Self-absorption effects of laser-induced breakdown spectroscopy under different gases and gas pressures

Songning WANG(王崧宁),Dianxin ZHANG(张殿鑫),Nan CHEN(陈楠),Yaxiong HE (何亚雄),Hong ZHANG (张红),*,Chuan KE (柯川),Tao XU (许涛),Yongliang CHEN (陈永亮) and Yong ZHAO (赵勇)

1 Superconductivity and New Energy R & D Center,Southwest Jiaotong University,Chengdu 610036,People’s Republic of China

2 School of Physical Science and Technology,Southwest Jiaotong University,Chengdu 610036,People’s Republic of China

3 School of Physics and Energy,Fujian Normal University,Fuzhou 350117,People’s Republic of China

Abstract The self-absorption effect is one of the main factors affecting the quantitative analysis accuracy of laser-induced breakdown spectroscopy.In this paper,the self-absorption effects of laserinduced 7050 Al alloy plasma under different pressures in air,Ar,and N2 have been studied.Compared with air and N2,Ar significantly enhances the spectral signal.Furthermore,the spectral self-absorption coefficient is calculated to quantify the degree of self-absorption,and the influences of gas species and gas pressure on self-absorption are analyzed.In addition,it is found that the spectral intensity fluctuates with the change of pressure of three gases.It can also be seen that the fluctuation of spectral intensity with pressure is eliminated after correcting,which indicates that the self-absorption leads to the fluctuation of spectral intensity under different pressures.The analysis shows that the evolution of optical thin spectral lines with pressure in different gases is mainly determined by the gas properties and the competition between plasma confinement and Rayleigh-Taylor instability.

Keywords: self-absorption coefficient,ambient gases,gas pressure,laser-induced breakdown spectroscopy

1.Introduction

As a potential qualitative or quantitative analysis technique which uses a high-energy pulsed laser to focus on the surface of a substance to generate plasma,laser-induced breakdown spectroscopy(LIBS)has many advantages,such as fast analysis speed,remote online detection of multiple elements,suitability for objects in various states,and low requirements of sample preparation[1,2].Therefore,LIBS is widely used in the fields of national defense,industry,environment,medical care,and space[3-5].In terms of qualitative analysis,LIBS has unique advantages.However,in terms of quantitative analysis,since the formation and evolution of laser-induced plasma is a complex process,the collected plasma spectra have great uncertainty and poor repeatability,which leads to poor precision of quantitative analyses.In addition to the characteristics of the laser itself and experimental conditions,matrix effects,uneven temporal and spatial distribution of the plasma,and the self-absorption effect[6]are all reasons for the low accuracy of quantitative analysis of LIBS.Among them,the self-absorption effect is a serious influence which will reduce the intensity and distort the shape of the spectral line,causing a lower fit of the calibration curve when using a calibration method and a large error of electron density and plasma temperature in a calibration-free method [7,8].

Up to now,various methods have been used to decrease the effects of self-absorption on LIBS.The main correction methods are summarized [9]from the perspectives of experiment parameter optimization [10-15],a physical assist device [16-18],and a self-absorption model and correction algorithm [19-21].Some studies have shown that the self-absorption effect can be reduced by changing the atmosphere gas and its pressure when optimizing experimental parameters [14,15,22,23].Ning [14]et alstudied the self-absorption effect of Cu spectral lines under different air pressures,and found that moderately reducing the air pressure can reduce the self-absorption effect.Haoet al[15]studied the effects of air pressure on the self-absorption effect of Cu and Mn spectral lines,and the results showed that the effect of self-absorption effect was basically eliminated at 1 kPa.Rezaeiet al[22]studied the effects of Ar and He on the selfabsorption for different Al spectra,and the results showed that the self-absorption in Ar was more serious than that in He.However,Zehra Ket al[23]studied the effects of pressure on the intensity of Si lines in Ar,He,and Ne respectively and found that the line in Ar has the highest intensity,followed by Ne,and the lowest in He.In other words,compared to He,Ar can enhance the signal,but it can also enhance the spectral selfabsorption effect.This means that if Ar is used to enhance the signal in experiment,there is a risk of increasing the selfabsorption of the spectra.Therefore,it is significant to study the mechanism of the influence of ambient gas and its pressure on the self-absorption effect and the influence of the self-absorption effect on spectral signal under different gas pressures.

