非平坦样品激光诱导等离子体光谱特性研究

激光诱导击穿光谱（LIBS）作为一种快速、实时的元素分析技术，由于其在痕量元素探测、地质环境监测等领域有着广阔的应用前景，而受到人们极大的关注。在实际应用中，样品表面是影响等离子体产生及其特性的关键环境因素之一。在大气环境下，利用脉宽为8 ns、波长为1 064 nm的纳秒脉冲激光产生等离子体，对比研究了天然岩石样品在非平坦和平坦表面条件下等离子体的发射光谱。基于激光辅助辐射波模型，阐释了非平坦样品表面对其光谱特性的影响。通过对比等离子体时间积分光谱，发现非平坦样品的谱线强度相比于平坦样品的谱线强度减弱了近70%，该结果说明非平坦样品表面对LIBS真实测量数据的负面影响不可忽视。针对褐铁矿样品中的谱线Fe Ⅰ 404.58 nm和Fe Ⅰ 438.35 nm，研究了在平坦和非平坦样品表面下的峰值强度以及其衰减因子随激光能量的变化规律，结果表明非平坦样品表面条件下采集的光谱强度始终低于平坦样品表面的光谱强度。光谱强度的衰减因子先随激光能量增大而逐渐降低，并在激光能量33 mJ达到最小值，后随激光能量的进一步增大而增大。实验结果进一步表明在非平坦样品表面条件下产生了密度较低的等离子体，并且非平坦与平坦样品的电子密度的比值在激光能量33 mJ时达到最小，此结果与光谱强度的衰减因子随激光能量的变化趋势一致，这是源于非平坦样品表面会形成较大激光入射角度，使得激光等离子体能量吸收区厚度变薄，产生等离子体屏蔽效应所对应的激光能量阈值升高。此外，样品表面状态和激光能量对等离子体温度的影响甚微。阐述了非正入射时等离子体特征参数与正入射时等离子体特征参数的联系和差异，揭示了非平坦样品激光等离子体特征参量变化的内在物理机制，为室外LIBS探测技术在元素定性和定量分析中光谱强度的校正提供参考。

Investigation of the Spectral Characteristics of Laser-Induced Plasma for Non-Flat Samples

Laser-induced breakdown spectroscopy (LIBS), a fast and real-time tool for elemental analysis, has attracted great attention due to its broad applications in trace detection, geological environment monitoring, and other fields. The sample surface is one of the key environmental factors that affect the generation and characteristics of plasma. In this work, a 1 064 nm-laser beam with a pulse width of 8 ns is used to produce plasma in ambient air and comparatively investigate the emission spectra of a series of natural rock samples under non-flat and flat samples surfaces. Based on the laser-supported detonation wave model, the influence of non-flat sample surface on spectral characteristics of laser-induced plasmais discussed. For time-integrated spectra, the results show that the spectral intensities of the atomic lines of the non-flat sample are reduced by nearly 70% compared to those of the flat sample. This indicates that the negative effect of the non-flat sample surface on the LIBS cannot be ignored. According to the signal intensity of the spectral lines, Fe Ⅰ 404.58 nm and Fe Ⅰ 438.35 nm from limonite sample under different laser energies, the variation of their peak intensities and reduction factor with the change of laser energy were studied under the conditions of flat and non-flat sample surfaces. It is found that the spectral intensity under the condition of the non-flat sample surface is lower than that under the condition of the flat sample surface. It is worth noting that the reduction factor of spectral intensity first decreases gradually with laser energy, reaches the minimum value at 33 mJ, and then increases with the further increase of laser energy. Further observations show that laser-plasma with lower electron density is generated on the non-flat sample surface, and the ratio of the electron density of the non-flat sample to that of the flat sample reaches its minimum at the laser energy of 33 mJ, which is consistent with the changing trend of reduction factor with laser energy. This mainly arises because a thinner energy absorption region in laser-plasma is formed due to the large laser incident angle on the non-flat sample surface, thereby increasing the laser energy threshold corresponding to the plasma shielding. Moreover, it is found that the sample surface and the laser energy have little effect on the plasma temperature.