Dependence on the Lens-to-Target Distance and with the Laser Energy at Constant Irradiance of the Laser-Induced Breakdown Spectroscopy Signal

Abstract

The emission signal-to-noise ratio (S/N) of a laser-produced plasma on an aluminum target at different focusing distances and at fixed irradiances was investigated. The plasma was produced by a 1064 nm nanosecond-pulsed laser and the energy and irradiances were varied in the 6–110 mJ and 0.4–700 GW cm⁻² ranges, respectively. Regardless of the applied laser energy, adjusting the lens-to-target distance, best emission values were obtained for an irradiance of nearly 8 GW cm⁻². At lower irradiances, the signal decreases due to less matter removal, while at higher values, the plasma shielding effect prevents the laser from reaching the sample. This mechanism is surpassed when the lens-to-sample distance is close to the nominal focusing value at about 100 GW cm⁻². The enhancement of the signal with the focusing distance is due to a combination of an increment of the plasma temperature, electron density, and atomized mass. When the irradiance is kept fixed changing simultaneously the laser energy and the ablated area, an increment of the emission was observed. This is basically due to an increment of the ablated mass while both electron density and temperature do not show significant changes, even though the laser energy increased by more than one order of magnitude.

Keywords

Laser-produced plasmas laser-induced breakdown spectroscopy LIBS plasma characterization

Introduction

Laser-induced plasmas have been employed for several years for different applications including pulsed laser deposition of thin films, material micro- and nanoprocessing, nanoparticle production, plume filtering, and laser-induced breakdown spectroscopy (LIBS), among others.^1–5 The ablation process depends on several experimental factors including the ablated material, ambient conditions, and the type of laser. In particular, different lasers could be employed to perform the ablation with specific wavelength, pulse duration, different pulse spatial profiles, and laser repetition rate, according to the desired application and the employed target. The interaction among the laser emission, the target, and the subsequent plasma dynamics is a complex process that has been extensively investigated.^1,6–9 One of the most important experimental parameters is the laser irradiance over the target, which is an important issue in elemental analysis through LIBS. This analytical technique allows the spectrochemical analysis of any kind of samples including gases, liquids, and solids.^10,11

Laser-induced breakdown spectroscopy could be implemented on site, without any or little sample preparation, giving results in real time. In this technique, several experimental parameters need to be optimized to maximize the signal-to-noise ratio (S/N) such as the laser irradiance, typically reported as the laser fluence. Since most LIBS experiments are usually performed at atmospheric pressure, the distance between the focusing lens and the target is reduced below the nominal value to prevent the air breakdown.^10,12 Furthermore, to maximize traces detection, the emission S/N must also be improved. Hence, different experimental values need to be optimized including light collection, controlling the gate delay and gate width of the employed detector to minimize the signal noise, maximizing line intensities, and shot to shot repeatability,¹³ but various of these parameters are linked. Several works investigated the effect of the experimental parameters on LIBS signal and were reviewed in the work of Hahn and Omenetto.^14,15 To maximize the emission intensity at a fixed laser energy, different focusing positions have been investigated. Reducing the lens to target distance below the focusing point caused an increment of emission.^16–20 But, for major elements in a sample, this is translated in an enhancement of the self-absorption effect.²¹ Besides, these investigations were carried out for a single laser energy and the emission S/N was not considered.

The plasma characteristics were also investigated as the irradiance is changed. As the lens is moved toward steel samples below the focusing distance, the plasma expansion is reduced while the electron density grows.²² On the contrary, in the work of Li et al.,²³ it was reported a reduction of the electron density on copper samples. On the other hand, in the work of Multari el al.,¹⁹ it was reported an increment of the plasma temperature and removed material of metal and soil samples, as the lens-to-sample is reduced below the focusing distance. Additionally, Aragon and Aguilera¹⁷ investigated the emission intensity at a fixed irradiance of 40 GW cm⁻². In this work, after a spatial and temporal correction, it is concluded that the plasma analyzed parameters remain unchanged as the energy is increased.

All of these previous works focused on the variation in the intensity of emission lines, but not on their S/N which is a basic parameter to obtain the sample elemental composition. Moreover, in most of them, only the focal distance was varied while keeping the laser energy constant or a single fixed irradiance was considered. In this work, the effect on the emission S/N is investigated, varying the focusing distance for different laser energies and at constant irradiances, varying both, the laser energy and the focusing distance. The obtained results are discussed as a function of electron density, plasma temperature, and atomized material to contribute to the understanding of the physical processes involved.

