waves in laser–material interaction

In this work, the dynamics and internal structure of shock waves in picosecond laser–material interaction are explored at the atomistic level. The pressure of the shock wave, its propagation, and interaction zone thickness between the plume and ambience are evaluated to study the effect of the laser absorption depth, ambient pressure, and laser fluence. Sound agreement is observed between the MD simulation and theoretical prediction of shock wave propagation and mass velocity. Due to the strong constraint from the compressed ambient gas, it is observed that the ablated plume could stop moving forward and mix with the ambient gas, or move backward to the target surface, leading to surface redeposition. Under smaller laser absorption depth, lower ambient pressure, or higher laser fluence, the shock wave will propagate faster and have a thicker interaction zone between the target and ambient gas.     

脉冲激光沉积纳米硅晶粒流体模型的推广

Yoshida等人提出的惯性流体模型只能解释脉冲激光烧蚀制备纳米硅晶粒平均尺寸随环境气压的变化规律.在此模型基础上,考虑到烧蚀粒子的初始速度分布(Maxwell分布),得到了纳米硅晶粒尺寸分布的解析表达式,数值模拟结果与Yoshida等人在不同环境氦气压下制备样品的晶粒尺寸分布的实验统计数据基本相符.还利用修正后的模型对不同环境气体种类(氦、氖、氩)中制备的纳米Si晶粒尺寸分布进行了模拟,模拟结果与实验数据相符.结论可为实现纳米硅晶粒尺寸的均匀可控提供理论依据. 关键词： 纳米硅晶粒 脉冲激光烧蚀 惯性流体模型 尺寸分布     

Material ablation and plasma plume expansion study from Fe and graphite targets in Ar gas atmosphere

Study of expansion dynamics of pulsed-laser ablation plasmas from Fe and graphite targets is presented. A 532 nm Q-switched Nd:YAG laser with fluence of 30 J cm⁻² is used to ablate the Fe and graphite targets in various Ar ambient gas pressures. Plasma ablation parameters for the two target materials are estimated using snow-plow and shock-wave models, which show that the laser beam energy deposited to ablated species remains at 70% for both targets at all ambient pressures. The plume splitting was observed, more prominently, for Fe plasma as it moves faster compared to graphite plasma. The difference in plasma plume fronts’ speeds for different targets was attributed to the significant difference in mass of the ablated plasma for two targets, as estimated from simulation results.     

On the influence of the mass ablated by a laser pulse on thin film morphology and optical properties

To know and to control experimental parameters that play a role in laser ablation is vital to define film properties. Among the others, laser fluence is commonly used. Yet, when plasma expansion dynamics takes place through an ambient gas, the relation between the ablated mass per pulse and gas mass is critical and till now it was poorly investigated. While the gas mass is fixed by the pressure in the deposition chamber, the ablated mass is not unequivocally determined by the laser fluence. For a given fluence value the ablated mass changes as a function of the irradiated target area. Here, we show that nanostructured silver thin films deposited keeping unaltered the laser fluence, while changing in a controlled way the irradiated area and hence the ablated mass per pulse, display markedly differentiated morphological and optical properties, as evidenced by electron microscopy and UV–Vis and Raman spectroscopies.     

Brownian motion in granular gases of viscoelastic particles

A theory is developed of Brownian motion in granular gases (systems of many macroscopic particles undergoing inelastic collisions), where the energy loss in inelastic collisions is determined by a restitution coefficient ɛ. Whereas previous studies used a simplified model with ɛ = const, the present analysis takes into account the dependence of the restitution coefficient on relative impact velocity. The granular temperature and the Brownian diffusion coefficient are calculated for a granular gas in the homogeneous cooling state and a gas driven by a thermostat force, and their variation with grain mass and size and the restitution coefficient is analyzed. Both equipartition principle and fluctuation-dissipation relations are found to break down. One manifestation of this behavior is a new phenomenon of “relative heating” of Brownian particles at the expense of cooling of the ambient granular gas.     

气压对激光烧蚀Al等离子体中粒子速度的影响

在背景气压为8× 10-3— 100Pa范围内 ,通过测量脉冲激光烧蚀平面Al靶产生的等离子体辐射谱的时间分辨特征 ,比较空间不同点辐射的飞行时间轮廓的相对延迟 ,从而得到辐射粒子速度及其空间分布 .利用绝热膨胀的理论和激波模型分别对背景气压小于 0 .6Pa的结果和 5Pa时的结果作了分析 ,并得出激波的波面基本上为柱对称. With the ambient gas pressure in the range from 8×10 -3 to 10 2 Pa, Q-switched YAG laser ablates plane aluminum target and plasma are produced. Optical emission spectroscopy is used to carry out time-resolved analysis of atomic particles. Using the resonance transition of Al I 396.1 nm (3p 2P-4s 2S), the spatiol velocity distribution of Al I has been obtained under the laser energies of 160 mJ/pulse when the ablating size is about 200 μm. The velocity is at the order of 10 6 cm/s...     