In order to decrease the effect of self-absorption on the accuracy of LIBS analysis,and to obtain the best environmental gas and pressure conditions,three gases,air,Ar,and N2,were used as environmental gases to study the influences of their properties and pressures on LIBS.In this paper,the degree of self-absorption is quantified though calculating the self-absorption coefficient (SA) [20],and the influences of three gases and their pressures on self-absorption of Al alloy plasma emission spectra are studied.In addition,the influences of gas species and pressure on the spectral signal are discussed in depth by correcting the line intensity usingSA.These results provide an important theoretical reference for LIBS to determine suitable gas species and pressure in experiments and practical applications.

2.Experimental setup

A schematic of the experimental setup is shown in figure 1.A picosecond Nd:YAG laser (BLAZER-30P,Grace Laser Technology Co.,Ltd) emits a laser at 1064 nm,which is focused on the sample surface in a closed gas chamber through a focusing lens with 79 mm focal length.The pulse width is about 30 ps and the ablation energy is 31.9 mJ per pulse.The radiation emitted by the plasma is collected by an optical fiber with a diameter of 400 μm and transmitted to a spectrometer (Aryelle-200,LTB Photonicstech China Limited) with a wavelength resolution of 12 500.The spectrum is detected by an enhanced charge-coupled device(ICCD,18H-13,Oxford Instruments PLC) with a minimum gate width of 50 nm.In this experiment,a coaxial LIBS system is used to collect the signal.Compared with a paraxial signal collection system,the optical path of a coaxial system is simpler.Different signal collection systems will only affect the collection efficiency of the signal,but there is no effect on our experimental conclusions.The closed gas chamber system regulates gas pressure through a vacuum pump and is driven by a stepper motor which can be adjusted in three dimensions.

A 7050 Al alloy produced by Shanghai Bixuan Metal Material Co.,LTD is used as the test sample in this experiment.Table 1 shows the nominal concentration of each element in the sample.

Table 1. Nominal element concentrations of 7050 Al alloy (wt%).

Table 2.Physical parameters of Ar,air,and N2.

Table 3. Parameters of four Al spectral lines.

In this experiment,plasma emission spectra of the 7050 Al alloy are collected by the LIBS system under air,Ar,and N2.During the experiment,the pressure of the gas chamber is first pumped below 500 Pa by a vacuum pump.The gas chamber is then filled with gas and the gas pressure is controlled from 5 to 95 kPa.The spectra are collected at an interval pressure of 5 kPa.While collecting each spectrum,the exposure time of ICCD is set to 15 000 ms and the laser repetition rate is 10 Hz.The delay time is set to 0.5 μs and the gate width is set to 1 μs which can provide a relatively high signal-to-noise ratio.Therefore,each LIBS spectrum is obtained by accumulating 150 pulses at the same location of the sample which can help eliminate the influence of energy fluctuations.Considering the influence of ICCD noise,the dark noise is collected before each spectrum acquisition and subtracted from the spectrum.In addition,in order to reduce the influence of signal uncertainty and sample heterogeneity,five spectra are collected under the same gas pressure and analyzed using their average data.The sample is moved by adjusting the three-dimensional step of the sample stage after each data collection.Before the experiment,the signal intensity of the ICCD is calibrated using a deuterium halogen source (DH-3plus-CAL Calibration,Ocean Insight).

3.Results and discussion

Figure 2 shows a typical LIBS spectrum of the 7050 Al alloy,including some spectral lines of Mg and Al at different wavelengths,as well as the spectral lines of Hα.In this experiment,two spectral lines,396.15 nm(Al I)and 518.36 nm(Mg I),are used as the analytical lines.In order to understand the influence of ambient gas on the spectrum,some physical parameters of the three gases are shown in table 2.As shown in table 2,the ionization energies of Ar and N2are close.Since air is a mixture of multiple gases,its ionization energy is not available.

Figure 1.Schematic of the experimental setup.

Figure 2. Typical LIBS spectrum of the 7050 Al alloy.

Figure 3.Variation of intensity of (a) Al I 396.15 nm and (b) Mg I 518.36 nm with the pressure in three gases.

Figure 3 shows the variation of the spectral intensity of the Al I 396.15 nm and Mg I 518.36 nm with the pressure in three gases.The spectral intensity mentioned in this paper is the fitting intensity after Lorentz fitting.It can be seen from figure 3 that the spectral intensity of Al I 396.15 nm and Mg I 518.36 nm is the highest in Ar,followed by air,and the lowest in N2.This is because Ar has the largest relative molecular weight among the three gases,which causes Ar to have the strongest confinement effect on the plasma under the same pressure [22,24].This effect makes the plasma have a smaller volume and a higher density,which intensifies collisions within the plasma to enhance the signal.