Experimental

The ablation of the samples was performed using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Surelite III, Continuum), which emitted at the fundamental wavelength of 1064 nm and was operated at a pulse rate of 1 Hz. Pulses had a Gaussian profile, a beam divergence of 0.6 mrad, and a pulse duration of 5 ns. The energy output was kept fixed at 110 mJ and was controlled by means of a half-wave plate and a high-power polarizing cube beam splitter to keep the laser temporal profile and polarization. The target was a commercial aluminum 6061 alloy containing 97.4% Al, 1.2% Mg, 0.6% Si, 0.3% Fe and Cu, 0.09% Cr, 0.05% Mn, 0.03% Zn, and 0.014% Ti, and minor traces of Ni, Pb, Sn, and Bi. Laser emission passes through a 4.8 mm in diameter diaphragm and was concentrated at the target surface by a plano-convex lens with the convex side of the lens facing the laser. The lens has an effective focal length of 47 mm measured from the plane face for the employed laser wavelength. In this work, all distances, d, have been reported with respect to the employed lens focal length, where negative d values correspond to distances from the lens to the target below the nominal focal value. The lens and the target were mounted onto linear translation stages to accurate control d and to have a new ablation region between acquisitions; the estimated error in the d position due to the focusing process is about 200 µm.

The spatially integrated plasma emission was collected by a 5 mm in diameter quartz collimating lens located at about 45° with respect to the laser emission and at 6 cm from the ablation point. Light was sent by using an optical fiber bundle to a Czerny–Turner spectrograph (Spectra Pro 2500i, Acton Research) equipped with an 1800 lines/mm grating. Dispersed light was detected by an intensified charge-coupled device (CCD) camera (PiMAX 1024 × 1024, Princeton Instruments). The intensified CCD (ICCD) was synchronized with laser pulses through a pulse/delay generator (575-8C, Berkeley Nucleonics). The entrance slit was 10 µm wide giving an instrumental resolution of 0.05 nm. Five different spectral regions spanning about 10 nm each were employed in this work centered at 263, 282, 301, 327, and 409 nm; this allowed for the monitoring of different species present in the sample including Al, Mg, Si, Ti, Fe, Cu, Cr, and Mn. Monitoring minority elements below 1% in content allows to study the emission, minimizing the self-absorption effect. Each spectrum at constant energy or constant irradiance was obtained from the average of 20 laser shots, and the target was moved after each acquisition. Under the experimental conditions used, all pulses produced ablation of the target surface for d < 0; however, air breakdown was observed in some cases for d > 0. For time integrated measurements, the ICCD acquisition time delay time was kept fixed at 500 ns to avoid the initial bremsstrahlung and the acquisition time was set to 30 µs to measure the whole temporal evolution. The S/N was obtained as the ratio of the line peak adjusted through a Lorentz profile over the spectrum noise adjacent to the line.

Time-resolved spectra were acquired to obtain the temperature evolution of the plasma, where the gate width was varied from 10 ns for the shortest acquisition delays and up to 1 µs for the largest one. The irradiance was calculated using the pulse duration given by the laser manufacturer and the beam diameter at the sample surface obtained theoretically and experimentally corroborated through a knife edge and the 1/e² rule.

Results and Discussion

Emission Results

The optimization process of the LIBS experimental parameters were aimed at maximizing the trace detection and was based on maximizing the S/N for the different species present in the sample.

In this work, the dependence of the S/N with the laser energy and the distance between the lens and the target was investigated; all the remaining experimental parameters were kept fixed. The effect of the focusing distance on the emission intensity for the Mg(II) (280.27 nm) transition as a function of the shot number is shown in Fig. 1. Here, the energy was kept fixed at 30 mJ for the first 200 laser shots and intensities values were calculated at the peak to base adjusted through a Lorentz profile. As can be seen, for all distances, a larger variation was obtained for the first pulses, probably due to the aluminum oxide deposited over the target surface. It was also observed that the signal stability was reached after some tens of laser shots, but this was dependent on the focusing distance. The higher intensities values were obtained about 3–4 mm below the nominal focusing distance. As the distance is further decreased up to d = −8 mm, the LIBS signal is reduced by a factor of five. On the other hand, relative low intensity values were observed when the laser is focused at distances larger than the focal length since occasionally the breakdown is produced in air. Furthermore, it was also observed that depending on the focusing distance, the intensity increases for the first tens of laser shots. The irradiance over the surface produced by a laser with a beam propagation ratio M² can be obtained calculating the beam radius W at the distance d, through^10,24

