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Abstract
The onset of self-absorption of laser-induced plasma poses a problem for converting emission line intensities to concentrations, which is one of the main bottlenecks in quantitative laser-induced breakdown spectroscopy (LIBS) measurements. In this paper, the effects of atmosphere and laser fluence on self-absorption reduction of the plasma induced on tungsten-copper alloy target were investigated with nanosecond infrared (1064 nm) laser pulse over a range of 2.9 to 18.2 J/cm2. The time-resolved features of self-absorption, and temperature and electron density of the plasma were characterized in atmospheric air and argon, respectively. The experimental results show the effect of self-absorption can be significantly reduced by increasing the laser pulse energy. The argon atmosphere is more helpful for self-absorption reduction. The time-resolved diagnostics of emission spectra in the early stage of the plasma formation are very effective to prevent self-absorption to improve the LIBS analytical performance.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
During the past few years, laser-induced breakdown spectroscopy (LIBS) has emerged as a powerful real-time, chemical analysis tool [1,2]. This technique is based on the detection of spectral emission originating from a plasma formed following the interaction of a pulsed laser beam with a sample. Study of characteristic line emission from the plasma can give information about the composition of the sample material. LIBS enables three-dimensional chemical imaging, remote sensing and quantitative multi-elemental analysis of samples without any pre-treatment [3–5]. Due to its unique features, LIBS has been successfully applied in material science [6], geochemical fingerprinting [7], biomedicine [8], environmental monitoring [9], space exploration [10], and so on. Tungsten was selected as divertor plate material in ITER tokamak due to its favorable mechanical and refractory properties [11]. The composition of surface layer of divertor plate can be significantly modified by sputtered material re-deposition and implantation of plasma ions during the fusion plasma discharges, which affects the actual tokamak operation. LIBS is a straightforward in situ tool to monitor fuel retention and dust deposition on the divertor plates [12,13].
Under ideal LIBS conditions, the plasma is assumed to be optically thin. The spectral line emitted from the center of the plasma is acquired without any absorption [14]. Hence, the observed intensity of a spectral line increases nearly linearly with the elemental concentration in a specimen. However, it is often not optically thin for strong lines (e.g. resonance lines) when the elemental concentration is high [15]. For optical thick laser-induced plasma, the emission light from the plasma center is re-absorbed by relatively cold atoms of the plasma periphery [16,17]. This self-absorption effect weakens the peak of a spectral line, even making a dip in the center (called self-reversal), which causes a pronounced non-linearity in the calibration function at increasing concentration [18,19]. This subtle onset of self-absorption poses a problem for converting line intensities to concentrations, which is one of the main bottlenecks in quantitative LIBS measurements. Some researchers reduced ambient gas pressure to weaken the influence of self-absorption effect [20,21]. There are few investigations focused on restraining the self-absorption in atmospheric environment [22,23].
The aim of the experiment reported in this paper was to comparatively study the emission characteristics of laser-induced plasma in different ambient gases of air and argon at one atmosphere pressure. The ambient surrounding is a significant factor to affect the properties of plasma. A simple and conventional configuration of ablation with a nanosecond pulse of a metallic target (tungsten-copper alloy) was used to study the behavior of self-absorption of the plasma during its expansion into different ambient gases. The effect of laser energy and time-resolved spectroscopic measurement on self-absorption reduction were investigated to obtain an optimized experimental condition for quantitative analysis in the open air.
2. Experimental set-up
A schematic view of the experimental setup is shown in Fig. 1. A Q-switched Nd:YAG laser (Brilliant Eazy, Quantel), delivering up to 100 mJ at 1064 nm with a 5 ns pulse duration, was used as the ablation source. The laser beam was directed normal to the sample surface by 45° mirrors and focused using a plano-convex lens (f = 75 mm) onto a certified tungsten-copper alloy plate (W 80 wt.% and Cu 20 wt.%). The focus position of the incident laser was optimized at 6 mm below the sample surface, which was chosen to avoid background gases breakdown. The spot diameter was estimated to be about 600 μm by measuring that of the ablation crater.
