Characterization of laser ablation of copper in the irradiance regime of laser-induced breakdown spectroscopy analysis

Spectrochimica Acta Part B 101 (2014) 164–170

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Characterization of laser ablation of copper in the irradiance regime of laser-induced breakdown spectroscopy analysis☆ J. Picard a,⁎, J.-B. Sirven b, J.-L. Lacour b, O. Musset c, D. Cardona a, J.-C. Hubinois a, P. Mauchien b a b c

Commissariat à l'Energie Atomique, DAM, Valduc, F-21120 Is-sur-Tille, France Commissariat à l'Energie Atomique, DEN/DANS/DPC/SEARS/LANIE, Saclay, F-91191 Gif-sur-Yvette, France Université de Bourgogne, Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR CNRS 5209, F-21000 Dijon, France

a r t i c l e

i n f o

Article history: Received 6 December 2013 Accepted 8 August 2014 Available online 27 August 2014 Keywords: Laser ablation 266 nm Irradiance Ablation crater LIBS

a b s t r a c t The LIBS signal depends both on the ablated mass and on the plasma excitation temperature. These fundamental parameters depend in a complex manner on laser ablation and on laser–plasma coupling. As several works in the literature suggest that laser ablation processes play a predominant role compared to plasma heating phenomena in the LIBS signal variations, this paper focuses on the study of laser ablation. The objective was to determine an interaction regime enabling to maximally control the laser ablation. Nanosecond laser ablation of copper at 266 nm was characterized by scanning electron microscopy and optical profilometry analysis, in air at 1 bar and in the vacuum. The laser beam spatial profile at the sample surface was characterized in order to give realistic values of the irradiance. The effect of the number of accumulated laser shots on the crater volume was studied. Then, the ablation crater morphology, volume, depth and diameter were measured as a function of irradiance between 0.35 and 96 GW/cm². Results show that in the vacuum, a regular trend is observed over the whole irradiance range. In air at 1 bar, below a certain irradiance, laser ablation is very similar to the vacuum case, and the ablation efficiency of copper was estimated at 0.15 ± 0.03 atom/photon. Beyond this irradiance, the laser beam propagation is strongly disrupted by the expansion of the dense plasma, and plasma shielding appears. The fraction of laser energy used for laser ablation and for plasma heating is estimated in the different irradiance regimes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Laser-induced breakdown spectroscopy (LIBS) consists in focusing a laser pulse onto the surface of a sample and in measuring the emission of the resulting plasma with a spectrometer in order to do elemental analysis [1]. For a given emission line, the LIBS signal is proportional to the population of atoms in the upper line level, which depends on two fundamental parameters: the number of free atoms in the plasma, assumed to be proportional to the number of ablated atoms, and the plasma electron temperature [2,3]. The first one is related to material sampling by laser ablation, while the second one is related to the excitation of the ablated material. The peculiarity of the LIBS technique is that these two processes, sampling and excitation, are simultaneous and antagonistic. Indeed, a certain fraction of the pulse energy is used for laser ablation and vaporization of the ablated matter, while the remaining energy is used to excite and ionize atoms of the plasma. Thus, the proportion of energy used for ablation is not available for plasma heating, ☆ Selected paper from the 7th Euro-Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2013), Bari, Italy, 16–20 September 2013. ⁎ Corresponding author. E-mail address: [email protected] (J. Picard).

http://dx.doi.org/10.1016/j.sab.2014.08.029 0584-8547/© 2014 Elsevier B.V. All rights reserved.