W^{2} (d) = {(\frac{{λfM}^{2}}{πR})}^{2} + (\frac{d^{2} R^{2}}{f^{2}})

(1)

in which, the first term corresponds to the beam waist radius at d = 0, where λ is the laser emission wavelength and R the illuminated radius at the lens. The M² parameter is given by¹⁰ $M^{2} = π ϕ Θ / 4 λ$ , where $ϕ$ the laser beam diameter and $Θ$ is the full beam divergence of the laser. For the experimental conditions of Fig. 1, at d = 0, the beam radius is 28 µm which corresponds to an irradiance of 245 GW cm⁻².

The effect on the LIBS sensitivity must also consider the signal noise.²⁵ Thus, neglecting the first 20 shots, the signal intensity was averaged over 180 shots, for different distances d; results are shown in Fig. 2a using data reported in Fig. 1. The error bars were obtained from the standard deviation corresponding to the averaged laser shots on the same spot. The highest intensity was obtained about 8 GW cm⁻² both for ions and neutral species, while the relative standard deviation was the lowest for this irradiance value.

In a similar way, the S/N values were calculated for each laser shot (see Fig. 2b); the relative standard deviation of the S/N is also shown in the figure. The S/N error is larger than the obtained from intensity measurements only since it also includes the variation from the spectrum noise even though the emission S/N reaches its maximum value when the error reaches its minimum at d = −3 mm. These results agree with the normal practice in LIBS where the lens-to-sample distance is kept below the lens focusing distance. The obtained behavior may be explained in terms of the plasma shielding effect reported in various works.^17,20,26 Thus, only the initial part of the pulse ablates the sample, and afterward, the plasma becomes opaque to laser radiation. This is due to inverse bremsstrahlung and photoionization of the excited plume called plasma shielding. This phenomenon combined with the laser-supported detonation is reported to occur from a very low threshold of less than 1 GW, but it has a significant effect on the emission for irradiances larger than 10 GW cm⁻².^20,27–29 Consequently, as the lens is moved from d = −9 mm away from the target, the irradiance is enhanced which is translated in a higher S/N. This increment is directly proportional with the irradiance increment and could be related with an enhancement of the removed material. When the distance is further reduced up to d = −2 mm (17 GW cm⁻²), the plasma shielding effect becomes significant and the S/N begins to decrease. However, a relative increment could be observed near the focusing distance where it has been reported that a high irradiance regime saturates the plasma shielding mechanism.²⁰

One experimental parameter frequently used to improve the S/N is the laser energy, keeping fixed the focusing distance. Assuming that at d = −3 mm the emission reaches its maximum, the S/N as a function of the laser energy at that distance was investigated for several neutral and ions transitions belonging to different species present in the target (see Fig. 3). The reported values were obtained using a fixed gate delay and integration time of 0.5 and 30 µs, respectively. However, the optimal delay to maximize the emission S/N depends on the species and even on the concentration of the analyzed element. For instance, at the highest energy employed, the S/N increased from 10 to 30% for neutral transitions using an ICCD gate delay of 1.5 µs but for ions, this delay results in a reduction in S/N. Results show that S/N increases nearly linear with the deposited irradiance up to 6–8 GW cm⁻² keeping fixed the distance d; this was observed for all investigated transitions belonging to various species present in the alloy. For higher irradiance values, a slower growth rate in the LIBS signal was observed and could be attributed to plasma shielding effect, as discussed above. Therefore, it is necessary to perform an optimization for both, the laser energy and the focusing distance. However, this procedure is time consuming, and this is not normally performed in most LIBS experiments.

The emission intensity depends on various experimental factors and has been reported to increase with the square of the laser energy.^21,30 In addition to the increase in ablated mass, this could be attributed to an enhancement of the plasma temperature and the interaction of the laser beam with the plasma plume. On the other hand, at a low irradiance regime, it was observed a linear dependence with the laser energy.¹³ In that work, this behavior is attributed to a linear increase of the removed material. In any case, the dependence of the LIBS with the laser energy also depends on the focusing distance as will be seen below.