Spectral emission of the laser-induced plasma was collected with a quartz lens (f = 100 mm), and transported via an optical fiber of 200 μm core diameter. The fiber was coupled to an Echelle spectrograph (with spectral resolving power λ/△λ = 12500) equipped with a time gated ICCD (iStar DH334T, Andor). The collection optical axis was fixed at a 45-degree angle to the laser propagation axis.
The experiments were conducted under two different ambient gases of air and argon at the atmospheric pressure. Since the laser ablation was not in an airtight environment, an argon flow at~5 L/min was introduced using a tube with 3 mm diameter positioned above the sample surface and several millimeters away from the ablation crater. Such fixed flow of argon was able to prevent air diffusion into argon background, because emission of nitrogen was eliminated for all detection time windows in our experiments.
The alloy plate (10 × 10 × 3 mm3) was subsequently mechanically polished with silicon carbide (SiC) papers of the different grades to ensure a flat surface and then ultrasonically cleaned using ethanol. The sample was mounted on a high precision X-Y-Z translation stage to provide a fresh sample surface for each test, and the sampling position and laser ablation process were monitored via a CMOS camera.
3. Results and discussion
3.1. Characteristics of LIBS plasma
A spectroscopic study of the influence of laser fluence on laser-produced plasma was performed in air and in argon. Strong continuum emission was generated at the beginning of plasma formation, which was caused by bremsstrahlung and recombination radiation from the plasma as free electrons and ions recombine. To eliminate interfere of this strong continuum light, the line emissions were measured with a time delay of tdelay = 2 μs and an integration width of tint = 4 μs, firstly. The panoramic Echelle spectra in the spectral range 200-780 nm show many atomic and ionic emission lines of copper and tungsten. Figure 2(a) presents a portion (300-525 nm) of laser induced plasma spectra obtained at the laser fluence of 15.3 J/cm2. The ion and neutral line emission intensities recorded in argon were significantly higher than those in air. In each spectrum, the strongest emission line is that at 324.75 nm, at which the lower level of the transition is the ground state. Normally self-absorption or self-reversal would be more observable for these resonance lines during the measurements of major elements in solid samples [24]. A non-resonance line of Cu I transition (3d94s5s-3d94s4p) at 465.11 nm, without severe self-resonance, was used to investigate the influence of the ambient gas on the laser-induced plasma. Figure 2(b) presents plots of the net peak areas of Cu I 465.11 nm lines versus laser fluences in air and argon. Increasing laser fluence from 2.9 to 18.2 J/cm2 lead to increased plasma emission in both ambient gases. Comparing the results obtained, the emission intensities in argon were always higher than those in air. Similar experimental observations on the effect of surrounding atmosphere on the plasma emission have been reported [25–27]. In Fig. 2(c), the enhancement ratios of Cu I 465.11 nm emission in argon to that in air shows an approximately linear increase with increasing laser fluence. It is about 6.8 when the laser fluence is 18.2 J/cm2.
Fig. 2 (a) Spectral emission from tungsten-copper LIBS plume generated in an argon and an air atmosphere. The gate delay and width were 2 μs and 4 μs, respectively. (b) Variation of emission intensity of Cu I 465.11 nm line versus incident laser fluence. (c) Variation of Line intensity ratio of Cu I 465.11 nm emission in argon to that in air versus laser fluence. (d) Plasma parameters (excitation temperature and electron density) as a function of laser fluence.
The spectral emission is related to excitation/de-excitation processes in lase-induced plasma. Further, the intensity can be increased by the re-excitation of atoms by the collision with energetic particles [28]. The ambient surrounding is a significant factor to affect ablation process and the properties of plasma, which either enhances or prevents the coupling of laser energy into plasma. It is well known that excitation temperature (Texc) and electron density (ne) can strongly influence the intensity and spectral distribution of the plasma emission.
