采用高能量钕玻璃激光器输出的单脉冲激光重复作用于土壤样品表面同一位置,利用光谱采集系统对序列激光脉冲作用下形成的等离子体发射光谱进行采集。实验结果显示:在熔穴孔径的约束作用下,激光诱导产生的土壤等离子体辐射不断增强,谱线强度和信背比都随着作用脉冲个数的增加有不同幅度的提高。通过采集等离子体像及熔穴形貌的图片, 对等离子体形状、激光诱导熔穴形貌以及激光烧蚀样品质量进行了研究,初步探究了激光诱导熔穴对等离子体辐射增强的内在机理。在序列激光脉冲作用下,所形成等离子体的体积先是逐渐增大,尾焰发生畸变,然后体积慢慢变小,并且颜色从浅黄色逐渐变为白色,意味着温度在不断升高。激光诱导熔穴的形状比较规则,从孔穴底部到顶部直径逐渐增大,基本成圆锥形。激光烧蚀样品质量随着脉冲个数的增加而递减。借助熔穴的约束作用、孔壁多次反射过程所产生的Fresnel(菲涅耳)吸收以及逆韧致辐射吸收提高了蒸发物质的原子化程度,增大了等离子体辐射强度。

Single pulses exported from high-energy neodymium glass laser were used to act on the same position of soil sample surface repeatedly,and the plasma emission spectra generated from sequential laser pulse action were collected by spectral recording system.The experimental results show that the laser-induced soil plasma radiation was enhanced continuously under the confinement effect of the crater walls,and the line intensities and signal-to-background ratios both had different improvements along with increasing the number of acting pulses.The photographs of the plasma image and crater appearance were taken to study the plasma shape,laser-induced crater appearance,and the mass of the ablated sample.The internal mechanism behind that laser-induced crater enhanced plasma radiation was researched.Under the sequential laser pulse action,the forming plasma as a result enlarges gradually first,leading to distortion at the trail of plasma plume,and then,its volume diminishes slowly.And also,the color of the plasma changes from buff to white gradually,which implies that the temperature increases constantly.The laser-induced crater had a regular shape,that is,the diameter increased from its bottom to top gradually,thus forming a taper.The mass of the laser-ablated substance descends along with increasing the amount of action pulse.Atomization degree of vaporized substance was improved in virtue of the crater confinement effect,Fresnel absorption produced from the crater walls reflection,and the inverse bremsstrahlung,and the plasma radiation intensity was enhanced as a result.