Thus, the dependence of the S/N with the distance d was investigated for different laser energies (see Fig. 4). Here, the error bars were obtained from the standard deviation of four different acquisitions. As it would be expected from the data of Fig. 3, the S/N grows with the laser energy at fixed focusing distance. The maximum S/N value depends linearly on the laser energy although the emission intensity grows at a faster rate. As a result, the obtained enhancement may not only be attributed to material removal but also to changes in the plasma temperature and density. Besides, the optimum distance is shifted to lower values up to d = −5 mm as the energy is increases to 90 mJ. As a result, the optimum distance, d, depends on the laser energy, but d could also depend on the employed focusing lens.²⁵ The maximum occurs for an irradiance of about 8 GW cm⁻² (see Fig. 4 inset) for the energies shown but for the case of 10 mJ the curve is rather plane from above 2 GW cm⁻². This result agrees with the values reported by Aguilera et al.²⁰ where it was observed a bend of the intensity curves when the irradiance surpassed 10 GW cm⁻². Therefore, based on previous obtained data, it is not enough to report only the laser fluence or the irradiance to characterize a LIBS experiment or any other ablation experiment. Both laser energy and the ablated spot area are essential to reproduce an experiment.

Finally, the effect of laser energy at constant irradiance was investigated and the most important results are shown in Fig. 5. Each point was obtained at a different focusing distance d, varying from approximately −1 mm to −4.6 mm for the 10 GW cm⁻² case. The reported values are obtained from the average of three spectra of 20 laser shots each and the error bars were obtained from the standard deviation of the obtained data from different spectra. Fig. 5a shows that as the laser energy is increased, the normalized S/N also grows. This enhancement was observed for both ions and neutrals corresponding to the different elements present in the target. This increment is alike to the observed at a constant distance as the laser energy is increased, but in a lower extent. Since the irradiance is kept constant, as the energy deposited by the laser is doubled, the ablated area increased by the same amount. Under these conditions the observed increment in emission should be related with an increment of removed mass from the target. Besides, Fig. 5b shows that the higher S/N value is obtained for the lowest irradiance. Therefore, for fixed laser energy, the S/N is reduced as the irradiance is increased from 10 to 40 GW cm⁻²; this is especially noted at high energies. To further investigate the involved processes that produce the observed behaviors, a study of the plasma physics is required.

Plasma Characterization

The temporal evolution of electron density and plasma temperature were obtained from the Stark broadening effect and Saha–Boltzmann plots, assuming local thermal equilibrium. Since laser-induced plasmas are dominated by collision between particles, the electron density can be obtained from the Stark broadening of the emission lines, with the expression proposed by Griem,³¹ neglecting the quasi-static broadening term. The Stark broadening parameter has a weak dependence on temperature and was obtained from Dimitrijević and Sahal-Bréchot³² and Bukvić et al.³³ The instrumental broadening was estimated to be 0.05 nm and subtracted from the linewidths. Subsequently, the electron density was calculated from the average of the values obtained from the following transitions: Mg(I) (285.21 nm) and Mg(II) (279.57 and 280.27 nm). The plasma temperature was obtained using the Saha–Boltzmann method that relates the emission of ions and neutral species.^30,34 In this work, magnesium emission lines around the spectral region of 277–286 nm were used. The analysis included all observed transitions of Mg(I): 277.67, 277.98, 278.14, 278.30, and 285.21 nm, and of Mg(II): 279.08, 279.55, 279.80, and 280.27 nm. Spectroscopic parameters such as ionization energy, transition probability, level degeneracy, and upper-level energy were taken from the NIST database.³⁵ Plasma characterization was performed in a time-integrated manner (30 µs) and with a time resolution of up to 10 ns. The temperature was also measured using an aluminum alloy with a magnesium content of less than 100 ppm (parts per million), so that self-absorption could be safely ruled out.

Figure 6 shows the time-integrated electron density for selected conditions of delivered laser energy at different focusing distances d, and for two different values for the irradiance (varying the laser energy and the distance d). The error bars were obtained from the line width error discarding the systematic error from the Stark width parameter. Traces depicted in Fig. 6a show a similar trend to the observed in the Fig. 4. There is a clear correlation between the electron density maxima and the corresponding S/N values obtained from spectra. The electron density maximum for the 90 mJ case was observed in the range of 5–13 GW cm⁻² (considering the error bars), while the peak of the S/N occurs at 8 GW cm⁻². Besides, the electron density grows more than twofold when the energy rises from 30 to 90 mJ. Additionally, a second rise was observed near the focusing point where it was also observed an increment of the emission. Conversely, at constant irradiance (see Fig. 6b), the electron density increases with the deposited laser energy by less than 50%. The time integrated values obtained for low laser energies of up to 20 mJ do not show significant differences. But at higher values, the experiment performed at 10 GW cm⁻², reach a higher electron density compared to the one at 40 GW cm⁻². Intermediate values were measured at 20 GW cm⁻², not shown in the figure to avoid overlapping of the traces.