The excitation temperature (Texc), estimated using the Boltzmann plot method, is given by [29]:
where Iij, λij, Aij, gi, and Ei are the intensity, wavelength, transition probability, statistical weight, and energy of the upper state i, K is Boltzmann constant, U(Texc) is the partition function and N(Texc) is the total number density. Non-resonant atomic Cu lines were chosen to calculate the plasma excitation temperature. The parameters for these Cu I lines are given in Table 1
Table 1. Neutral Cu parameters used for plasma temperature calculation [30]
Plots for the estimation of plasma temperature versus laser fluence in air and argon are shown in Fig. 2(d). The temperature was found to be significantly higher in argon than that in air, and the temperature gradually increased with increase of laser fluence from 2.9 to 15.3 J/cm2. There was a slow decrease of temperature at 18.2 J/cm2. A possible reason may be that with higher vertical expansion velocity the hot plasma partly escaped from our observation area at longer delay times.
The electron density can be determined from the spectral line shape, and is affected by the broadening mechanisms originating from the surrounding plasma environment. Under the present experimental conditions, the contribution from Doppler broadening due to thermal or directed motion of the emitting atoms was estimated to be lower than 0.6 pm, which is much less than the observed Stark broadening and can therefore be ignored. This shows the contribution of Stark broadening to be dominant. The Stark-broadened line was extracted from the measured line width by simply subtracting the instrumental broadening [31]. The instrumental broadening width was 0.02 nm measured from emission lines of a mercury light source. For typical LIBS conditions, the contribution from ion broadening is negligible, and then the full width at half maximum (FWHM) ∆λ of Stark-broadened lines, as related to the electron density ne, is given by [32,33]:
where ω is the half-width Stark broadening parameter. The Cu I transition (3d94s5s-3d94s4p) at 465.11 nm was used to calculate electron density of plasma. Lorentz curve fitting was used to determine the FWHM of each line. The ω parameter of the selected Cu I 465.11 nm is 0.0435 Å, and the uncertainty of ω is within 50% [34]. In Fig. 2(d), a significant increase in electron density was observed as the laser fluence increased from 2.9 to 18.2 J/cm2 in both atmospheres.Figure 3 presents temporal evolution of Cu atomic emission at wavelength 465.11 nm line in argon and air. The emission spectra were recorded at different gate delays, ranging from 0.5 to 19.5 μs, with gate width of 1μs. In this measurement, the intensity of the atomic emission line decayed exponentially with an increase in gate delay. Similar variations were noted, whether spectra were acquired in argon or in air. The lifetime for the line emission from laser-induced plasma is defined as the time when the intensity falls to 1/e of the maximum [35]. It was found that the measured lifetime of Cu I 465.11 nm in argon was about 1.90 μs, much longer than that of 1.13 μs in air.
Fig. 3 Temporal variation of peak area of Cu I 465.11 nm line in argon and air. The acquisition gate widths are 1μs. Points: experimental data. Solid lines: exponential fits to the data.
The argon ambient leads to a hot and dense plasma, and therefore stronger spectral emission from the plasma than that in the atmospheric air. It can be explained as follow: (i) There are smaller thermal conductivity and higher atomic mass of argon gas in comparation to those of air. Argon has a smaller thermal conductivity (0.0387 cal/cm s deg) and a smaller specific heat (0.0763 cal/g deg) than do N2 and O2 [28,36]. Such differences in thermal properties result in higher temperature plasma, leading to stronger and longer plasma emission in argon ambient; (ii) For laser ablation in a gas with higher atomic mass (e.g., Ar), the vapor plume expanded more slowly due to the stronger momentum drag provided by the background gas, which leads to a higher electron density plasma [37]; (iii) The ambient gas plays an important role in chemistry in the laser ablation, e.g. through the formation of oxides in laser-induced plasma [38]. Using of argon as the ambient atmosphere can protect the excited atoms from forming oxides, which could reduce the plasma emission and lifetime.
