Laser Induced Breakdown Spectroscopy Enhancement Characteristic And Application Research

Laser Induced Breakdown Spectroscopy（LIBS）is an elemental analysis method based on atomic emission spectroscopy.A high-energy laser is used to excite the sample to generate a plasma,and the radiation spectrum of the plasma is analyzed to infer the composition and content of the elements in the sample.LIBS technology has been widely used in industrial production,agricultural products,food safety,environmental protection monitoring,archaeological relics,space exploration and other fields since its inception,because it has many advantages such as no sample preparation,simultaneous multi-element detection,and almost no damage to samples.With the continuous improvement of the technical level of lasers and spectroscopic instruments,LIBS technology has shown good development potential.However,the current technology still has problems such as detection limit difference,low measurement accuracy,and low stability.Enhancing the laser induced breakdown spectrum by various means is an important development direction of LIBS technology.In this thesis,the Double-pulse excitation and CO₂ laser irradiation methods are used to enhance the laser-induced breakdown spectroscopy,and the plasma spectral emission intensity and detection accuracy are improved.The main research results are as follows:（1）A dual-pulse laser enhanced laser-induced breakdown spectroscopy technique was proposed.The relationship between the double-pulse focusing structure,the laser wavelength combination,the double-pulse time interval and the plasma spectral intensity was studied in detail,and the physical mechanism of spectral enhancement was analyzed.In the experiment,pre-ablative orthogonal double pulse,reheat orthogonal double pulse and collinear double pulse laser were used to induce the excitation of Al metal samples,and the atomic lines of Al 309.2 nm were obtained three times and five times respectively.20 times enhanced.The spectral stability of three kinds of double-pulse laser configurations was studied.Under the condition of pre-ablative orthogonal double-pulse laser configuration,the spectral stability of LIBS was effectively improved,and the relative standard deviation was reduced by 2%.The enhancement of the laser plasma by the combination of the double-pulse laser wavelength is studied.When the double-pulse laser wavelength modes are 1064 nm+532 nm,1064 nm+1064 nm,532 nm+532 nm,and 532 nm+1064 nm,the spectrum of the Al 256.7 nm atomic line is excited.The intensity is increased by 2,2.2,3,and 4.8times,respectively,compared to the single pulse laser（532 nm）excitation.In the double pulse laser excitation mode,the short-wave laser is more favorable for the initial plasma generation,and the long-wave laser is more effective for re-excitation of the plasma.At the same time,the enhancement relationship between the double pulse laser delay time and the laser induced plasma signal is analyzed.（2）A continuous CO₂ laser and Nd:YAG laser dual-beam enhanced plasma spectroscopy scheme was proposed.After irradiating the Al metal sample with continuous CO₂ laser for a period of time,the Nd:YAG laser was triggered to enhance the plasma excitation.The effects of CO₂ laser spot size,irradiation power and irradiation time on the plasma spectrum were analyzed.The spectral line enhancement factor is maximized when the preheated CO₂ laser irradiation time is 1000 ms and the laser power is 100 W,which is 5 times that of the Nd:YAG laser alone.By preheating the CO₂ laser,the electrons in the sample absorb the long-wave laser energy under the action of the inverse tough absorption effect,and the surface temperature of the sample increases.Since the surface reflectance of the sample is affected by temperature,the increase in temperature causes the reflectance of the sample surface to the Nd:YAG laser to decrease,and the coupling efficiency between the laser and the sample surface increases,which promotes the formation of plasma and enhances the spectral intensity of the plasma.（3）The dual-pulse laser induced breakdown spectroscopy（DP-LIBS）technique was used to enhance the spectrum of heavy metal plasma in the atmosphere.The five atomic lines of 393.4 nm,396.2 nm,422.6 nm,430.2 nm and 455.3 nm of heavy metals such as Fe,Cr and Ni in atmospheric samples were selected as the analysis objects.In the DP-LIBS spectrum,the intensity of the atmospheric heavy metal sample line is enhanced,and the maximum value is reached at a pulse delay time of about 15 us,and the best plasma line enhancement is obtained.The intensity of the main heavy metal lines such as Fe,Cr,Ni is about three times that of single-pulse laser excitation.The spectral line stability was improved by using DP-LIBS technology.The relative standard deviation of the single-pulse excitation line was 6%,and the relative standard deviation of the line obtained by the double-pulse laser technique was reduced to 3%.This result has important scientific significance for improving the accuracy and stability of atmospheric environmental monitoring.