Moreover,as shown in table 2,among the three gases,the thermal conductivity and heat capacity of Ar are the smallest.Therefore,lower thermal conductivity and specific heat capacity will reduce the plasma cooling rate and lead to higher plasma temperature and signal enhancement effect.However,in general,the lifetime of plasma is of the order of microseconds,and the influence of the thermodynamic properties of the gas in such a short time still needs to be further studied [25].In addition,the inertness of Ar can prevent the composition of samples from forming metal oxides that are difficult to dissociate at high temperature[26],thus enhancing the spectral signal of the metal sample.It can also be found from figure 3(a) that the spectral line intensity of Al I 396.15 nm in air and N2varies similarly with pressure.Similarly,in figure 3(b),the intensity of Mg I 518.36 nm varies similarly with pressure in air and N2.This is due to the fact that the percentage of N2is 78% in air.Compared with N2,the spectra intensity in air is increased,which may be related to 21% O2in air.O2has a much lower ionization energy of 12.5 eV than N2.The lower ionization energy makes the ambient gas more susceptible to ionization by strong laser fields or high-energy electrons sputtered from the target surface,resulting in the formation of a so-called gasphase plasma [24].Due to the inverse bremsstrahlung radiation,the electrons absorb energy and have high kinetic energy.During the frequent collisions,the electrons convert the kinetic energy into excitation energy of the atoms and ions sputtered during the ablation process,thus enhancing the emission spectral line intensity.

However,as shown in table 2,the ionization energy of the three gases is close,but the relative molecular weight of Ar is much larger than those of air and N2.Hence,the enhancement effect of Ar on spectral line intensity is mainly attributed to its larger molecular weight.Therefore,which of these two factors has a more obvious enhancement effect on the intensity needs further study.Moreover,the effect of the thermodynamic properties of the gas needs to be further studied by using different gases whose relative molecular weights and ionization energies differ little but whose thermodynamic properties differ greatly.In addition,as shown in figure 3,the variation of the intensity of the spectral lines in three gases with pressure shows a fluctuation.In order to study this phenomenon,the effects of the pressure on the selfabsorption of the spectra need to be investigated.

Figure 4 shows the shapes of Al I 396.15 nm and Mg I 518.36 nm in air and Ar at different pressures.Many studies[27,28]have confirmed that the self-absorption effect will reduce the intensity of the spectral line,distort the shape of the line,and cause line reversals.In addition,the selfabsorption will also cause spectral line broadening and line shift [29].In figures 4(a)-(d),it can be seen that the spectral lines have a large difference at different pressures.In figure 4(a),the line shape appears slightly distorted at 80 kPa and 90 kPa,and the intensity of the line in air decreases significantly at 30-50 kPa.However,the lines in figure 4(b)have no obvious distortion or decrease.In addition,in figures 4(a)-(d),the widths of the spectral lines all increase with increasing pressure which may be caused by the selfabsorption effect and the electron density.Moreover,in figure 4(d),as the pressure increases,the spectral lines appear red-shifted.The influence of the self-absorption effect on these phenomena is also a subject worthy of study.

Figure 4.Shapes of Al I 396.15 nm line in (a) air and (b) Ar and Mg I 518.36 nm in (c) air and (d) Ar varying with pressure.

Figure 5.Variation of the self-absorption coefficient of (a) Al I 396.2 nm and (b) Mg I 518.4 nm with pressure in three gases.

Usually,the degree of self-absorption is difficult to determine from observing the shape of the line.Therefore,the self-absorption effect is quantified by theSAand the degree of self-absorption under different gas pressures is investigated.This method is proposed by Sherbiniet alin 2005 [20]and the scientific analysis in this paper is based on their results for primarily optically thin considerations.The aim is to quantify the degree of self-absorption effects of the spectral line by calculating the self-absorption coefficient.According to the expression ofSAdefined by Kunze [30]:

whereI(λ)is the intensity of the measured spectral line,I0(λ0)is the intensity of the spectral line free of selfabsorption,andk(λ0)lis the optical depth at the center of the plasma.When the self-absorption effect is enhanced,theSAwill gradually decrease until the plasma is in the optically thick state,at which timeSAis infinitely close to 0.WhenSAis equal to 1,it can be considered that the line is free of selfabsorption.SAcan also be expressed as [20]:

where α is -0.54,Δλ0is the intrinsic full width at half maximum (FWHM) of the Lorentzian component of the spectral line in an optically thin plasma,Δλis the FWHM of the spectral line measured in the experiment after Lorentz fitting,ωSis the Stark broadening parameter [31],andneis the electron density in the optically thin state.Since,in the LIBS experiment,water vapor (H2O) in the environment above the sample is usually ablated together with the sample surface,thus the spectral line of H can be obtained in the spectrum.Many studies[32,33]have shown that the Hα and Hβ lines of the Balmer series of H atomic lines can be used to calculate the electron density.Since the intensity of Hβ line is too small to be observed in this experiment,Hα line is used to calculate the electron density.The study by Pariggeret al[32]confirms that when the delay time is larger than 400 ns,the Hα line is almost free of the self-absorption effect and can be used as a reliable indicator of the plasma electron density.In addition,some studies [32,34]give empirical formulas to calculate the electron density using the Hα line.In this paper,the electron density is calculated by equation (3) [32]:

where ΔλHis the Stark width of the Hα line,withnein(m-3).In this paper,equation(2)is used to calculate the selfabsorption coefficient because the parameters in equation (2)can all be obtained from experiments or papers.The calculation results ofSAare shown in figure 5.

Figure 5 shows the variations of theSAof Al I 396.2 nm and Mg I 518.4 nm with pressure in three gases.It can be seen in figure 5 that a change in pressure has a significant effect onSA.The values ofSAof Al I 396.2 nm and Mg I 518.4 nm in Ar both increase and then decrease with increasing pressure,while theSAof Al I 396.2 nm in air and N2fluctuate with increasing pressure.However,theSAof Mg I 518.4 nm increases with increasing pressure,growing to 1 at 60 kPa in air and 80 kPa in N2.Moreover,the values ofSAin all three gases show the same change at pressure below 45 kPa,and there is little difference in the values,while when the pressure is greater than 45 kPa,there is an obvious difference in the variation ofSAin the three gases.As the pressure rises,the confining effect of the ambient gas on plasma increases,which makes the plasma have smaller volume,higher temperature,and higher density.This may have an impact onSA.In order to clarify the mechanism,the values of plasma temperature (T) and electron density (ne) under different pressures in the three gases are investigated.In this work,Tvalues are obtained from a Boltzmann plot of four Al lines with wavelengths of 396.15 nm,704.20 nm,705.66 nm and 706.36 nm,and some parameters of these four Al lines are taken from the NIST database [35],as shown in table 3.The values ofneare calculated by equation (3) using the Stark broadening of the Hα line.

Figure 6. Variations of (a) plasma temperature and (b) electron density with pressure in three gases.

Figure 7.Variations of the intensities of(a)Al I 396.15 nm and(b)Mg I 518.36 nm with gas pressure after correction using SA in three gases.

The variations ofTandnewith pressure in the three gases are shown in figure 6.The results show that the plasma temperature and electron density increase gradually with increasing pressure in the three gases.In Ar,Tis the highest andneis the largest,followed by air and then N2.This is due to the fact that Ar has the largest relative molecular weight and the smallest thermal conductivity among the three gases.In figure 5,it can be found that the values ofSAin all three gases show the same change at pressure below 45 kPa,and there is little difference in the values.This is because the number of molecules in a gas is very small at low pressure,the properties of the gas have no obvious influence on the self-absorption effect,and the number of low-energy particles of the same element in the path of photon radiation has a direct influence of the self-absorption effect.At 5-45 kPa,when the pressure increases,the gas compresses the plasma,the electron density increases,and the total number of particles along the photon radiation path increases,which enhances the self-absorption effect.However,with increasing pressure,the plasma temperature also rises which reduces the number of particles at lower energy levels in the plasma,thus reducing the self-absorption effect.These two competing processes work together to affect the number of low-energy particles along the photon radiation path,makingSAof Al I 396.2 nm increase first and then decrease andSAof Mg I 518.4 nm increase from 5 to 45 kPa.This finding suggests that when the air pressure is less than 45 kPa,no matter what kind of ambient gas it is,their influence on the self-absorption effect is basically the same.However,when the pressure is greater than 45 kPa,different gases have different effects on the self-absorption effect of spectral lines.In addition,it can be seen in figure 5 that the values ofSAof the two spectral lines have a considerable difference.This is due to the difference in the concentrations of Al and Mg.Many studies[36-38]have confirmed that the higher the element concentration,the more obvious the self-absorption effect of its spectral lines.

