A numerical investigation of the dependence of the threshold irradiance on the wavelength in laser-induced breakdown in ?
A numerical investigation of the dependence of the threshold irradiance on the wavelength in laser-induced breakdown in ?
复制标题
阈值辐照度对激光诱导击穿波长依赖性的数值研究？
DOI：
10.1088/0022-3727/32/4/012
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
Jamal M Daoud
中科院分区：
文献类型：
--
作者：
Y. Gamal;Mahmoud Shafik;Jamal M Daoud
对氮分子的激光击穿进行了研究，研究了不同气压下阈值辐照度与波长的关系。该分析是基于电子能量分布函数（EEDF）和一组速率方程描述的激发态人口的变化率的时间依赖性玻尔兹曼方程的数值解。在分析过程中引入了速率系数和截面随电子能量的变化关系，以探讨各个物理过程对击穿现象的确切贡献。计算是在Davis等人的实验条件下进行的。在该实验中，在1064、532、355和266 nm的波长下，在25-760 Torr范围内的气压下，用在t0范围内的激光辐照度测量氮的击穿。计算的阈值被认为是在所有波长的测量结果吻合良好。计算的EEDF及其参数表明，在nm处，振动损耗占主导地位。基态和激发态分子的碰撞电离被发现在532，355和266 nm处的击穿现象作出较小的贡献。然而，在1064 nm处对该过程的贡献更有效。因此，击穿现象通过电子级联过程进行，该过程仅将分子转换为激发态，其中多光子电离起作用。
Studies of laser-induced breakdown in molecular nitrogen were carried out to investigate the dependence of the threshold irradiance on the wavelength at various pressures. The analysis was based on the numerical solution of the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations describing the rate of change of the excited states population. The rate coefficients and cross-sections as functions of the electron energy were introduced into this analysis in order to probe the exact contribution of each physical process to the breakdown phenomenon. The calculations were performed under the experimental conditions of Davis et al. In this experiment the breakdown of nitrogen was measured at wavelengths of 1064, 532, 355 and 266 nm, over gas pressures in the range 25-760 Torr, with laser irradiances in the range to . The computed thresholds were found to be in good agreement with the measured ones at all wavelengths. The calculated EEDF and its parameters showed that, at nm, vibrational losses are dominant. Collisional ionization of ground and excited state molecules was found to make a minor contribution to the breakdown phenomenon at 532, 355 and 266 nm. However, the contribution to this process at 1064 nm was more effective. Therefore, the breakdown phenomenon proceeds via an electron-cascade process that converts the molecules only into the excited states, whence multiphoton ionization plays its role.