3.2 Self-reversal effect in laser-induced plasma
The presence of self-absorption adversely affects peak intensity, which poses an interference for LIBS quantitative analysis. When there is too much of the analyte present in the plume, the plasma itself could absorb its own emission and produce peaks with a flat topped or dip. Figure. 4 compares the emission profiles obtained at different laser fluences in atmospheres of air and argon. In Fig. 4(a), typical dips in the central frequency of the Cu atomic resonance lines at low fluence in air are evidence of self-reversal, which will give rise to a doubt of there being two lines. The reason is that laser-induced plasma is thermally inhomogeneous and optically thick at this detection time window. There is a relatively high plasma temperature and electron density existed in the center the plasma. The temperature exhibited an obvious drop at the outer edges of the plasma, where the electron density was also relatively low [29]. The outer edges of the plasma will be populated by ‘cool’ atoms, residing mostly in the ground state. As the excited atoms in the plasma center decay to the ground state, the emitted photons corresponding to resonance transitions will have a high probability of being absorbed by the ‘cooler’ atoms in the outer edges, thereby reducing the observed peak intensity [16]. In Fig. 4(a), it was found that the effect of self-reversal decreased obviously with increasing laser fluence in air atmosphere. It seems clear that laser ablation using high laser fluence can improve the plasma parameters (like temperature and electron density) and reduce the self-absorption process. Sweeping the sample surface with an argon flow, the plasma emission was enhanced by many orders of magnitude without self-reversal as shown in Fig. 4(b).
Fig. 4 Cu I resonance lines at 324.75 and 327.40 nm acquired at different fluences in air (a) and argon (b). The gate delay and gate width of the measurement were 2 and 4 μs, respectively.
Figure 5 shows an overview of the occurrence of self-reversal in different laser fluences in both ambient gases. The black symbols for data points 1-4 represent acquired spectra with obvious self-reversal, whereas the red symbols for data points 5-12 indicated no self-reversal. The results show that self-reversal can be reduced by increasing laser ablation influence, and the argon atmosphere was a very effective approach to prevent self-reversal.
Fig. 5 Electron density versus plasma temperature at different laser fluences in air and argon. The numbers close to the data points in the graph refer to the following laser fluences: 1, 2.9 J/cm2; 2, 5.9 J/cm2; 3, 9.2 J/cm2; 4, 12.4 J/cm2; 5, 15.3 J/cm2; 6, 18.2 J/cm2; 7, 2.9 J/cm2; 8, 5.9 J/cm2; 9, 9.2 J/cm2; 10, 12.4 J/cm2; 11, 15.3 J/cm2; and 12, 18.2 J/cm2. The open symbols (1-6) and solid symbols (7-12) represent LIBS measurements in air and argon atmosphere, respectively.
3.3 Time-resolved analysis of self-absorption
The effect of laser energy on self-absorption of Cu I resonance lines was investigated using time-resolved spectrum. Figure 6 illustrates typical emission spectra acquired in air at atmospheric pressure at different gate delays after the irradiation of the laser pulse. When using the laser pulse of 12.4 J/cm2 as shown in Fig. 6(a), a clear dip was observed on the Cu I 324.75 nm line after 2.5 μs, whereas at low fluence of 5.9 J/cm2 as shown in Fig. 6(b), significant self-reversal phenomena appeared at all selected times. The shift between the absorption and emission maximum varies with time, which is attributed to the electron density difference between plasma core and plasma shell [18]. As Stark shift being a function of electron density, the higher the electron density of plasma core is, the greater the Cu I 324.75 nm line emission will shift. In order to understand the plasma evolution, the temporal variation of electron density and excitation temperature were investigated and shown in Fig. 7(a) and 7(b), respectively. The attenuation of electron densities and temperature are both proportional to t -a. At early times, the self-reversal is relatively weak as shown in Fig. 6, which can be mainly explain from two aspects: (i) higher electron density causes this line emission shifted towards longer wavelength; (ii) higher excitation temperature lowers the number of ground-state atoms in outer edges of the laser-induced plasma and reduces intensity of self-absorption.
Fig. 6 Time evolution of Cu I resonance lines in air under 12.4 J/cm2 (a) and 5.9 J/cm2 (b). During this experiment, gate width was fixed at 1 μs.