and vice versa. And the energy distribution between these two phenomena depends in a complex manner on the laser beam characteristics at the surface (wavelength, irradiance, numerical aperture, intensity spatial distribution…) [4,5] and on the material physico-chemical and thermo-physical properties [6–9], including its surface properties (roughness, porosity, concentration gradients…). This issue has been theoretically addressed by different authors. Thus, in the case of nanosecond ablation of copper, Bleiner et al. found an absorption of laser energy by the plasma of 48%, for a 1 GW/cm² irradiance at 266 nm [10]. Clair et al. found ≈75% at ≈2 GW/cm² for a 532 nm pulse [11]. Finally, in a recent article, Autrique et al. estimated a 70% absorption by the plasma, for a 532 nm laser and an irradiance of 1.7 GW/cm². Experimental results obtained in the same conditions confirm the validity of the model [12]. These different results obtained from modeling approaches show that the plasma absorption is already high for an irradiance significantly lower than those commonly used in LIBS. From 1 GW/cm², there is a very strong coupling between laser ablation and plasma excitation. Moreover, for a given application, optimization of operating conditions is difficult because we do not know the optimum balance between these two phenomena from an analytical point of view. Of course the LIBS signal is all the more so intense as the ablated mass and plasma temperature are higher. But on the other hand, the optical thickness

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depends on the plasma density and temperature distribution, and it may involve self-absorption effects that limit the linearity of calibration curves [3]. However, as a first step, several elements lead us to focus on the study of laser ablation compared to the plasma description. Indeed, Chaléard et al. and Gornushkin et al. showed that matrix effects are dramatically corrected by normalizing the LIBS signal by the ablated mass [13,14]. Similarly, several articles also showed a significant reduction of matrix effects by normalizing the signal by a line of a major element of the matrix, which is equivalent to correcting by the ablated mass [15,16]. Finally, other studies showed that the LIBS signal is practically linear with the pulse energy or with the fluence, at least in a certain range [6,17–19]. As the intensity of a line depends linearly on the ablated mass and non-linearly on the temperature, we can assume that in this range the influence of temperature variations is negligible compared to variations in ablated mass. Therefore, these studies suggest that laser ablation processes play a predominant role compared to plasma heating processes in the LIBS signal variations. Let us add that from a practical point of view, in order to control the measurement, the user can only impose the experimental parameters of the laser and of the beam focusing. By comparison, the detection parameters (particularly the gate delay and width) are rather driven by the initial state of the plasma and by its dynamics. Then, it is justified to seek first to control laser ablation as such, before describing the temporal evolution of the plasma characteristics. In this article, we experimentally characterized the ablated mass in irradiance conditions relevant for LIBS analysis in order to determine an interaction regime enabling to maximally control the laser ablation. To minimize the coupling between laser ablation and absorption by the plasma, we chose to use a UV laser at 266 nm. Indeed, the laser photon absorption cross section by inverse Bremsstrahlung depends on λ2 for neutral atoms and on λ3 for ions [20]. Furthermore, the absorption decreases with the pressure because the plasma confinement by the ambient gas decreases and tends to lower its density [21]. Therefore, we compared the results obtained in air at atmospheric pressure and under vacuum to study the effect of plasma on the laser ablation as a function of the irradiance regime. 2. Experimental setup A diagram of the experimental system is shown in Fig. 1. The laser source was a Q-switched Nd:YAG laser (Quantel, Brio) operating at 266 nm with 5 ns pulse duration (FWHM) (20 Hz, M2 = 2.5). The laser pulse energy was controlled with a variable attenuator (λ/2 plate and polarizing beam splitter at 266 nm) and was measured from shot to shot with a pyroelectric detector and a joulemeter (Gentec, Maestro) after a 5% reflection on a beam splitter. The beam was focused by a planoconvex quartz lens (f = 250 mm) inside a vacuum chamber