When the pressure is larger than 45 kPa,there is an obvious difference in the variations ofSAin the three gases.TheSAin Ar is the minimum and decreases with the rise of pressure.TheSAin air is the maximum which increases first and then decreases with increasing pressure.TheSAin N2is slightly lower than that in air which increases with increasing pressure.This is because when the pressure is larger than 45 kPa,as the pressure increases further,the density of gas molecules increases,and the effect of the gas properties on self-absorption begins to appear.Ar enhances the confinement effect on plasma due to its large molecular weight,causing an increase in the plasma density and the number of low-energy particles along the radiation path,resulting in a self-absorption effect that continues to increase with increasing Ar pressure.However,because of the lower relative molecular weights of air and N2,the increase inTplays a dominant role in reducing the self-absorption effect which makesSAincrease with increasing pressure,and the slightly higherSAin air than that in N2is due to the lower ionization energy of air.The lower ionization energy makes it easier for the gas to ionize electrons,which collide with lower-level particles outside the plasma,returning these particles to higher energy levels and reducing the number of lower-level particles in the photon emission path.Therefore,the selfabsorption effect is reduced.

To further check the influence of the self-absorption effect on spectrum,equation(1)is used to obtain the intensityI0(λ0) of analysis lines in the optically thin situation by dividing the intensity of the lines measured in the experiment by theirSA.Figure 7 shows the variations of the intensities of Al I 396.15 nm and Mg I 518.36 nm with gas pressure after correction usingSA.

Comparing figures 3 and 7,it can be found that the fluctuation of variation of the corrected intensity with increasing is almost eliminated.It shows that the selfabsorption effect is the main reason for the fluctuation.However,at 5-20 kPa,the intensity decreases with increasing pressure in all three gases,while when the pressure is larger than 20 kPa,the intensity keeps increasing in Ar,slightly increases in air,and remains stable in N2.Yuet al[25]used the Rayleigh-Taylor instability (RTI) theory [39-41]to explain this phenomenon.As the density difference between plasma and surrounding gas increases,the interaction between the plasma and surrounding gas is enhanced.As a result,collisions increase and result in a higher energy transfer rate between the plasma and gases.By colliding,these gas particles absorb the plasma energy,reducing the intensity of the spectral lines.Usually,the degree of RTI is described by the Atwood numberAt:

whereρbis the density of the ambient gas andρpis the plasma density.The largerAt,the higher the degree of RTI.The total number density of a laser-induced plasma is generally about 1017cm-3[24].It can be calculated by the ideal gas law combined with the volume parameters of the gas chamber used in this experiment and the temperature during the experiment that the molecular number density of the gas is about 2.4 × 1017cm-3at 1 kPa and 1.2 × 1018cm-3at 5 kPa.Therefore,when the gas pressure rises,Atwill also increase.

Combined with the theories above,as the gas pressure increases,on one hand,the confinement effect of the ambient gas on the plasma becomes stronger which can enhance the signal intensity.On the other hand,the increase in the gas pressure makesAtlarger and the RTI process more serious which reduces the signal intensity.The competition between these processes finally makes the line intensity first decrease and then increase in the optically thin situation.Moreover,when the pressure is larger than 20 kPa,it can be seen that compared with air and N2,the spectral line intensity in Ar increases with increasing pressure.This is because the confinement effect caused by the larger molecular weight of Ar is much greater than the RTI process.Compared with N2,the slight increase in the line intensity in air may be caused by the lower ionization energy of O2in air.In addition,since the pulse width of the laser used in this experiment is at the picosecond level,the shielding effect of the plasma on the laser is not considered.

4.Conclusion

In this paper,three types of gases,air,Ar,and N2,are used as the ambient gas to study the effects of their properties and pressure on an LIBS experiment.By comparing the spectral lines intensities in three gases,the physical mechanisms of the influences of ionization energy,thermal conductivity,relative molecular weight,and heat capacity of gas on the LIBS signal are discussed.It is found that the signal enhancement in Ar is mainly caused by the relatively large molecular weight and its inertness,and the influence of the thermodynamic properties of the gas on signal needs further study.Compared with air and N2,Ar significantly enhances the spectral signal.Although Ar has such a strong enhancement of the spectral intensity,it also enhances the self-absorption effect of the spectrum,which may negatively affect the quantitative analysis.The theoretical spectral line intensity free of selfabsorption is obtained through correcting the spectral intensity usingSA.The fluctuation of spectral intensity with pressure transformation is almost eliminated.This shows that the self-absorption is a main factor which leads to the fluctuation of spectral intensity.The analysis shows that the evolution of optical thin spectral lines with pressure in different gases is mainly determined by the gas properties and the competition between plasma confinement and Rayleigh-Taylor instability.

Acknowledgments

This work was supported by National Key Research and Development Program of China (Nos.2017YFE0301306,2017YFE0301300,and 2017YFE0301506).Fujian Province Industrial Guidance Project (No.2019H0011).