【作者】 樊建梅；

【导师】 崔执凤；

【作者基本信息】 安徽师范大学 ， 原子与分子物理， 2010， 硕士

【摘要】 利用聚焦的高功率密度脉冲激光束入射样品表面产生的等离子体叫激光诱导等离子体。激光诱导等离子体动力学的实验和理论研究一直备受科学研究者的关注,这不仅是由于激光等离子体提供了一个很好的光源,而且激光诱导等离子体在化学、医学、生物物理、原子分子物理、天体物理和等离子体物理等学科发展中起到很重要的作用。近年来兴起的利用激光诱导等离子体发射光谱进行在线痕量分析的激光诱导击穿光谱技术,同时以激光诱导等离子体为基础的激光同位素分离技术、激光制备纳米材料技术、激光加工与激光处理技术的发展,需要对激光诱导等离子体动力学有更深的认识和理解。激光诱导等离子体的动力学过程十分复杂,涉及到激光和物质的相互作用,因此研究样品表面的物理化学性质、所处的环境和激光的各种性质(波长、能量、脉冲宽度)对等离子体的形成动力学过程有重要影响。电子温度和电子密度是描述等离子体性质的两个重要物理参数,而激光诱导等离子体是非稳态等离子体,其动力学过程主要是由这两个参数的演化特性来描述。在实验研究中,虽然研究动力学过程的手段很多,但通过测定激光等离子体发射光谱的时间演化特性来研究其形成动力学,被公认为是最有效的研究手段。多年来人们对纳秒脉冲激光诱导等离子体动力学特性进行了广泛和深入的研究,但对飞秒激光诱导等离子体动力学的研究正在兴起,特别是双飞秒脉冲激光诱导等离子体动力学的研究才刚刚起步。由于飞秒脉冲激光与样品相互作用机理与纳秒脉冲激光有很大的不同,许多特性有待研究,因此开展飞秒脉冲激光诱导等离子体发射时间分辨光谱特性的研究,对揭示飞秒脉冲激光诱导等离子体动力学具有重要作用。本论文研究工作中,我们分别利用波长为800nm,脉宽为30fs的单飞秒脉冲激光和共线型双飞秒脉冲激光作为光源,产生了飞秒脉冲激光诱导Ni等离子体,测定了单飞秒脉冲激光诱导Ni等离子体中Ni原子时间分辨发射谱,分析得到了其电子温度的时间演化特性;研究了相对于采用相等能量的单飞秒脉冲激光束而言,采用共线型双飞秒脉冲激光诱导Ni等离子体中Ni原子发射谱强度的增强情况,得到了双飞秒脉冲情况下发射谱强度增强因子与双脉冲之间延时的关系。论文第一章,首先给出了论文选题的研究背景和意义,其次综述了激光等离子体的产生动力学以及激光等离子体发射谱的实验研究进展。第二章阐述了激光等离子体的基本理论。第三章介绍了本论文研究中采用的实验装置和实验方法。第四章给出了单飞秒脉冲激光诱导Ni等离子体中Ni原子的时间分辨发射光谱的实验测定结果,由测定的谱线相对强度得到了等离子体的电子温度的时间演化特性;同时,还测定了等离子体中Ni原子发射光谱线Stark线宽和Stark线移的时间演化特性。实验结果表明,当延时在110~610ns范围内变化时,等离子体的电子温度变化范围为7500~4500K,与纳秒激光诱导等离子体相比,其电子温度数值较小,等离子体的寿命较短,且等离子体的特性参数随时间演化更快。第五章,在总能量与单飞秒脉冲激光能量相等的条件下,通过调节两脉冲间的延时,测定了不同脉冲间延时下,共线型双飞秒脉冲激光诱导Ni等离子体发射谱强度的增强情况,得到了发射谱强度增强因子与两光束间延时之间的关系。通过比较拟合计算得到的单、双飞秒脉冲激光情况下的等离子体发射光谱寿命、电子温度、发射谱Stark增宽的时间演化特性,分析了双飞秒脉冲激光等离子体发射光谱信号增强的原因。实验结果表明,两光束间延时td在0~310ps范围内,等离子体发射光谱信号强度增强因子经历了先增大后减小再增大的过程,当td在0~50ps范围内变化时,发射光谱信号强度持续增强,在td=90ps时,增强因子出现拐点,在td>310ps后信号增强因子基本维持在同一个值(5~8)。双飞秒脉冲情况下等离子体光谱信号的增强主要是由于等离子体电子温度的增加引起的。最后,在总结所做工作的基础上,对今后的工作开展进行了展望。更多还原