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equipped with a primary pump and a secondary pump. Positioning of the sample was realized by XYZ translation stages. Two collimated laser diodes on each side of the focusing lens were set to intersect in the focal plane in order to adjust the height of the target surface in this plane. The sample was irradiated in air at atmospheric pressure and in the vacuum (10−4 mbar). The pulse energy on the sample surface was adjusted from 26 μJ to 7 mJ (irradiance from 0.35 to 96 GW/cm²), with a maximum energy fixed at 70% of the nominal laser energy to compensate for a possible drift during the experiments. The ablation was performed for 10 to 500 laser shots per crater. Four different craters at each energy and each number of laser shots were made. The crater depth, diameter, and volume were measured using a whitelight interferometric profilometer (Bruker, ContourGT1) equipped with a ×20 objective (0.4 numerical aperture) with a lateral resolution of 0.67 μm and a depth resolution lower than 1 nm. Scanning electron microscope (SEM) images were also taken to analyze the craters morphology. A pure copper sample was used for all experiments. Its surface was polished prior to the ablation. The laser beam intensity distribution in the focal plane was measured with a UV-sensitive charge coupled device (CCD) camera (FireWire BeamPro, 2523). The intensity profile is shown in Fig. 2. We note that it is very close to a Gaussian profile. The best Gaussian fit gives a beam diameter of 30 μm at 13.5% (1/e2) of the maximum intensity, but this value is underestimated since a fraction of laser energy is measured in the wings of the beam. Then, to calculate the irradiance in the focal plane, we used the real beam diameter at 13.5% of the maximum intensity. We found an average diameter of 43 μm. 3. Results and discussion 3.1. Crater morphology SEM images of craters obtained on pure copper at 25 GW/cm² and 75 GW/cm² are shown in Fig. 3 for a single laser shot in air and in the vacuum. The laser beam diameter at 13.5% (43 μm) is shown as a black dotted circle. Nanosecond laser ablation is thermal, and given the Rayleigh length of the focused beam (2.1 mm), the laser–matter interaction can be seen as a piston on the surface that successively melts, and then vaporizes the target. On the SEM pictures, three zones associated to the laser ablation processes can be defined. Their diameters are given in Table 1. The crater bottom (zone 1) has a smooth structure. This zone undergoes the strongest heating by the laser; hence its diameter is the same order of size than the diameter of the central part of the laser spot. Zone 1 can be seen as a reservoir of melted, then vaporized matter, which feeds the plasma. The following zone (zone 2) has a radial structure corresponding to the ejection of melted matter from zone 1, due to the recoil pressure induced by the expanding plasma. Its diameter is close to that of the crater, as measured by the profilometer, and at

Mirror

Joulemeter

Mirror

Beam splitter

Lens Lens f = 100mm f = -300mm V Mirror

Mirror V Lens f = 250mm

Nd:YAG laser 266 nm

Sample XYZ

Variable attenuator at 266 nm

Vacuum chamber Fig. 1. Experimental setup for laser ablation in air at 1 bar and in the vacuum.

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Table 1 Diameter of zones 1, 2 and 3 (see Fig. 3) obtained for single-shot craters at 25 and 75 GW/ cm², in air at 1 bar and in the vacuum.

Horizontal axis Fit

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26 μm 68 μm 191 μm

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Fig. 2. Laser beam intensity profiles in the focal plane (line: experimental data; dot: Gaussian fit).

25 GW/cm² it is very close to the laser beam diameter. The third zone (zone 3) is concentric, with a diameter approximately equal to twice that of zone 2. Zones 2 and 3 are asymmetric relative to the center, and this observation is reproducible. It is due to the laser beam spatial profile, which is also slightly asymmetric.

Vacuum

75 GW/cm2

25 GW/cm2

Air 1 bar

Table 1 shows that the ambient pressure has no significant influence on the morphology of single-shot craters at 25 GW/cm². This means that the laser–surface interaction remains the dominant mechanism of ablation, and argues in favor of a weak plasma shielding even at atmospheric pressure. On the other hand, at 75 GW/cm², zone 3 is clearly larger in air at 1 bar compared to the vacuum. This thermal affected zone seems too small to be attributed to a plasma–surface interaction. It is most probably due to the wings of the laser beam seen in Fig. 2, for which the local irradiance is high enough to induce thermal damages to the sample. This table also provides information on the effect of irradiance on the different diameters. Overall, they slightly increase with irradiance. In air at 1 bar, zone 3 is significantly larger at 75 GW/cm² compared to 25 GW/cm². This supports the existence of plasma shielding at high irradiance, which may increase the plasma temperature [22]. In addition, as shown below, this increase in diameter can be due to the enlargement of the laser beam diameter above the ablation threshold when the irradiance increases. Fig. 4 shows the crater profiles obtained after ten accumulated laser shots in the vacuum, and for 6 different irradiances (0.35; 1; 4.7; 11.7; 24.7; 97.2 GW/cm²), measured by the optical profilometer. The measured laser beam profile (horizontal profile on Fig. 2) is superimposed

Fig. 3. SEM images of single-shot craters made on pure copper in air at 1 bar and in the vacuum at 25 GW/cm² and 75 GW/cm². The black dotted circle shows the laser beam diameter in the focal plane (43 μm). The three zones defined in the text are also indicated. The scale and the image orientation relative to the laser beam are not the same for all images.