Modeling of plume dynamics with shielding in laser ablation of carbon

The process of laser ablation of carbon in presence of background gas is simulated numerically. The plume dynamics in laser ablation is important to study for many reasons including temperature of plume particles and shielding of target by previously ablated plumes. Shielding leads directly to the change in energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. Carbon ablation is studied for single and multiple laser hits typical for synthesis of nanotubes. Two models of correction of ablated velocity and pressure resulting from shielding effect are proposed and investigated. Numerical modeling of this plume dynamics and its integral effect of shielding is challenging due to inherent high nonlinearity of the problem. Some of available numerical techniques handles nonlinearity but are dissipative, e.g. Godunov type schemes. Other techniques are less dissipative but fail to account for strong nonlinearity typical for initial stages of ablation, e.g. the ENO-Roe. To effectively model this highly nonlinear plume dynamics a combination of two of above mentioned schemes is developed so as the numerical evaluation of fluxes is close to their physical values and the scheme has minimum dissipation. The non-monotonic behavior of ablated mass as a function of time duration between two laser pulses is studied.     

Investigation of laser ablated plumes using fast photography

Fast photography is used to investigate the expansion dynamics of the laser ablated plasmas in various ambient atmospheres and laser energies. Dependence of plasma parameters such as velocity, temperature, density, and pressure on time and ambient atmosphere is presented. The measured vapor pressure and temperature decrease with the increase in ambient gas pressure. The images of the expanding plumes are used to locate the shocked region and hence to estimate the plasma parameters in the shocked regime. The calculated plasma parameters are used to optimize target-substrate distance, a key parameter for laser ablation deposition of thin films     

Propagation of LaMnO3 laser ablation plume in oxygen gas

The propagation of LaMnO₃ laser ablation plume in oxygen background has been investigated using fast photography of overall visible plume emission and time-resolved optical emission spectroscopy. The plume expansion was studied with ambient oxygen pressures ranging from vacuum level to 100 Pa. Free-expansion, splitting, sharpening and stopping of the plume were observed at different pressures and time delays after the laser pulse. Time-resolved optical emission spectroscopy showed that oxides are mainly formed through reaction of the atomic species ablated from the target with oxygen in the gas-phase. These reactions mainly affect the content of lanthanum oxide in the plume, while emission of manganese oxide is barely observed in all the range of pressure investigated.     

Molecular dynamics simulation study of deep hole drilling in iron by ultrashort laser pulses

Classical molecular dynamics simulation technique is applied for investigation of the iron ablation by ultrashort laser pulses at conditions of deep hole for the first time. Laser pulse duration of 0.1 ps at wavelength of 800 nm is considered. The evolution of the ablated material in deep hole geometry differs completely from the free expansion regime as two major mechanisms are important for the final hole shape. The first one is the deposition of the ablated material on the walls, which narrows the hole at a certain height above its bottom. The second mechanism is related to ablation of the material from the walls (secondary ablation) caused by its interaction with the primary ablated particles. Properties of the secondary ablated particles in terms of the velocity and the angular distribution are obtained. The material removal efficiency is estimated for vacuum or in Ar environment conditions. In the latter case, the existence of well-defined vapor cloud having low center of the mass velocity is found. The processes observed affect significantly the material expulsion and can explain the decrease of the drilling rate with the hole depth increase, an effect observed experimentally.     

Spectroscopy of laser-produced cerium plasma for remote isotope analysis of nuclear fuel

A cerium oxide sample was ablated by 2nd harmonic radiation of Nd:YAG laser at a power density of 0.1 GW/cm². Time evolution of the ablation plume was investigated by laser absorption time-of-flight (TOF) measurement. It was found that the ablated ionic plume in vacuum consisted of two components having different velocities whereas the ablated neutral atoms had mainly a single component. The flow velocity perpendicular to the sample surface in vacuum was determined to be 3.5 km/s for neutral atoms, and 4.7 km/s and 9.3 km/s for singly charged ions. From the detailed plume evolution in ambient atmosphere with several pressures we obtained some experimental conditions suitable for isotope analysis of atomic cerium.     

Temporal evolution of laser-ablated Co ions probed by time-of-flight mass spectrometry

A pulsed-field time-of-flight mass spectrometric (TOFMS) technique was used to investigate the expansion dynamics of the ionic species ejected from the visible (λ=532 nm) laser ablation of cobalt target at low laser fluence less than 1 J/cm². The temporal evolution of Co⁺ ions was studied by varying the delay time of the ion repelling pulse with respect to the laser irradiation, which provides significant information on the ablated plume characterization. The obtained TOF mass spectra were well fitted by shifted Maxwell–Boltzmann distributions on a stream velocity, commonly used to describe the measured velocity distributions. The TOF distribution of Co⁺ ions showed a bimodal distribution with fast and slow velocities. These velocities show a decreasing tendency with delay time, which is attributed to the gas collisions between the plume ejecta and to the related gas dynamics. The present results suggest that the in situ measurements of the most probable velocity of ablated ions along the normal to the solid target can be accomplished by the simple technique of a laser ablation/TOFMS.     

