Study On The Thermal Electron Emission Behavior Of Laser Ablation On Metal Surfaces

As a spectroscopic analysis method,laser-induced breakdown spectroscopy(LIBS)technology has been applied in many fields.LIBS qualitative analysis scheme has been relatively developed,but the quantitative analysis scheme still needs comprehensive exploration,especially improvement of calibration.Quantitative calibration method for LIBS includes standard sample method,internal standard method and calibration-Free method(CF-LIBS).The former two schemes need adequate standard samples,but it is difficult to prepare suitable standard samples for all application scenarios because of LIBS’s wide range of application.The calibration-free method is based on local thermal equilibrium(LTE)state of LIBS plasma.The initial conditions of plasma formation are the key factors for LIBS plasma to reach local thermodynamic equilibrium while the pulsed thermal electron emission induced by laser ablation is one of the initial conditions for the evolution of LIBS plasma.Based on the background,this paper carried out research on the behavior of thermal electrons emission on metal surface.In order to track the pulsed thermal electron emission,a circuit method is proposed for measurement of thermal electron emission current based on the circuit measurement of current resource.Aiming at the ablation process initiated by Nd:YAG pulse laser under vacuum condition,we designed two measurement modes which namely inductance mode and capacitance mode.For the modes,the measurement scheme and calculation method of thermal electron emission current intensity are established respectively.Both modes can successfully track the evolution of electron emission current.By comparing the two modes,it is proved that capacitance mode may achieve higher accuracy of electron current than inductance mode if the used capacitor can process large enough capacitance and small enough parasitic inductance.Although the results of capacitance mode showed a trend of higher accuracy with better capacitor used,the accuracy of inductance mode is reached higher than capacitance mode due to the limitation of capacitor qualities.Then,the temporal characteristics of thermal electron emission are studied using inductance measurement mode.It is indicated that the peak of the thermal electron emission intensity is later by 18 ns than the laser pulse,which is far ahead of the formation moment of the ablation plasma.The typical rising edge of electron emission pulses is about 18 ns and the width is about 24 ns,which is obviously earlier than the formation of ablation plasma.By the way,multiple ablation test on certain surface site shows that the thermal electron emission pulses generated by different ablations are almostly similar to each other in shape within certain ablation times(50 times).Therefore,the measurement error can be eliminated by average measurement on multiple ablations to improve the accuracy.In addition,the vacuum condition of the experiment chamber has certain influence on the thermal emission electrons.Theoretical calculation demonstrates that the influence of background gas can be ignored when the vacuum is 3 Pa.The influence of laser spot size,material type and laser energy on the thermal electron emission behavior was studied also using inductance measurement mode.The results show that the spot area and laser energy jointly affect the thermal electron emission intensity by changing the laser energy density impacting on the surface.If the laser energy keeps constant,the peak of thermal electron current density decreases abruptly with of spot area.By comparison of three different targets,with the same laser energy density,the ablation thermal electron emission current density of titanium target is the largest,followed by copper target and aluminum target.According to the thermal electron emission law,the emission current density is determined by the emission region’s temperature and matrial work function.The surface temperature rising process is the main factor determining the thermal electron current density,while the laser pulse energy and spot area determine the surface temperature rising process and the melting point of the material determines the peak current density of the thermal electron emission.