Fig. 7 Time-resolved evolution of the electron density (a) and excitation temperature (b) of plasmas produced in air and in argon.
The self-absorption coefficient (SA) is determined as the ratio of measured peak height IO(λ0) to the value IT (λ0) of line peak in absence of self-absorption, and expressed as [39]
With this definition, SA is 1 if the line emission is not self-absorbed and decreases toward 0 if the self-absorption increases. The value of line width is also affected by the self-absorption of the radiation. It was found that the observed FWHM △λO of an emission line in presence of moderate self-absorption was related to the true line width △λT, through the empirical relation [15]:Under typical LIBS conditions, the main contribution to the line width comes from the Stark effect [32]. Neglecting the contribution from ion broadening, the true FWHM △λT can be estimated with the real plasma electron density ne (cm−3) according to Eq. (2). To investigate the self-absorption of Cu I 324.75 nm line, with substitution of △λT(324.75) in the Eq. (4), we obtain:where △λO(324.75) is the measured FWHM of Cu I 324.75 nm line, ω324.75 is the half-width Stark broadening parameter of Cu I 324.75 nm line. The value of ω324.75 is 0.0072 Å, and the uncertainty of ω324.75 is within 30% [40].The real plasma electron ne can be determined via measurement of the Stark broadening of non-self-absorbed emission lines. The non-resonance Cu atomic transition (3d94s5s-3d94s4p) at 465.11 nm was used to calculate plasma electron. Some simulation works also reported there is no self-absorption of Cu I 465.11 nm line emission when the Cu absolute density is lower than 1017 cm−3 in LIBS measurements [15]. As mentioned above, based on Eq. (2), the following linear approximation can be adopted:
where △λO(465.11) is the measured FWHM of Cu I 465.11 nm line, ω465.11 is the half-width Stark broadening parameter of this line. Using Eqs. (5) and (6), the self-absorption coeffective of the resonance Cu atomic line at 324.75 nm can be expressed asA comparison of time-resolved SA of Cu I 324.75 nm lines under different LIBS experimental conditions is shown in Fig. 8. Our experimental results show that in air environment laser fluence is a more relevant parameter to reduce the self-absorption. Increased laser fluence from 2.9 to 18.2 J/cm2 leads to significantly decreased self-absorption in the laser-induced plasma. At early times, high line intensity with low self-absorption is obtained, which also can be attributed to the hot and dense plasma state. Moreover, under the low laser fluence of 5.9 J/cm2 with a gate delay time of 0.5 μs, the values of SA are 0.45 and 0.12 in argon and in air, respectively. The obtained results show that the argon atmosphere is very helpful to improve the plasma parameters and minimize self-absorption in LIBS.
Fig. 8 Self-absorption coefficient (SA) of Cu I 324.75 nm line under different experimental conditions. Gate width was fixed at 1 μs.
4. Conclusion
In this work, we comparatively studied time-resolved analysis of laser-induced plasma in different ambient gases (air and argon) and with different laser fluences. It shows that significant self-reversal of Cu I resonance lines in the open air appeared at the laser fluence lower than 12.4 J/cm2, while self-reversal phenomenon can be decreased obviously by increasing the laser fluence. Time-resolved measurements of spectra in the early stage of laser-induced plasma formation are very effective for reducing the effect of self-absorption in both gases. In the open air, the optimum conditions for preventing self-absorption are a laser fluence of 18.2 J/cm2, and a gate delay time of 0.5 μs with a gate width of 1 μs. With the same experimental conditions, sweeping the sample surface with an argon flow is more effective to self-absorption reduction and the subsequent LIBS analytical performance improvement. The results will help improve the LIBS quantitative analysis of tungsten materials in EAST tokamak.
Funding
National Natural Science Foundation of China (11705020); National Magnetic confinement Fusion Science Program of China (SQ2017YFE03112-04); China Postdoctoral Science Foundation (2018M630285); Natural Science Foundation of Liaoning Province (20170540153).
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