偏振分辨激光诱导击穿光谱的铝铁合金信背比改善效果研究

偏振分辨激光诱导击穿光谱(PRLIBS)以其低成本压制背景的特点在LIBS降低检测限上有着重要意义,但是理论上的争议和信背比改善效果不稳定限制了其应用前景。为深入分析等离子体产生偏振特性的原理和PRLIBS信背比的改善效果,采用1 064 nm纳秒脉冲激光器和光纤光谱仪对铝铁合金样品进行了偏振分辨LIBS的信背比改善效果及偏振原理的探索性实验。通过对辐射能量的估算,大胆地推测轫致辐射在背景辐射中的占比是由大变小的,并据此解释了背景辐射强度在时间上的变化。通过改变能量密度、检偏角度、探测角、延时时间、积分时间等因素,采集光谱强度和波长数据、计算偏振度和信背比,观察到铝铁合金的等离子体光谱中背景谱和分立谱均有偏振且存在偏振度和偏振方向的差异,发现偏振LIBS改善信背比的效果与实验参数包括能量密度、延时时间、检偏角度以及波长有关。PRLIBS信背比关于能量密度的变化与一般LIBS类似,在能量较大时会趋于饱和。检偏角度会影响信背比大小,与光谱的偏振方向和偏振度有关,推导了信背比改善效果关于偏振度、检偏角度、偏振方向夹角的公式。在波长上连续谱偏振度趋势较为稳定而分立谱偏振度随光谱强度增大而减小。随延时时间增加,偏振度变化不明显,原因是相比于积分时间延时的改变量很小,而信背比的变化趋势与无偏振LIBS一致。总结了国内外对PRLIBS机理具有代表性的解释并进行了讨论,排除了激光光场、菲涅尔反射、各向异性电子速度分布等因素对等离子体偏振特性的决定性作用。研究结论是在ns-LIBS实验中,复合辐射在可见光和紫外光范围内占背景辐射的大部分,其偏振特性主要源于等离子体复合阶段产生的各向异性复合过程,原子谱偏振特性可能源于该过程中受激原子的磁性支能级间数量的不平衡,而背景谱与原子谱的偏振度及偏振方向的差异主要取决于偏振特性产生机理的不同。研究发现, PRLIBS并不总能提升元素的信背比,尤其对弱光谱信号改善效果有限,要获得较好的压制背景的效果,可以对PRLIBS的能量密度、检偏角度、延时与积分时间等条件加以控制,在能量密度20 J·cm-2积分时间30μs检偏角度20°下FeⅠ407.12 nm处的信背比由4.86提升至12.97。偏振度与探测角相关性较小,原因可能是导体的菲涅尔反射效应很弱。研究结果对PRLIBS的原理研究和应用提供了有效的理论基础。

The Improvement of Signal-to-Back Ratio in Polarization Resolved Laser-Induced Breakdown Spectroscopy of Al-Fe Alloy

Polarization-resolved laser-induced breakdown spectroscopy(PRLIBS) has important significance in reducing the detection limit of LIBS with its advantage of background suppressing. However, the controversy in theory of polarization and the unstable improvement of the signal-to-back ratio limit its application prospects. In order to investigate the theory of polarization characteristics and the improvement of the signal-to-back ratio, this study used 1 064 nm nanosecond pulsed laser and fiber optic spectrometer to conduct an exploratory experiment on the improvement of polarization-resolved LIBS signal-to-back ratio and polarization mechanism of aluminum-iron alloy samples. According to estimation of the energy of bremsstrahlung radiation, it’s confirmed that proportion of bremsstrahlung radiation in background reduces with time. By varying the energy density, detection angle, analyzer angle, delay, integration time and other factors, the spectral intensity and wavelength data were collected, and polarization degree and the signal-to-background ratio were calculated. It was observed that the background and the discrete spectrum of the plasma spectrum of the aluminum-iron alloy was partial polarized. There were phenomena of polarization of spectrum and there were differences in polarization degree and directions of polarization. It was found that the effect of polarized LIBS to improve the signal-to-back ratio is related to experimental parameters including energy density, delay time, detection angle and wavelength: signal-to-back ratio of PRLIBS varies with respect to energy density similar to that of LIBS. SBR becomes low and flat when the energy density is large. The analyzer angle affects the SBR, which is related to the polarization direction and degree of polarization of the spectrum. The formula for improvement about the polarization degree, the angle of detection and the polarization direction is derived. The polarization degree of continuum is flat at all wavelengths and polarization degree of the discrete spectrum decreases as the spectral intensity increases. Though not obvious, the polarization degree changes with the delay increasing. The reason is that the amount of change in the delay time is too little compared to the integration time. The trend of the signal-to-back ratio is consistent with non-polarized LIBS. The representative explanations of the PRLIBS mechanism at home and abroad are summarized and discussed. It’s proved that laser field, Fresnel reflection effect, anisotropic electron velocity distribution and other factors play an inconclusive role in the polarization properties of the plasma. The conclusion is that in the ns-LIBS experiment, most of the background in the visible and ultraviolet spectrum comes from the recombination radiation. The polarization characteristics are mainly due to the anisotropic recombining process during the recombination stage of plasma. During this process, the number of magnetic sublevel of the excited atoms is imbalanced, and the difference between the polarization degree and direction of the background and the atomic spectrum mainly depends on the mechanism of polarization. Studies have shown that PRLIBS does not always improve the signal-to-back ratio of elements, especially for weak discrete spectrum. When the energy densities, analyzer angle, delay and integration time of PRLIBS are controlled, a better background effect can be obtained. With the energy density of 20 J·cm-2 and the integration time at 30 μs, the SBR at Fe 407.12 nm increased from 4.86 to 12.97. It was found out that polarization degree is less correlated with the variable detection angle, for which the reason is probably that the Fresnel reflection effect of the conductor is too weak. The research results provided an effective theoretical basis for the research and application of PRLIBS.