纳秒脉冲激光诱导空气等离子体的近红外辐射特性

开展纳秒激光诱导空气等离子体近红外辐射特性的实验研究,对波长为532 nm的脉冲ns激光诱导产生的空气等离子体的近红外光谱进行测量.结果表明:空气等离子体的近红外辐射在光谱范围为1100-2400 nm内由连续谱和线状谱组成,光谱指认表明线谱主要来源于N,O原子的中性原子谱和氮分子的振动光谱.通过对连续谱的分析得知,黑体辐射是连续辐射的主要来源.空气中波长1128 nm附近的辐射,可能是N和O中性原子谱的贡献.保持真空腔内气压不变,改变腔内氮气和氧气气体组分含量,分析测得的红外光谱数据,可知混合气体中氧气和氮气含量变化只对波长为1128 nm附近的辐射有影响.利用二元线性回归分析对数据进行分析后得知,氧气对波长为1128 nm附近的辐射贡献较大.最后从电离难易的角度分析造成这一结果的原因.

Near infrared characteristics of air plasma induced by nanosecond laser

The near infrared emission from laser induced air plasma has been investigated in a range of 1100–2400 nm.The infrared spectra of air plasma consist of linear spectral and continuum radiation.Most of the spectral features observed are identified,including atomic lines of O I and N I and molecular bands of N2.The spectra show trace of blackbody background emission and the plasma temperature is estimated from Planck law.We find that the continuum radiation is mainly origins mainly from the blackbody emission of plasma.There is a limitation of plasma temperature estimation by using Boltzmann method.For example,the local thermodynamic equilibrium must be satisfied,and the trend of change in plasma temperature can be estimated within a few microseconds after the laser shot.In this paper,the plasma temperature in 15μs after laser irradiation is estimated from the Planck law,and the temperature of air plasma is estimated to be about 3900 K,which can compensate for the shortcomings of Boltzmann method.It is found that the neutral atomic spectra of N and O both may contribute to the radiation of the air plasma at 1128 nm.Then we keep the air pressure in the vacuum chamber at 80 k Pa,and change the nitrogen and oxygen content in the chamber.The infrared spectrum data show that the oxygen content in the mixed gas only affect the radiation of 1128 nm wavelength.The binary linear regression analysis shows that oxygen contributes much to the radiation of 1128 nm wavelength.This can be explained by the difference in ionization potential between molecule O2 and N2.The infrared radiation intensities of the air plasma at 1128 nm under 20-80 k Pa are obtained,and they are compared with the calculated results obtained with the fitting formula.The predicted value is very close to the experimental value and the relative error is negligibly at the pressure of 30-80 k Pa.The study of the characteristics of infrared emission from laser induced plasma is of great significance for understanding and using the physical mechanisms of laser-matter interaction.