Study On Laser-induced Breakdown Spectroscopy Assisted With Laser-induced Fluorescence

Laser-induced breakdown spectroscopy(LIBS)is a laser-ablation-based spectroscopy method,in which elemental information is deduced by analyzing spectra emitted from laserinduced plasmas.Though LIBS has the advantages of minimal sample preparation,fast response,in-situ and remote detection,its drawbacks of spectral interference,self-absorption effect,and poor sensitivity significantly hinder its further development.Therefore,LIBS assisted with laser-induced fluorescence(LIBS-LIF)was investigated to improve analytical performance of LIBS in this thesis work,and three novel methods for resonant excitation in LIBS-LIF were proposed,which further improved analytical sensitivity and accuracy.The detailed contents are as follows:An experimental setup for LIBS-LIF analysis was built,and an optical system was designed,integrating laser control,laser monitoring,spectral collecting,sample surface lighting and imaging.For convenient data analyses,a software was developed for spectral processing,integrating exploring,pre-processing,background removal,normalizing,peak searching,and quantitative analysis,etc.A method for selecting resonance-excited target atoms was studied.The relation between fluroscent intensity and number of target atoms was discussed and demontrated by detecting trace cobalt element in steels with stimulating cobalt atoms in different states.The result showed that the atoms with a higher population offered higher fluroscent intensity and analytical sensitivity.Silicon,copper,and manganese in steels,as well as ytterbium,aluminum,and phosphorus in silica glass were used to validate this method of selecting target atoms in different element.Spatially selective excitation for reducing excitation interference was proposed.Inhomogeneity of the laser-induced plasmas was investigated with resonant excitation.Taking chromium and nickel elements in steels as examples,it was discovered that optimal excitation locations were the center of the plasmas for the matrix iron,but the periphery for Cr and Ni.By focusing an excitation laser at the optimal locations,both the excitation efficiency and the analytical sensitivity were improved.A novel method using laser-stimulated absorption was proposed to reduce the selfabsorption effect.Namely,a non-focused wavelength-tunable laser was used to radiate the whole plasma,and the cold atoms in plasma periphery which cause the self-absorption were dramatically reduced by the excitation.The self-absorption effect was reduced.The feasibility of this method was proven by observing the extent of self-reversal disappearing in spectral lines of matrix elements.The improvement of analytical linearity and accuracy was demonstrated by analyzing low-concentration elements in steels using this method.For solving the problem that some hard-to-excite elements cannot be resonantly excited in open air,a method of forming and resonant exciting molecular radicals was proposed,by which,the transient radicals in plasmas could be obtained by chemical reactions in open air.The laser-induced fluorescences from these radicals can be observed and deduced to provide elemental information.The excitation and observation windows are moved to VIS-NUV(200-800 nm),which make it feasible to detect hard-to-excite elements in open air.The physical mechanism and experimental process were discussed by taking carbon-nitrogen radicals as an example.High enhancement factor,analytical sensitiviy and accuracy in quantitative analyses were demonstrated by detecting low-concentration carbon in steels.Universality of this method was proven by resonant exciting dicarbon,aluminum monoxide,silica monoxide,and boron monoxide radicals.In summary,the LIBS-LIF technique was systemically investigated in this thesis.Methods of selecting target atoms,spatially selective excitation,laser-stimulated absorption,and molecular radical excitation were proposed to reduce spectral interference and selfabsorption effect,by which analytical sensitivity and accuracy were significantly improved.This thesis provides a guide of theory and experiment for further development of LIBS applications.