环境气压对激光诱导等离子体内靶材元素与空气元素光谱时间分辨特性的影响

Effect of Ambient Pressure on Spectral Time Resolution Characteristics of Target and Air Elements in Laser⁃Induced Plasma

激光诱导击穿光谱已被广泛应用于物质分析,但是目前的研究多关注于靶材料元素,极少有对环境气体元素进行分析的公开报道,而激光诱导等离子体与环境空气混合的过程对于理解等离子体膨胀过程极其重要。鉴于此,本团队研究了激光诱导击穿光谱中靶材元素铜和气体元素氮、氢、氧的特征光谱随环境气压、延迟时间变化的规律。实验结果表明:铜元素的特征光谱强度与环境气压不呈单调关系,气体元素的光谱强度与环境气压呈单调关系,其中氮元素只有在环境气压大于10 Pa时才可以探测到,氢和氧元素可以在10^(-2) Pa时探测到;气体元素的光谱强度随环境气压升高而单调增强,随延迟时间增加而快速下降,信噪比随延迟时间增加而先增大后下降。本工作有助于促进对激光诱导等离子体膨胀过程的理解和实验参数的优化。

Objective Laserinduced breakdown spectroscopy(LIBS)technology has attracted the attention of numerous researchers since its inception.It has been widely used in material analysis owing to its advantages such as high detection speed,small sample damage,no sample preparation,and online analysis.It has applications in fields such as material detection,element analysis,deep space exploration,and various other fields.However,current studies on LIBS mainly focus on the LIBS signals of target material elements,and there is little analysis of the LIBS signals of environmental gas elements.In the laserinduced plasma process,the highenergy pulse laser will not only excite the target material elements but the ambient gas elements near the target surface will also be excited by the highenergy pulse laser,resulting in a unique LIBS signal of the ambient gas elements.Therefore,the laserinduced plasma process is related to not only the target material elements but also environmental gas elements.The laser plasma expands rapidly after it is generated and interacts with the ambient gas during the expansion process.The plasma expansion process is significantly influenced by the types of gases and air pressure environments.Highpressure ambient gases restrict the expansion of the plasma and affect its spectral radiation.Both the laserinduced plasma and the plasma expansion processes can be significantly impacted by ambient gas.The variation in the target element LIBS and the gas element LIBS signal with air pressure and delay time is of major significance for understanding the influence of ambient gas on laserinduced plasma.Methods Nd∶YAG laser with wavelength of 1064 nm and pulse width of 10 ns was used to irradiate Cu target without O and with Cr,Fe,Cr elements to generate plasma.The pulse laser energy and repetition frequency were set to 50 mJ and 1 Hz,respectively.Pulsed laser was reflected by two mirrors and then focused by a focal lens with a focal length of 100 nm on Cu target in the vacuum cavity.Mechanical and molecular pumps were used to realize the experimental vacuum degree of 5.5×10^(-5)‒1.0×10^(5) Pa.The characteristic spectra of the target elements Cu,N,H,and O in laser plasma were analyzed by collecting plasma spectra from the normal direction of the laser incident plane.The variation in the characteristic spectra of the target element Cu and gas elements N,H,and O in the laser plasma with ambient pressure and delay time was studied.Results and Discussions The experimental results show that the spectral intensity of the characteristic line CuⅠ521.8 nm does not show a monotonic relationship with the ambient pressure,and there is an obvious interval effect,as shown in Fig.4.The spectral intensity decreases with the ambient pressure in the range of 10-2‒1 Pa,increases with the ambient pressure in the range of 1‒104 Pa,and decreases again with the increase in ambient pressure in the range of 104‒105 Pa.The radiation lifetime of the characteristic line CuⅠ521.8 nm is analyzed,and the results show that the radiation lifetime of the characteristic line CuⅠ521.8 nm increases with the ambient pressure.As shown in Fig.8,the spectral intensities of the characteristic spectra of the gaseous elements N,H,and O are monotonically related to the ambient pressure.The spectral intensities of the characteristic spectra of the gas elements increase monotonically with the ambient pressure owing to the increase in the particle number density of the ambient gas and the enhancement of the restriction effect of the ambient gas.However,the characteristic spectral intensities of the gas elements N,H,and O have different attenuation rates with delay time owing to the different molecular bond energies of different gas elements.Among them,the characteristic spectral intensities of N have the fastest attenuation with the delay time,followed by H,and O has the slowest attenuation.The results show that the radiation lifetime of the characteristic spectra of gaseous elements increases with an increase in atmospheric pressure.Furthermore,it is found that the lowest pressure at which the characteristic spectra of gaseous elements can be detected is not the same,in which the characteristic spectra of N can be detected only when the ambient pressure is greater than 10 Pa,while the characteristic spectra of H and O can be detected at a pressure of 10-2 Pa.The signaltonoise rate(SNR)analysis of the gas element LIBS signal shows that the SNR of the N,H,and O gas element LIBS signal changes with the delay time in the same way,which increases first and then decreases with the delay time.However,the time required to obtain the maximum SNR of the gaselement LIBS signals is not completely consistent but is roughly between 500 ns and 1000 ns.The maximum SNR of the LIBS signals of each gas element is inconsistent at different ambient pressures,and the maximum SNR of the O element reached 19 when the air pressure was 105 Pa and the delay time was 1000 ns.Conclusions This work can help understand the influence of ambient gas elements on laser plasma and on the optimization of the experimental parameters of LIBS technology.