飞秒强激光场中氢原子团簇的各向异性膨胀

利用LAMMPS程序研究了氢原子团簇在飞秒强激光场下的动力学行为, 讨论了引起小氢原子团簇各向异性膨胀的原因.通过对外电离过程中团簇内部电子的行为以及团簇各个方向上最外层质子距离团簇中心的距离随时间的变化情况的分析, 发现团簇的膨胀呈现各向异性,且引起这种各向异性的根源在于团簇内部电子的抖动以及逃逸.对氢原子团簇与强激光场相互作用过程中质子各能量分量以及各向异性程度随时间变化情况进行了研究,发现各向异性程度是随时间变化的, 这种各向异性程度首先随着激光电场的增强而增加,随后又逐渐减小,直到最后趋于某一大于1的稳定值.分析了激光脉冲结束后质子的平均能量与观测角之间的关系, 并将分析结果与Ditmire小组的实验结果进行了比较,发现我们的模拟结果在定性上与实验相符合.     

超强激光脉冲作用下的Au等离子体L-X射线辐射测量

基于SILEX-Ⅰ激光器,利用单光子计数型电荷耦合器件,在超强超短脉冲激光与高纯度Au靶相互作用中,通过改变入射激光的能量,测量了不同激光功率密度下的Au等离子体L-X射线发射谱。实验结果表明:在超强超短脉冲激光作用下,Au等离子体L-X射线发射过程中由于高速电子存在,会诱发很强的热辐射和轫致辐射,并且Au等离子体特征L-X射线发射强度、热辐射和轫致辐射随激光功率密度增加而增强。     

超强激光脉冲作用下的Au等离子体L-X射线辐射测量

基于SILEX-Ⅰ激光器,利用单光子计数型电荷耦合器件,在超强超短脉冲激光与高纯度Au靶相互作用中,通过改变入射激光的能量,测量了不同激光功率密度下的Au等离子体L-X射线发射谱。实验结果表明:在超强超短脉冲激光作用下,Au等离子体L-X射线发射过程中由于高速电子存在,会诱发很强的热辐射和轫致辐射,并且Au等离子体特征L-X射线发射强度、热辐射和轫致辐射随激光功率密度增加而增强。     

分子的飞秒强光电离

文章以一个实验者的角度，介绍了分子的飞秒强光电离的研究现状。文章从对比飞秒激光电离质谱与纳秒激光电离质谱开始，接着介绍分子在激光场作用下的可能电离机理，着重描述了几个处理分子场致电离的理论模型和实验验证，最后对飞秒激光导致的分子在激光脉冲作用后取向研究进行了简单介绍。     

分子的飞秒强光电离

文章以一个实验者的角度,介绍了分子的飞秒强光电离的研究现状.文章从对比飞秒激光电离质谱与纳秒激光电离质谱开始,接着介绍分子在激光场作用下的可能电离机理,着重描述了几个处理分子场致电离的理论模型和实验验证,最后对飞秒激光导致的分子在激光脉冲作用后取向研究进行了简单介绍.     

X-ray emission from femtosecond laser micromachining

We characterize the spectral properties of X-rays generated from selected metal and semiconductor targets when 120-fs laser pulses are focused to intensities of∼10¹⁴–3×10¹⁵ W/cm² during laser micromachining in air. High fluxes of multi-keV-energy X-rays could be obtained with 280-μJ pulses at a 1 kHz repetition rate. The yield and spectral composition of the X-rays are found to depend sensitively on the processing conditions, and thus the X-ray emission is expected to be a novel indicator of optimal laser machining. Received: 17 July 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Excited ions in intense femtosecond laser pulses: laser-induced recombination

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.     

氢团簇在飞秒强激光场中的动力学行为

利用分子动力学方法研究了氢团簇在飞秒强激光场中的动力学行为. 与库仑爆炸模型所预言的不同, 团簇的膨胀是各向异性的, 质子平均动能沿激光场极化方向上的分量要明显大于垂直于激光场极化方向上的分量. 讨论了团簇各向异性膨胀产生的原因, 分析了激光和团簇参数对各向异性程度的影响.     

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.     

Chirping-dependent ionization and dissociation of methane in an intense femtosecond laser field

We studied the ionization and dissociation of polyatomic molecule methane in an intense femtosecond laserfield with wavelength of 810 nm and intensities ranging from 1.4×10~(14)to 2.6×10~(15) W/cm~2 by massspeetroscopy.Abundant fragment inos were observed in addition to the strong parent ion.The effect offrequency chirp was investigated and it was found that the negatively chirped pulses dramatically enhancedthe dissociation probability,which might be used to control the dissociation pathways.     