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Fig. 4. Craters (solid line) and laser beam (dotted line; horizontal profile on Fig. 2) profiles at different irradiances for 10 accumulated laser shots in the vacuum. The amplitude of the laser beam profile is normalized to the crater depth.

to each crater profile. We can see that the crater profile is narrower than the laser beam one at low irradiance due to the ablation threshold, then it continuously widens with irradiance. At moderate irradiance, both profiles are highly similar: the crater shape is totally representative of the laser beam profile, like a fingerprint; therefore, no effect of plasma shielding is observed. This is consistent with the previous comparison of SEM pictures at 25 GW/cm², in air at 1 bar and in the vacuum. Then, for these irradiances, we can say that the laser fully controls the interaction with the sample. At 97.2 GW/cm², the crater profile is clearly wider than the laser beam profile. This endorses the idea that laser ablation occurs as soon as the irradiance is higher than a certain threshold that depends on the material, the laser wavelength, temporal profile and focusing conditions [23,24]. Then, the actual laser beam diameter that contributes to the ablation increases with irradiance.

1E+6

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From an analytical point of view, a better control of the ablated mass is favored by a moderate irradiance regime. Thus laser ablation is driven by the properties of the laser and of the focusing system, which the user

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2. At moderate irradiance, no significant difference is detected between craters made at 1 bar and in the vacuum, which is consistent with the assumption of weak plasma shielding in this regime; 3. At atmospheric pressure and high irradiance, plasma shielding may play a role in the ablation process.

4.2 mJ 

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1. At moderate irradiance, the crater morphology, hence the ablated mass, is mostly due to the laser beam energy distribution at the sample surface;

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To summarize, our observations shore up different phenomena:

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Fig. 5. Crater volume in air at 1 bar vs. number of laser shots for 26 μJ, 95 μJ, 0.37 mJ, 1.8 mJ and 4.2 mJ. Error bars represent ±2 standard deviations.

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can fully control, and uncontrolled effects of the laser–plasma interaction shall be minimized. 3.2. Influence of the number of laser shots on the crater volume Fig. 5 shows the crater volume as a function of the number of laser shots for different pulse energies between 26 μJ and 4.2 mJ (irradiance between 0.36 and 58 GW/cm²), in air at 1 bar. For each energy, up to 50 laser shots, we see an approximately linear increase of the crater volume with the number of pulses. This shows that the ablation rate (in μm3/pulse) remains overall constant in this range: the surface modifications and the variation of the crater aspect ratio induced by successive laser shots, seem to have a negligible influence. Beyond 50 shots, the crater volume increases more slowly, indicating a modification of the ablation rate. Several mechanisms, already mentioned in the literature [24], can be invoked to explain this observation. As the crater gets deeper, the plasma forms at the crater bottom and can be significantly confined inside it, hence changing its expansion, temperature and density. This may induce an increase of laser shielding by the plasma and a disruption of the laser beam propagation toward the target surface. In addition, it is possible that a certain mass of ablated matter cannot be ejected from the crater bottom or condenses on the colder crater walls. All these phenomena are likely to limit the ablated volume. Due to surface modifications induced by the accumulation of laser shots (roughness, porosity, concentration gradients…), the study of laser ablation processes is in principle simpler when performed on single-shot craters. However, the linearity observed below 50 laser pulses shows that in our experimental conditions, the ablation rate remains unchanged. Therefore we can assume that physical phenomena are the same between 1 and 50 accumulated pulses. Then, in the following, the number of accumulated laser shots was fixed to 10 in order to improve the crater measurement accuracy compared to single-shot craters. 3.3. Influence of the pulse energy on the crater volume, depth and diameter Fig. 6 shows the crater volume as a function of the pulse energy, in air at 1 bar and in the vacuum, for 10 accumulated laser shots. The energy range is extended up to 7 mJ (96 GW/cm²), so that almost three decades are covered. In the vacuum, a monotonous increase of the ablated volume is observed. It is remarkably fitted over the whole range by a power law, with Vcrater ∝ E1.24 ± 0.02. At 1 bar and low energy (up to 0.1 mJ), the ablated volume is very similar to that measured in the