The growth of Y2SiO5:Ce thin films with pulsed laser deposition

Y₂SiO₅:Ce phosphor thin films were grown onto Si(100) substrates with pulsed laser deposition (PLD) using a 248-nm KrF excimer laser. Process parameters were varied during the growth process and the effect on the surface morphology and cathodoluminescence (CL) was analysed. The process parameters that were changed included the following: gas pressure (vacuum (5×10⁻⁶ Torr), 1×1⁻² Torr and 1 Torr O₂), different gas species (O₂, Ar and N₂ at a pressure of 455 mTorr), laser fluence (1.6±0.1 J cm⁻² and 3.0±0.3 J cm⁻²) and substrate temperature (400 and 600°C). The surface morphology was investigated with atomic force microscopy (AFM). The morphology of the thin films ablated in vacuum and 10 mTorr ambient O₂ showed more or less the same trend. An increase in the pressure to 1 Torr O₂, however, showed a definite increase in deposited particle sizes. Ablation in N₂ gas resulted in small particles of 20 nm in diameter and ablation in O₂ gas produced bigger particles of 20, 30 and 40 nm as well as an agglomeration of these particles into bigger size clusters of 80 to 100 nm. Ablation in Ar gas led to particle sizes of 30 nm and the particles were much more spherically defined and evenly distributed on the surface. The higher fluence deposition led to bigger particle and grain sizes as well as thicker layers with respect to the lower fluence. The particle sizes of the higher fluence vary mainly between 130 and 140 nm and the lower fluence sizes vary between 50 and 60 nm. The higher fluence particles consist of smaller particles ranging from 5 to 30 nm as measured with AFM. The surface structure of the thin film ablated at 400°C substrate temperature is less compact (lesser agglomeration of particles than at 600°C). The increase in substrate temperature definitely resulted in a rougher surface layer. CL was measured to investigate the effect of the surface morphology on the luminescent intensities. The increased O₂ ambient (1 Torr) resulted in a higher CL intensity compared to the thin films ablated in vacuum. The thin film ablated in Ar gas showed a much higher CL intensity than the other thin films. Ablation at a high fluence resulted in a higher CL intensity. The higher substrate temperature resulted in better CL intensities. The more spherically shaped particles and rougher surface led to increase CL intensities.     

Large-time behavior of the Broadwell model of a discrete velocity gas

We study the behavior of solutions of the one-dimensional Broadwell model of a discrete velocity gas. The particles have velocity ±1 or 0; the total mass is assumed finite. We show that at large time the interaction is negligible and the solution tends to a free state in which all the mass travels outward at speed 1. The limiting behavior is stable with respect to the initial state. No smallness assumptions are made.Partially supported by N.S.F. Grant No. NSF-DMS-84-08393     

Plume dynamics during film and nanoparticles deposition by pulsed laser ablation

The gas dynamics of pulsed laser ablation of silicon target in the helium gas ambient is investigated via direct simulation Monte Carlo method with a real physical scale of target-substrate configuration. A shock driven process is clearly observed. It is shown that the interaction of the shock front with the target surface and the vapor front induce significant backward flux of ablated particles and oscillating behavior of vapor front. A confined layer mixed with high density Si and He atoms is formed around the contact front. Its behavior is important to the nanoparticle formation and deposition.     

Ion current generation in the field ion microscope: I. dynamic approach

The numbers of gas particles arriving at unit tip surface in unit time from a field free region are derived as functions of velocity components for a spherical tip. It is shown that a considerable fraction of the gas particles arrives at the tip having large tangential velocities. The simple model of collision of a particle with a metal surface is used and the trajectories and rebounds of particles are tracked. The principal method to calculate the total ion current is shown. The capture probability of particles by the dipole attraction potential is shown to increase when the tip temperature is lowered, the field strength is increased, the mass ratio of the gas atom to the metal atom increases and the gas temperature is lowered.     

脉冲激光烧蚀制备纳米Si晶粒成核区位置的确定

为了确定纳米Si晶粒气相成核的位置，采用XeCl准分子激光器，在10Pa氩气环境下，烧蚀高阻抗单晶Si靶，在距离等离子羽正下方2.0cm处、与其轴线平行放置一系列单晶Si或玻璃衬底，沉积制备了纳米Si薄膜. X射线衍射、Raman散射、扫描电子显微镜和原子力显微镜结果均显示，纳米Si晶粒只在距靶约0.5—2.8cm平行距离范围内的样品上形成，在此范围内，随着离靶平行距离的增大，所形成的纳米Si晶粒的平均尺寸逐渐减小，并且晶粒尺寸的分布也发生变化. 根据成核区起始和终止的突变特征，结合晶粒形成后的平抛运动规律，对晶粒气相成核的位置进行了估算. 关键词： 纳米Si晶粒 脉冲激光烧蚀 成核区