Hard X-ray generation from microdroplets in intense laser fields

Microdroplets of 15-μm diameter are subjected to ultra-short laser pulses of intensities up to 10¹⁵Wcm⁻² to produce hot dense plasma. The hot electrons produced in the microdroplet plasma result in efficient generation of hard X-rays in the range 50–150keV at an irradiance as low as 8×10¹⁴Wcm⁻². The X-ray source efficiency is estimated to be about 2 ×10⁻⁷%. A prepulse that is about 11ns ahead of the main pulse strongly influences the droplet plasma and the resulting X-ray emission. For a similar laser prepulse and intensity, no measurable hard X-ray emission is observed when the laser is focused on a solid target of similar composition and this indicates that liquid droplet targets are best suited for hard X-ray generation in laser–plasma interactions.     

Anisotropic explosions of hydrogen clusters under intense femtosecond laser irradiation

We report on measurements of ion energy distributions from hydrogen clusters irradiated by intense laser pulses of duration 40 and 250 fs. Contrary to the predictions of a simple Coulomb explosion model, we observe a pronounced spatial anisotropy of the ion energies from these explosions with the highest energy ions ejected along the laser polarization direction. The origin of the anisotropy is distinct from that previously seen in clusters of high Z atoms such as Ar and Xe. Furthermore, a measured increase in H+ ion energy when longer, lower intensity pulses are employed suggests that multiple-pass, vacuum heating of the cluster electrons is important in the deposition of energy by the laser.     

Coulomb explosion of CS_2 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.     

Fluorescence emission from excited molecular ions in intense femtosecond laser fields

Strong fluorescence emissions were observed for nitrogen, carbon monoxide, and carbon dioxide molecules in intense femtosecond laser fields. These emissions can be assigned to the transitions of the molecular ions from the excited electronic states to the ground electronic states. The formation mechanisms were discussed and the lifetimes were measured for these excited molecular ions in intense laser fields.     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002     

Sampling measurement of soft-x-ray-pulse shapes by femtosecond sequential ionization of Kr+ in an intense laser field

We propose a sampling technique for measuring the shape of ultrashort soft-x-ray pulses. The technique uses the transient state of Kr+ ions that is produced by the femtosecond sequential evolution of Kr ions during optical-field-induced ionization as an ultrafast x-ray-absorption sampling gate. We demonstrate the technique by measuring the pulse shape of the 51st harmonic (15.6 nm) generated by a 100-fs titanium:sapphire laser pulse. The measured pulse duration is 220 fs. Our experimental result confirms that the sequential evolution of Kr+ ions from neutral Kr to Kr2+ is the dominant contribution to the ionization process from the aspect of time-domain measurement.     

Asymmetric high-energy ion emission from argon clusters in intense laser fields

Clusters of 2x10(3) to 4x10(4) Ar atoms are Coulomb exploded in intense (up to 8x10(15) W cm(-2)) laser fields. The dependence of multiply charged argon ion energies on the polarization state of light is probed. A directional asymmetry in the ion-explosion energies is observed for the highest charge states. The ion-energy distribution consists of a low-energy isotropic component, and a high-energy anisotropic one. The results are discussed in terms of an asymmetric Coulomb-explosion scenario.     

超短脉冲强激光与固体靶相互作用中Kα射线的实验研究

在相对论激光强度下,对p偏振30 fs激光与固体Cu靶相互作用中产生的Kα射线进行了实验研究.采用刀边成像技术和单光子计数X射线CCD相结合的探测装置,在单发激光脉冲打靶时同时得到X射线源的尺寸、能谱以及Kα光子的转换效率等多种信息.实验结果与Reich等人的理论计算结果有明显的差异,Kα光子的能量转换效率在激光功率密度为1.6×1018W/cm2的条件下达到最大值7.08×10-6/sr.根据这一结果并结合蒙特卡罗程序,推断出在这一聚焦光强下激光能量转换为前向超热电子的效率约为10%.     

Studies of ion energy and yield from argon clusters heated by intense femtosecond laser pulses

Using time-of-flight spectrometry, the interaction of intense femtosecond laser pulses with argon clusters has been studied by measuring the energy and yield of emitted ions. With two different supersonic nozzles, the dependence of average ion energy on cluster size in a large range of has been measured. The experimental results indicate that when the cluster size , the average ion energy Coulomb explosion is the dominant expansion mechanism. Beyond this size, the average ion energy gets saturated gradually, the clusters exhibit a mixed Coulomb-hydrodynamic expansion behavior. We also find that with the increasing gas backing pressure, there is a maximum ion yield, the ion yield decreases as the gas backing pressure is further increased.