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vacuum. Below ≈ 3 mJ (40 GW/cm²), the crater volume follows a power law, with Vcrater ∝ E1.12 ± 0.07. This trend is slightly different from that observed in the vacuum, and the measured volume is smaller. In this irradiance regime, we assume that this difference is mainly due to the plasma density. As pointed out by several authors [25–27], it is higher at 1 bar because of the confinement by the ambient gas, then the plasma is more dense and absorption of laser photons by the plasma is higher than in the vacuum. Another possible phenomenon is that in the vacuum, only heat conduction inside the target can occur during the ablation, whereas at 1 bar, conduction losses due to the ambient atmosphere are not negligible. In both cases, the laser energy coupling to the target may be less efficient at atmospheric pressure than in the vacuum, leading to a lower ablation efficiency. Overall, the obtained power laws of the crater volume as a function of pulse energy are not very far from a linear regime, which means that the fraction of pulse energy used for laser ablation, and that used for plasma heating, do not vary much in this irradiance range, though their values slightly depend on the pressure. At 1 bar and high irradiance, two phenomena are likely to limit the penetration of laser energy toward the surface, hence the ablation efficiency: plasma shielding, and modification of the laser beam propagation due to the dense plasma expansion. Then, beyond ≈3 mJ, we see that these mechanisms become dominant, so much so that the crater volume does not increase any more. In this regime, the crater volume is 2–4 times lower than in the vacuum. Therefore, assuming that at 1–2 GW/cm² the plasma absorption coefficient is 50–80% [10–12] and that this value is the same in the vacuum over the whole irradiance range, we obtain a plasma absorption coefficient of 75–95%. Then, the fraction of laser energy used for ablation is 30–50% at moderate irradiance and 5–25% at high irradiance. The crater depth is widely studied in the field of laser ablation to characterize the material ablation rate [28,29], but it is also important to know its relationships with the laser parameters [30,31]. Fig. 7 shows the crater depth as a function of the pulse energy in air at 1 bar and in the vacuum, for 10 accumulated laser shots between 26 μJ and 7 mJ. In the vacuum, a square root law fits very well the experimental data over the whole range. This trend was also observed by Liu et al. [30], and Salle et al. [7], and it is fully consistent with the model of Phipps et al. [32] on laser ablation in the vacuum. As this model is based on the hypothesis of a one-dimensional expansion of an opaque, ideal gas plasma heated by inverse Bremsstrahlung, we conclude that these assumptions correctly describe the dominant phenomena in our experimental conditions. The ablated depth follows the same law in air at 1 bar up to ≈3 mJ (40 GW/cm²). As seen previously, at moderate irradiance laser ablation produces craters with a very similar morphology in air at 1 bar and in the vacuum. The laser–target interaction weakly depends on the

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Pulse energy (mJ) Fig. 6. Crater volume vs. pulse energy for 10 accumulated laser shots, in air at 1 bar and in the vacuum. Error bars represent ±1 standard deviation.

Fig. 7. Crater depth vs. pulse energy for 10 accumulated laser shots, in air at 1 bar and in the vacuum. For the 1 bar case, the last three points are not taken into account to calculate the fit. Error bars represent ±1 standard deviation.

