Time evolution of emission spectra from plasmas produced by irradiation of seawater surfaces by a femtosecond laser

Abstract

543.423;533.9 The spectral and temporal characteristics of plasmas produced by femtosecond laser pulses acting on seawater surfaces are studied. The time dependence of the decay in the intensities of emission lines and the continuum are determined and the electron density is estimated using the three-body recombination time. It is shown that an atomic line with a low excitation potential is the most sensitive for femtosecond laser induced breakdown spectroscopy.Introduction. The increasing anthropogenic effect on the world's oceans has created a need for the development of new methods for monitoring marine ecosystems. Laser induced breakdown spectroscopy (LIBS) is definitely one such operational technique. LIBS does not require advanced preparation of samples, can be used for spectral analysis in real time, is a contactless, remote analysis technique, and can be used to determine the concentrations of a wide range of elements with relatively small sample sizes. The use of LIBS for determining the elemental composition of seawater was first proposed in [1,2]. Further studies have established the optimal parameters for detection of the emission lines of the elements dissolved in seawater with breakdown by nanosecond laser pulses [3-10]. The detection limits for elements by this method of laser plasma excitation is ~10

The relationship of detection limits with electron impact excitation rate constants

“…As an example of spectrum with a large number of emission lines, we take one from work [11], in which plasma is generated by femtosecond laser pulse on the sea water surface. The most intense lines in spectrum are of Na I 589, Ca II 393.4, Ca I 422.7 and Mg II 279.6 nm.…”

“…Therefore, if the gate delay exceeds the recombination time of ions then the population of the upper level N 2 will be determined mostly by collision of the electron with atom (ion) in the ground state. The characteristic recombination times for the femtosecond plasma are approximately 20 ns [11], i.e. Let's define excitation rate constant for Ca I-II, Mg II and Na I at temperatures T = 6000 and 4000 K which corresponds to experimental conditions of work [15].…”

“…Herewith the elemental analysis and spectral characteristics of laser breakdown on a liquid surface are scarcely investigated. It should be noted work [10] in which the distribution of Ca 2+ ion in water and biological samples is investigated, as well as works [11][12][13] in which temporal behavior of emission of plasma generated on the sea water surface is described and detection limits of elements is determined [13].…”

“…For LIBS of industrial, waste and sea waters, the emission spectrum of plasma consists of a large number of lines belonging to a single or a few chemical elements [11][12][13][14]. Thus, during the elemental analysis it is necessary to compare the LOD of several elements.…”

Recent development and application of laser induced breakdown spectroscopy

“…结果 显示, 消融区表面的fs-LIBS强度很高, 而深处的较 低, 由此表明fs-LIBS适合于表面污染物的高灵敏度 检测. Ilyin等人 [7] 采用Ti:Sapphire激光器(800 nm, 42 fs, 100 Hz, 1.1 mJ), 研究海水表面的等离子体发射光 谱和时间特性, 证明了低激发电位的原子谱线具有 更高的灵敏度. Bonis等人 [8] [10] 分别用SP-LIBS和DP-LIBS对低气压环境下钢样品的 真空紫外光谱进行研究, 结果也证明, 双脉冲激光方 法能够显著提高谱线强度, 并在两脉冲延迟为100 ns 得到了最强的谱线.…”