皮秒laser产生Cr等离子体的时间分辨发射光谱研究
Time-resolved optical emission spectroscopic studies of picosecond laser produced Cr plasma
摘要：
采用时间分辨的光发射光谱法测量了60 ps和300 ps激光脉冲辐照Cr靶产生的等离子体，研究了中性粒子和离子发射的线宽（$δλ$）随环境气压的变化。从10 $^{-6}$ Torr到10 $^2$ Torr范围内的测量结果表明，对于两种辐照，中性粒子（Cr I）的$δλ$与单离子化Cr（Cr II）的$δλ$相比有明显不同的变化。对于Cr II，$δλ$随压力单调增加，但对于Cr I，在中等压力下振荡明显。这种振荡不依赖于所使用的激光脉冲宽度。尽管在等离子体形成机制的差异，它是实验发现，有一个最佳的中间背景压力的中性$δλ$下降到最小值。重要的是，这些结果强调了这样一个事实，即对于中等压力，从中性粒子的$δλ$计算等离子体数密度的通常做法需要谨慎地进行，以避免得出不准确的结论。
英文摘要：
Time-resolved optical emission spectroscopic measurements of a plasma generated by irradiating a Cr target using 60 picosecond (ps) and 300 ps laser pulses is carried out to investigate the variation in the linewidth ($δλ$) of emission from neutrals and ions for increasing ambient pressures. Measurements ranging from 10$^{-6}$ Torr to 10$^2$ Torr show a distinctly different variation in the $δλ$ of neutrals (Cr I) compared to that of singly ionized Cr (Cr II), for both irradiations. $δλ$ increases monotonously with pressure for Cr II, but an oscillation is evident at intermediate pressures for Cr I. This oscillation does not depend on the laser pulse widths used. In spite of the differences in the plasma formation mechanisms, it is experimentally found that there is an optimum intermediate background pressure for which $δλ$ of neutrals drops to a minimum. Importantly, these results underline the fact that for intermediate pressures, the usual practice of calculating the plasma number density from the $δλ$ of neutrals needs to be judiciously done, to avoid reaching inaccurate conclusions.
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