Research On The Characteristics And Mechanism Of Spatial Confinement Spectral Enhancement Of Laser Plasma Under Atmospheric Pressure

Laser-induced breakdown spectroscopy is an excellent analytical technique which is proposed in 1960s.Compared with other chemical analytical tool,LIBS shows many advantages such as rapid testing,multielement detection,remote detection,normally without the need for sample preparation and so on.Therefore,LIBS has already been applied in a wide range of applications,which include environment monitoring,soil detection,biological agents,artwork diagnostics and so forth.However,by contrast,the limit of detection(LOD)of LIBS is relatively poor and the emission intensity is low,which prevent a wide range of applications of LIBS in industry and agriculture.Thus,some methods has been proposed by researchers in order to enhance spectral signal,and increase the repeatability of experiments.One of the methods called spatially confined LIBS performs its unique advantages:(1)easy implement,and slight modification for system;(2)stabilizing plasma plume,and reducing experimental error from laser pulse fluctuation;(3)enhancing spectral signal and improving LOD.In this work,we study the spectral signal enhancement from nanosecond laser-induced plasmas confined in cavities.At different laser energy condition,the plasma plumes were confined in cylindrical cavities and conic cavities various diameters,and the spectra signal,enhancement factor and plasma electron density and temperature were collected.The main content is divided into the following two parts.(1)Discuss the influence of cylindrical cavity on Al plasmas,corresponding to the third chapter of this paper.By utilizing cylindrical cavities of various diameters to contrast aluminum plasma,it is found that different diameters cavities may cause different effect.When the cavities with diameters of 2 and 3 are implemented,the plasma is quenching and the spectra enhancement effect disappears.When the cavities of diameters from 4 to 9 mm are applied,enhancement factor can reach 5 and above by optimizing the laser energies and cavity diameters.With the cavity diameter increasing to more than 10 mm,the primary and secondary enhancements of Al lines could be observed.The main reason for the secondary enhancement is due to the fact that the shockwaves are reflected more than one time when arriving at the cavity inner wall and the secondarily reflected shockwaves once again compress plasma into a smaller core,leading to an increasing collision rate and an elevated population of excited atoms at high state.(2)Discuss the influence of conical cavity on Al plasmas,corresponding to the fourth chapter of this paper.By comparing the time-integrated and time-resolved spectra and plasmas electron density and temperature,enhancements of spectral signal only appear in Mode I,and plasmas electron density and temperature are increasing by 27%and 23%,respectively.In the cases of Mode II and without any confinement,these physical parameters are rapidly decreased during the plasma cooling.The underlying cause is probably that the shockwave reflects downward to the plasma center and makes plasmas flattened and near to the sample surface rather than to make plasma plume denser and narrower.