The time-resolved experiment showed larger differences at the sub-microsecond timescale (see Fig. 6b inset). At 200 ns, the electron density grows in a factor of three when the energy is increased from 6 to 90 mJ for the 10 GW cm⁻² case and a factor of two for 40 GW cm⁻². Besides, at low laser energy (6 mJ) there are no significant differences between the two shown irradiances, while at 90 mJ, there is a slight increment of the electron density when using the lower irradiance.

The plasma temperatures were obtained using the electron density results shown in Fig. 6 and the error bars were calculated from the fit slope error of the Saha–Boltzmann plots. At a constant distance d and below the lens focusing length, the temperature grows with the laser energy (see Fig. 7a). This behavior has been reported by several authors under similar experimental conditions, as stated in the review of Zhang et al.³⁶ These curves have a similar behavior to the observed in Fig. 4; hence, the temperature reaches its highest value near 10 GW cm⁻². Fig. 7b shows that at constant irradiances the plasma temperature remains almost constant when the laser energy is increased. Under these conditions, the deposited energy density is the same except for the physical size of the produced plasma. Thus, variations in both electron density and temperature cannot explain the observed increase in emission S/N at constant irradiance.

The dependence of plasma temperature and electron density on the laser energy at constant focusing distance has been analyzed in several works and reviews.^14,15,36 The obtained values in this work show an increment of both the electron density and temperature. The time-integrated electron density grows by twofold and the temperature in about 1000 K when the irradiance is increased from 1.5 to 30 GW cm⁻².

Atomized Mass

There are various methods to obtain the removed material. Crater size measurements under the conditions of this experiment showed that the volume of the outer rim is comparable or even larger than the produced crater since the debris around the crater probably has a different density as was previously reported.^37,38 Besides, it was observed that melted material was re-deposited inside the produced cavity. On the other hand, the direct measurement of the weight after several thousands of laser shots was employed, but differences were not clearly appreciated. Finally, it was decided to estimate the atomized mass from the obtained spectroscopic data. This analysis assumes stoichiometric ablation, that plasma is homogeneous and under LTE conditions, and that the collected emission is directly proportional to the atomized mass. Two approximations were employed to obtain the removed mass with similar trends. The first method is based on measuring the y-intercept of the Saha–Boltzmann plots. Thus,

Y_{0} = l n [\frac{hc N^{I} F}{Q^{I} (T)}]

(2)

where N^I is the density population of the neutral species present in the plasma, Q^I is the partition function of neutral species at the temperature T, ℎ is the Planck constant, c the speed of light, and F is the factor that takes into account the efficiency of the employed detector and the geometry of the light collecting system. Since the F factor is constant in all cases and we are interested in the atomized material variation, it is not necessary to know this value.

Furthermore, the relation between the percentage of neutral and ion species was obtained through the Saha equation³⁴

\frac{N^{II}}{N^{I}} = \frac{1}{N_{e}} \frac{2 Q^{II} (T)}{Q^{I} (T)} {(\frac{2 πmk T}{h^{2}})}^{3 / 2} e^{- \frac{E_{\infty}^{I} - Δ E_{\infty}^{I}}{k T}}

(3)

where I and II indicates neutrals and ionic species, N^z is the density population, N_e is the electron density, k the Boltzmann constant,

E_{\infty}^{I}

is the ionization energy of neutral species, and

Δ E_{\infty}^{I}

is the correction of the ionization energy due to interactions in the plasma. The spectroscopic parameters such as the partition function are taken from the NIST database. Thus, it is possible to determine the degree of ionization of the plasma and the total number density N = N^I + N^II. Since the light from the plasma was spatially integrated and acquired at a large distance compared with the plasma dimensions, then it is not necessary to measure the plasma volume.²⁶ In this way, a value can be calculated that is proportional to the total population of magnesium atoms present in the plasma.

