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     

纳秒激光电离产生Kr17 + 的研究

随着激光技术 ,尤其是超快、超强激光技术的发展 ,强光场与原子、分子及团簇的相互作用 ,已成为近年来人们研究的热点[1 -8] .团簇分子与超快超强光场之间的相互作用 ,可以产生高价态离子以及高能电子 ,并产生强X射线发射甚至实现“台式核聚变” ,已引起广泛的重视 .Kr气及其团簇的强场电离 ,近年也有不少研究 .利用波长 1 .0 64μm的皮秒Nd YAG激光 ,Huillier等人在 1 0 1 3 ～ 1 0 1 4 W/cm2 光场强度下 ,研究了氪气的多光子电离过程 ,观察到 +4价的氪高价离子[7] .Castleman等人利用 80 0nm ,功率密度为 1 0 1 5W/cm2 的飞秒激光 ,对…     

团簇在超强超短脉冲激光场中的演化

 利用低温脉冲气阀产生氘团簇束,在SILEX-Ⅰ激光装置上开展实验,研究氘团簇在超强超短脉冲激光场中的演化过程,获得了数十keV的高能氘离子,这些氘离子的能谱分布与库仑爆炸模型计算结果一致。实验结果表明,在一定的激光功率密度条件下,团簇的平均尺度决定了释放出高能离子的能谱分布。激光辐照团簇后,通过阈上电离部分电子逃逸团簇,随着初始电离电子屏蔽作用加强,碰撞电离变成了团簇的主要电离机制。     

超短脉冲强激光与团簇的相互作用

超短脉冲强激光与团簇的相互作用已被证明能够产生能量高达ＭｅＶ量级的高能离子和中子以及非常强的Ｘ射线辐射。广阔好原子团簇在超短脉冲强激光场的作用下的加热、电离和膨胀等机制,对这种相互作用的深入研究可能会对激光核聚变和Ｘ射线激光等应用领域产生重大影响。     

双波长圆偏振光场中分子电离动力学的研究新进展

超短激光脉冲的出现为人们研究原子分子内电子的超快动力学过程提供了重要的技术手段。强激光诱导原子分子的光电离过程是光诱导物理过程的基石,也是目前强场物理领域的前沿热点之一。本文重点综述了双波长圆偏振光场中分子电离动力学的研究进展。首先,介绍了研究强场分子电离动力学的半经典模型,给出了电离电子波包的相位和振幅分布。然后,介绍了利用双波长圆偏振光场测量H2分子和CO分子的电离动力学的研究,发现电离电子的振幅结构以及隧穿后电子受到的长程库仑势都会影响电子的动力学过程。此外,电子波包的相位结构也会包含在光电子的发射角中,这个初始相位编码了电子吸收光子而电离过程中的时域信息。最后,对新型阿秒钟在分子光电离过程中的应用进行了总结,并展望了未来复杂分子体系的应用前景。     

超快强激光驱动的原子分子电离

强场电离是超快强激光与物质相互作用时发生的基本物理过程。强场驱动原子分子的电离电子动力学过程发生在一个光学振荡周期以内,是在阿秒时间尺度上研究电子超快动力学的典范。不仅如此,强场驱动下的超短电子束还为探测原子分子的结构及其超快动力学提供了重要的技术手段。文章首先简要阐述了超快强光场中原子分子电离的基本物理图像,在此基础上,介绍了近年来基于强场电离电子开展的超快过程研究的几个例子,最后简要讨论了强场电离研究的未来可能发展方向。     

一维氦原子非依次电离的经典模拟

利用经典系综理论研究了强超短激光脉冲作用下的（一维）氦原子的非依次电离过程．通过对先后电离的两个电子的平均库仑势能和距核的平均距离的动力学模拟发现：在脉冲长度不变的条件下,长波长和高强度的超短激光脉冲更有助于非依次电离的发生． 关键词：     

超短超强激光及微团簇与大尺度团簇相互作用

用2.5 MV静电加器提供的高品质微团簇束与固体、气体和大尺度团簇(100->1000原子/团簇)碰撞,超强飞秒激光与大尺度氘团簇相互作用,研究微团簇在大尺度团簇中的库仑爆炸特性,研究超短超强激光场中大尺度氘团簇的聚变机理.本文将介绍此项实验研究的进展.     

