In this manuscript, the time-resolved laser-induced breakdown spectroscopy (LIBS) on tungsten target in air and the coexistence of LTE among atoms and ions as well as the fulfillment of optically thin plasma condition are reported. The laser-induced plasma (LIP) of tungsten is generated by focusing the second harmonic of a Q-switched Nd:YAG laser of pulse width ~7 ns and repetition rate of 1 Hz on the tungsten target. The temporal evolution of LIP of tungsten is recorded at four different incident laser fluences of 60, 120, 180, and 270 J/cm2. The several atomic and singly ionized lines of tungsten are identified in LIP. For the estimation of plasma temperature via the Boltzmann plot, the transitions at 430.7, 449.4, 468.0, 484.3, 505.3, and 524.2 nm of Atomic transition of tungsten (WI) and that of the ionic transitions, First Ionic transition of Tungsten (WII) at 251.0, 272.9, and 357.2 nm are selected. The electron density is estimated using the Stark-broadened profile of WI line at 430.2 nm. The McWhirter criteria for the local thermodynamic equilibrium (LTE) condition is verified in present experimental conditions as well as the relaxation time and diffusion length are estimated to take into account the transient and inhomogeneous nature of the plasma. The optically thin plasma condition is studied by assessing the experimental intensity ratio of atomic lines and compared with that of the theoretical intensity ratio (branching ratio). The signal to noise ratio (SNR) is also obtained as a function of time with respect to laser pulse and incident laser fluence. All these observations indicate that the spectra should be recorded within the temporal window of 1–3.5 µs with respect to laser pulse where the plasma can be treated as optically thin as well as under LTE simultaneously along with the large SNR.

中文翻译：

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空气中激光诱导的钨等离子体的时间表征

在该手稿中，报道了对空气中钨靶的时间分辨激光诱导击穿光谱（LIBS）以及原子和离子之间LTE的共存以及光学上薄等离子体条件的实现。钨的激光诱导等离子体（LIP）是通过将Q开关Nd：YAG激光器的二次谐波聚焦在钨靶上而产生的，该二次谐波的脉冲宽度约为7 ns，重复频率为1 Hz。在60、120、180和270 J / cm 2的四种不同入射激光通量下记录了钨的LIP的时间演变。在LIP中可以识别出钨的几条原子和单离子化的谱线。为了通过Boltzmann图估算等离子体温度，钨（WI）的原子跃迁（WII）的离子跃迁在430.7、449.4、468.0、484.3、505.3和524.2 nm处的跃迁选择251.0、272.9和357.2 nm的波长。使用WI线在430.2nm处的Stark展宽的分布估计电子密度。在当前的实验条件下验证了局部热力学平衡（LTE）条件的McWhirter标准，并考虑了等离子体的瞬态和非均质性，估算了弛豫时间和扩散长度。通过评估原子线的实验强度比并与理论强度比（支化比）进行比较，研究了光学上稀薄的等离子体条件。相对于激光脉冲和入射激光通量，还随时间获得了信噪比（SNR）。所有这些观察结果表明，应将光谱记录在相对于激光脉冲的1–3.5 µs的时间窗内，在该范围内，等离子体可被视为光学上较薄，以及在LTE下同时具有较大的SNR。