【Abstract】 The plasma generated by focusing a high power density pulse laser beam on the sample surface is known as Laser-induced plasma. It is extensively concerned on the experiment and theoretical research on the laser induced plasma dynamics. The laser induced plasma not only can provide an atomic light source, but also can play an important role in promoting the development of the fields as chemistry, atomic and molecular physics, medicine, biophysics, astrophysics, and plasma physics. The presence of the laser induced breakdown spectroscopy technique applied in the online trace analysis based on the emission spectrum of laser induced plasma and the development of the techniques as laser isotopic element separation, laser producing nanomaterial, laser treating and laser based on the laser induced plasma make it necessary to have further knowing and understanding the laser induced plasma dynamics.The interaction between the laser beam and the sample surface leads to the complexity of the forming process of laser induced plasma. So the physical and chemical properties of the sample surface, and buffer gas environment, and the laser characteristics (such as wavelength, pulse energy, and pulse width) have great influence on the forming dynamics of laser induced plasma. The electron temperature and density is the most important physics parameters in description of the property of plasma. Due to the unstationary of the laser induced plasma, the forming processes of the plasma is described by this two parameters. Although there are many experiment methods to study the plasma production dynamics, the method by measuring the time-resolved emission spectrum of the plasma is well known as the most efficient research approach. In spite of the extensive investigation on the nanosecond pulse laser induced plasma dynamics, the study on the fs pulse and especially the dual fs pulse laser induced plasma were just interested in recent years. Because of apparent difference of the interaction mechanism between the nanosecond and femtosecond pulse laser induced plasma, the investigation on the time evolution property of the emission spectrum will play an important role in understanding the fs pulse laser induced plasma dynamics.In our experiment, the single pulse and collinear geometry dual pulse fs laser-induced Ni plasma under atmospheric environment are produced respectively by using the femtosecond laser beam at 800nm wavelength and 30fs pulse duration. The time resolved emission spectrum of Ni atom in the femtosecond laser induced plasma were measured and the time evolution properties of the electron temperatures were obtained. Comparing with the experimental results from the single pulse fs laser configuration with equal total pulse energy, the enhancement of the Ni atomic spectral line intensities created by the collinear geometry double pulse femtosecond induced plasma were investigated. Finally the dependence of the enhancement factors of the spectral line intensities from the double-pulse experiment on the inter-pulse delayΔt was obtained.In chapter one, the scientific background and significance of the research in this paper is described firstly,then the research progress of the production dynamics as well as the experimental study on the emission spectra of laser-induced plasmas were summarized. In chapter two, the basic theories of laser-induced plasma are reviewed. The chapter three is focused on the experimental setup and the measurement method.In chapter four, the time-resolved emission spectrum of Ni atom in single pulse fs laser induced Ni plasma was measured in air at atmospheric pressure using a femtosecond laser with 30fs pulse width and 800nm wavelength. The electron temperature of fs-laser induced Ni plasmas and its temporal evolution were obtained through the measured relative intensities of the Ni atomic emission spectra lines. In additions, the temporal evolution of Stark broadening and Stark shift of Ni atomic spectra lines were also obtained. It is shown that the electron temperature is varied from 7500 to 4500K when the time delay is in the range from 110 to 610ns, which is different with the dynamic characteristic of nanosecond pulse laser induced plasmas.In chapter five, the enhancement of the Ni atomic spectral line intensities created by the collinear geometry double pulse femtosecond laser induced plasma at different inter-pulse delayΔt were measured with the total pulse energy identical to the single pulse configuration. The dependence of the enhancement factors of the spectral line intensities from the double-pulse experiment on the inter-pulse delayΔt were obtained. By the comparison the fitted and calculated time evolution properties of the plasma emission spectrum lifetime, electron temperature, and the half height width of the emission spectral line between the two experiment configurations. The main causes for the enhancement of the spectral line intensity in dual pulse experimental regime were analyzed. It was shown from the experiment results that the enhancement factor was experienced the processes from increasing at first to decreasing and again to increasing finally when the inter pulse delay was in the range from 0 to 310 ps. At the first stage from 0 to 50ps, the enhancement factor increases until reaching a maximum, then decreased to the turning point before increased again in the second stage from 50 to 310 ps , and finally remained at a fixed value (5~8) from 310 to 1000ps. The enhancement of the spectral line intensity was mainly due to the increase of the electron temperature in the dual pulse fs laser induced plasma. However the increase of the electron density also can lead to the enhancement.In the last part of this thesis, some suggestions on the future research are presented based on the analysis of our experiment research.更多还原