我国在中红外强场物理领域获重要发现

我国科研人员近日在中红外强场物理前沿研究领域获得重要成果：利用上海光机所强场激光物理国家重点实验室新近建成的可调谐中红外波段的超强超短激光平台,开展了强场原子阈上电离的实验与理论的深入研究,从实验中发现了中红外新波段（例如,2000nm波长）强光场中,原子的阈上电离电子能谱在低能端出现了令人惊异的峰状（甚至双峰）新结构,并进而揭示了长波长条件下长程库仑相互作用起了重要作用．     

强激光场下原子分子量子隧穿实验测量

强激光场下原子分子的多光子电离和隧道电离是极端条件下的量子现象,也是强场光物理中基本的动力学过程.测量原子分子在强激光场中的超快动力学对探索微观粒子内部结构和相互作用有重要意义.本文简述强场光电离中的基本概念和现象,并介绍强激光与原子分子相互作用研究中的重要仪器——冷靶反冲离子动量成像谱仪(COLTRIMS),最后展示了基于COLTRIMS系统研究电子隧穿过程中势垒下相位对光电子动量谱的影响.     

纳秒光场下原子团簇产生高离化Kr离子的激光波长效应

利用不同波长和光强的纳秒激光,对Kr原子团簇进行了激光电离的飞行时间质谱研究,观察到Kr高价离子价态显著地依赖于激光波长,当分别用波长为1064,532,355和266 nm的激光照射Kr原子团簇时,可分辨的离子最高价态分别为+17,+11,+4和+2价;然而离子价态与激光功率密度的依赖关系并不明显。实验结果支持多光子电离-逆轫致吸收加热-电子碰撞电离三步电离模型,表明电子碰撞电离是高价离子产生的主要途径。     

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 1064\,nm nanosecond Nd-YAG laser with intensities varying from $0.8\times10^{11}$ to $5.6\times10^{11}$\,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 CS$_{2}$ neat clusters in the beam. An electron recolliding ionization model is proposed to explain the appearance of those multiply charged atomic ions under such low laser intensities.     

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.     

Application of the laser ablation for conservation of historical paper documents

Laser ablation was applied for surface cleaning and spectroscopic diagnostics of historical paper documents and model samples in the framework of the conservation projects. During cleaning the spectra of ablation products were recorded by means of the LIBS technique which allowed for nearly non-destructive identification of surface layers such as contaminants, substrate and pigments. For consecutive laser pulses a strong decrease of band intensities of the emission lines of Ca, Na, K, Al and Fe ascribed to contaminants were observed. The effect was used for monitoring of the cleaning progress of stained paper. For surface cleaning and spectra excitation the Q-switched Nd:YAG laser of 6 ns pulsewidth operating at wavelengths of 266, 355, 532, and 1064 nm and of fluence selected from the range 0.3-0.9 J/cm² was applied. The ablation parameters were optimized in agreement with the literature and the results were confirmed by surface studies and testing of the mechanical and chemical properties, and also by the response to the ageing process of the paper substrate. In case of the model paper irradiated in the UV range at 266 and 355 nm a visual inspection revealed local damages of the cellulose fibers accompanied by a decrease of the mechanical strength of the substrate. The effect was more pronounced after artificial ageing. The best results were obtained for samples irradiated at 532 nm and at laser fluence below the damage threshold of 0.6 J/cm², which is in agreement with literature.     

High-quality and high-efficiency machining of glass materials by laser-induced plasma-assisted ablation using conventional nanosecond UV, visible, and infrared lasers

This paper reports the micromachining of fused quartz and Pyrex glass by laser-induced plasma-assisted ablation (LIPAA) using a conventional nanosecond laser at wavelengths 266 nm, 532 nm, and 1064 nm, respectively. High-quality surface structuring can be achieved at each of these wavelengths. The micrograting formed has periods of 14 7m at 266 nm, 20 7m at 532 nm, and 30 7m at 1064 nm, respectively. The ablation rate using a 266 nm laser is much larger than that at longer wavelengths. The ablation thresholds of laser fluence are 0.7 J/cm² for 266 nm, 1.5 J/cm² for 532 nm and 3.7 J/cm² for 1064 nm, respectively. The 532 nm and 1064 nm lasers enable hole drilling in 0.5 and 2.0-mm thick fused quartz and Pyrex glass substrates of about 0.7-0.8 mm in diameter. However, the less destructive through channel can be only formed in Pyrex glass by using a 532 nm laser.     

