惰性气体在LIBS中的辅助测量研究

A Review of the Auxiliary Measurement of Nobble Gases in LIBS

激光诱导击穿光谱(LIBS)是一种发射光谱分析技术,因其实时、多元素、无损、远程等优势,在分析高危环境和重要文物等方面具有极大潜力。目前,LIBS测量技术仍受实验设备,测量环境等因素影响,其中,气氛环境对LIBS测量的影响尤为显著。惰性气体因其优秀的物理性质(比热容,导电率,稳定的化学性质,较大的相对分子质量等)有利于辅助LIBS测量,增强光谱信号,提高检测下限,在激光诱导击穿光谱测量中的应用愈发频繁。因此,在详细调研了惰性气体于LIBS测量过程中的应用基础上,解析了其对光谱信号的影响因素,如很低的化学活性有效降低了空气中其他气体(氧气,氮气等)对光谱信号的吸收湮灭、较大的原子体积限制了等离子体向外扩张,使得单位体积内的温度大幅度上升、丰富的核外电子使产生的等离子体中自由电子大幅度上升,提高了电子密度、亚稳态的存在促使惰性气体对待测样品的延迟激发,辅助激发某些较轻元素并大幅度增加等离子体寿命、能量传递更能特定增强某些特殊能级的发射谱线,获得更高的增强因子、还分析了氡气通过衰变对发射谱线的影响,惰性气体的发射谱线也可用于计算样品的等离子体温度和电子密度等,对LIBS的辅助测量有重要影响。列举了亚稳态在LIBS测量中的应用和惰性气体在不同测量对象中对谱线信号的增强因子。分析比较了不同惰性气体在信号增强,烧蚀质量,时间演化方面的差异,结果表明,氦气相比其他惰性气体较为特殊,归结于其特殊的物理性质和核外结构,而其他气体随着周期数的增加,信号增强因子逐渐增加而烧蚀质量却逐渐减小,等离子的时间演化也逐渐变长,为LIBS测量中选择合适的惰性气体提供理论支持和参考。

Laser-induced breakdown spectroscopy(LIBS)is an emission spectrum analysis technology which has great potential in the analysis of high-risk environments and important cultural relics due to the advantages of real-time,multi-element,non-destructive and long-range.Currently,LIBS measurement technology is still affected by experimental equipment and measurement environment,among which the influence of the atmosphere environment on LIBS measurement is particularly significant.Nobble gases are conducive to assisting LIBS measurement,enhancing the spectral signal,improving detection limit,etc.,because of their excellent physical properties(specific heat capacity,conductivity,stable chemical properties,large relative molecular mass,etc.),which are increasingly used in laser-induced breakdown spectroscopy.Therefore,based on the detailed investigation of the application of inert gas in the LIBS measurement process,the influencing factors on the spectral signal are analyzed.For example,the very low chemical activity effectively reduces the absorption and annihilation of spectral signals by other gases in the air(oxygen,nitrogen,etc.).The large atomic volume greatly limits the outward expansion of the plasma,causing the temperature per unit volume to rise significantly;The abundance of electrons causes the free electrons in the generated plasma to rise greatly,which increases the electron density;The metastable state promotes the delayed excitation of inert gas to the sample,assists in the excitation of some lighter elements and greatly increases the plasma lifetime;The energy transfer can enhance the emission spectral of some special energy levels to obtain higher enhancement factors;The effect of radon gas on emission line enhancement through decay was also analyzed.However,the emission of inert gases can also be used to calculate the plasma temperature and electron density of samples,which has an important influence on the auxiliary measurement of LIBS.In addition,the application of metastable state in LIBS measurement and enhancement factor of inert gas on the spectral signal in different studies are also listed.Finally,the differences in signal enhancement,ablation quality and time evolution of different inert gases are analyzed and compared.It turns out that helium is special compared to other inert gases because of its physical properties and extra-nuclear structure.However,with the increase of cycle number,the signal enhancement factor of other gases gradually increases.At the same time,the ablation mass gradually decreases,and the time evolution of plasma is also getting longer,which provides theoretical support and reference for the selection of appropriate inert gases in LIBS measurement.