On the other hand, the emissivity of a single line can be written as

ε_{ki}^{z} = \frac{hc}{λ_{ki}} g_{k}^{z} A_{ki}^{z} \frac{N^{z}}{Q^{z} (T)} e^{- E_{k}^{z} / k T}

(4)

where N^z is the density population of neutral or ionic species,

A_{ki}^{z}

is the transition probability,

g_{k}^{z}

is the level degeneracy, λ_ki is the transition wavelength, and

E_{k}^{z}

is the upper energy level. Here, the total number density was also obtained through the Saha equation. Since the target is mainly made of aluminum, the intensity of 281.62 nm from Al(II) and 305.01 nm from Al(I) was used to estimate the total amount of aluminum atomized material. These lines have low self-absorption and therefore can be used for this calculation.^26,37 The error in the ablated mass determination depends on uncertainties of the line intensity, plasma temperature and electron density and is about 30%.

Results from the atomized mass for different distances are shown in Fig. 8a. The values obtained as a function of distance were calculated for the same laser energies presented above. The highest value of the atomized mass was obtained near 8 GW cm⁻² in agreement with the maximum obtained for the emission S/N. Comparison of the three traces shows that at this irradiance, the increase of the atomized mass is nearly proportional to the laser energy employed. This result reveals that the main contribution to the enhancement of the S/N of the emission observed in Fig. 4 is the increase of the ablated mass. For example, the increase in intensity for the 90 mJ case when changing the distance d from −10 to −6 mm, varied by an order of magnitude depending on the transition, while the S/N increases by a factor of five, while the atomized mass increased sevenfold. The measured increase in the plasma temperature and electron density could explain the difference between the measured intensity increase and the calculated atomized mass. A similar correlation was obtained for the other energies investigated. The emission increase from ionic transitions is larger than that from neutral ones, probably due to the increase in ablation temperature. A second peak of the atomized mass (see inset of Fig. 8a) was also obtained near the focusing distance (for irradiances of about 100 GW cm⁻²). This increase is in accordance with the observed behavior for the emission and as also reported elsewhere.²⁰

Similarly, the atomized mass as a function of the laser energy was calculated for two constant irradiances 10 and 40 GW cm⁻² (see Fig. 8b). Here, it is observed that at constant laser energy, a larger amount of mass is removed using the lower irradiance regime, as would be expected when the plasma shielding effect is increased. The amount of atomized mass decreased by a factor of 1.7 for the irradiance values shown, even when the laser energy is increased by a factor of 15. It was also observed that the calculated increase in atomized mass is approximately the same as the increment of the laser energy. Evidently, this is the cause of the observed increase in S/N reported in Fig. 5, especially considering that the plasma temperature remains constant, and the electron density showed a modest increase. Therefore, the increment of the deposited energy produces a plasma of larger dimensions, but with similar physical properties.

Conclusion

In this work, the effect on the LIBS signal was investigated as a function of the lens-sample distance and at a constant irradiance, varying both the focusing distance and the laser energy. The experiment was performed using a 1064 nm nanosecond laser to ablate a metal target in air at atmospheric pressure. Under the conditions studied, it was obtained that at a fixed laser energy, the irradiance that maximizes the emission S/N is about 8 GW cm⁻². As the irradiance increases, the plasma shielding effect reduces the ablated mass and consequently the detected emission; but the plasma shielding could be surpassed at irradiances in the order of 100 GW cm⁻². At larger distances from the optimum value, the reduction of the removed mass is the cause in the emission drop. Besides, plasma analysis showed that both electron density and temperature also increase for the optimum irradiance value, which also results in an emission increase.

On the other hand, at constant irradiance, it was observed a rise in the emission S/N with deposited energy, which is also dependent on the delivered irradiance. This is due to the fact that, as the plasma shielding increases, the matter removed decreases. In contrast, the plasma temperature remains unchanged, and the plasma density showed a small increase. Thus, as the laser energy and the ablated area is increased, but keeping the laser fluence constant, the increment of removed mass is translated into a larger LIBS signal.

Since the experiment was performed using typical LIBS conditions, it is expected that similar results will be obtained using nonmetallic targets or other wavelengths to perform the ablation. Therefore, in a LIBS experiment, it is not enough to report only the laser irradiance on the sample but also the deposited energy or the ablated area since both parameters are needed to replicate a given experiment.

Footnotes

Acknowledgments

The authors are grateful to Dr. R. Sanginés de Castro for the critical reading of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Autonomous University of Mexico (DGAPA-UNAM: IN104421).

ORCID iD

Hugo Sobral

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