Generation of all-solid-state, high-power continuous-wave 213-nm light based on sum-frequency mixing in CsLiB6O10

Achievement of more than 100 mW of pure continuous-wave deep-ultraviolet radiation at 213 nm has been demonstrated in an efficient all-solid-state laser system that uses two Brewster-cut CsLiB6O10 (CLBO) crystals. The first crystal is used for 266-nm generation by external resonant doubling of 532-nm radiation from a frequency-doubled Nd:YVO4 laser. Subsequent sum-frequency mixing is performed in a second CLBO crystal placed in a diode-pumped Nd:YAG laser cavity to mix the single-pass 266-nm output with circulating 1064-nm light.     

Analysis of surface and material modifications caused by laser drilling of AlN ceramics

For the laser drilling of aluminum nitride ceramic the processing results and the effects related to pulsed irradiation were investigated. Images of the drilled surface revealed regular, cylindrically shaped holes of about 100 μm in diameter independently of the laser wavelength (1064/532/355 or 266 nm). The holes were surrounded by circular heat-affected zones of larger diameter. A comparison of the elemental compositions of the original material and the processed one indicated a decrease of the nitrogen concentration in the affected area. The spectral analysis of the ablated material composition revealed the presence of ions and neutrals in dependence on the laser intensity applied. It was found that at intensity values close to the ablation threshold the ejected material consisted mainly of neutrals, while doubling of the intensity resulted in appearance of single-ionized Al species, which were also observed together with Al clusters in the mass spectra of the UV-excited plasma. Their prevailing content was revealed for drilling at higher intensities around 15 GW/cm² at 532 nm. Results of model calculations indicated, in agreement with the experiment, that at the threshold the ceramic decomposes into gaseous nitrogen and solid Al particulates, while at a higher fluence the material particles vaporize and influence the quality of drilling.     

Laser-assisted decontamination—A wavelength dependent study

We present here the experimental results on cleaning of radioactive dielectric particulates, loosely deposited on stainless steel, by coherent light of 1064 nm wavelength and its three harmonics occurring at 532 nm, 355 nm and 266 nm, derived from an Nd-YAG laser. For the initial few exposures, the decontamination factor has been found to be highest when exposed to 1064 nm radiation. With increasing number of exposures, however, the radiation with reducing wavelength assumes a more important role as a cleaning agent. The observation of almost no cleaning with 1064 nm and much reduced cleaning with its harmonics when the contamination is deposited on a transparent substrate confirms the dominant role played by metal substrate towards expelling the loose particulates from its surface.     

Cluster-assisted generation of multi-charged ions in nanosecond laser ionization of pulsed hydrogen sulfide beam at 1064 and 532nm

The multi-charged sulfur ions of S^q= (q\le 6) have been generated when hydrogen sulfide cluster beams are irradiated by a nanosecond laser of 1064 and 532,nm with an intensity of 10¹⁰\sim 10¹²W1\cdotcm^-2. S⁶⁺ is the dominant multi-charged species at 1064nm, while S⁴⁺, S³⁺ and S²⁺ ions are the main multi-charged species at 532nm. A three-step model (i.e., multiphoton ionization triggering, inverse bremsstrahlung heating, electron collision ionizing) is proposed to explain the generation of these multi-charged ions at the laser intensity stated above. The high ionization level of the clusters and the increasing charge state of the ion products with increasing laser wavelength are supposed mainly due to the rate-limiting step, i.e., electron heating by absorption energy from the laser field via inverse bremsstrahlung, which is proportional to \lambda ², \lambda being the laser wavelength.     

Generation of CdS clusters using laser ablation: the role of wavelength and fluence

The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355, and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the mass spectra showed stoichiometric Cd _n S _n ⁺ and nonstoichiometric Cd _n S _n−1⁺, Cd _n S _n+1⁺, and Cd _n S _n+2⁺ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on wavelength, ranging from around 16 mJ/cm² at 266 nm to more than 300 mJ/cm² at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were identified: the “light” family containing S₂⁺ and Cd⁺ and the “heavy” clusterized family grouping Cd₂⁺ and Cd _n S _m ⁺. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of “heavy” cations as a function of fluence showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms of relative production, the contribution of the “heavy” family to the total cation signal was significantly lower for 266 nm than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm² has been identified as the optimum for the generation of large clusters in CdS.