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ambient pressure, and this is why the model of Phipps et al. remains remarkably valid in air at 1 bar below a certain energy threshold. Then the ablated depth saturates. This observation is consistent with the evolution of the crater volume with the pulse energy (Fig. 6), and the same phenomena can be put forward to explain it: as the irradiance increases, plasma shielding and modification of the laser beam propagation progressively limit the laser energy reaching the surface, until the ablated depth becomes constant. Those phenomena are not taken into account by Phipps' model. Fig. 8 shows the crater diameter as a function of the pulse energy for 10 accumulated laser shots in air at 1 bar and in the vacuum. We note here that the crater diameter is not significantly influenced by the ambient pressure. The data obtained in the vacuum are slightly higher than those obtained at 1 bar, but follow the same trend. We can see that the crater diameter increases with the pulse energy as a power law, with Φcrater ∝ E0.32 ± 0.03 in both cases. As shown in Section 3.1, this is qualitatively consistent with the fact that the actual laser beam diameter contributes to the ablation, i.e. above the ablation threshold, increases with irradiance. The crater volume is proportional to the crater depth and to the square of its diameter: Vcrater ∝ dcrater × Φ2crater. As dcrater ∝ E0.5 and Φcrater ∝ E0.32 ± 0.03, then we should obtain Vcrater ∝ E1.14 ± 0.06. In the moderate irradiance regime where the ablation is driven by the laser beam characteristics at the sample surface, this relationship is very well verified at 1 bar, with Vcrater ∝ E1.12 ± 0.07. A slight deviation is observed in the vacuum case (Vcrater ∝ E1.24 ± 0.02). Finally, in the vacuum, the crater volume, depth and diameter are fully correlated to the pulse energy, and the laser–surface interaction is the dominant mechanism of laser ablation. The same conclusions hold at 1 bar up to ≈40 GW/cm²: at moderate irradiance, we observe that the crater characteristics do not strongly depend on the ambient pressure. Beyond ≈40 GW/cm², plasma shielding and modification of the laser propagation by the expanding plasma appear. This laser–plasma interaction strongly limits the conversion of the pulse energy into ablated matter under atmospheric pressure. 3.4. Ablation efficiency of copper under atmospheric pressure From the data in air at 1 bar, we can determine the nanosecond ablation efficiency of copper at 266 nm. On the experimental points shown in Fig. 6, we can reasonably make a linear approximation in the central part of the range, i.e. between 0.3 and 3 mJ (5 and 40 GW/cm²), to estimate the ablation efficiency. Different approaches are mentioned in the literature to calculate this efficiency. Salle et al. [7] and Semerok et al. [31] defined it as the ratio of ablated volume to the laser pulse energy. Gojani et al. [8] divided the mass of the ablated material by the number of laser pulses while Zeng et al. [33] and Kononenko et al. [34] used the ratio between the crater depth and the number of pulses. In the 100

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framework of photo-fragmentation studies on nanoparticles, Stipe et al. [35] used the photon–atom ratio (PAR) which is the ratio of the number of photons striking the particle to the number of atoms in the particle. These expressions were reviewed by Hahn and Omenetto [2] as well as other relations. In our study, we chose to define the ablation efficiency as the photon–atom ratio, as its physical meaning is rather straightforward to apprehend. However, it is worth underlining that this ablation efficiency does not presume anything about the LIBS signal, which is emitted by free, vaporized atoms. All atoms ejected from the surface are taken into account here, but they are not all necessarily in vapour phase in the plasma. Then the ablation efficiency is measured from the slope a of the linear part of the curve shown in Fig. 6 (Eq. (1)): ηabl ¼

Total number of ablated atoms ρ  NA  hν 9 a ¼ 10  S Total number of laser photons MS  Np

ð1Þ

where ηabl is the ablation efficiency (atom/photon), ρS is the target density (g/cm3), NA is the Avogadro constant (6.02 × 1023 mol−1), hν is the laser photon energy (J), MS is the target molar mass (g/mol), Np is the number of accumulated laser shots and a in μm3/mJ units. The density and the molar mass of copper are 8.96 g/cm3 and 63.55 g/mol, respectively [36]. In our experimental conditions, we find an ablation efficiency of 0.15 ± 0.03 atom/photon (24 000 ± 4000 μm3/mJ). Thus, about seven 266 nm laser photons are needed to ablate one copper atom. This result is very consistent with the ablation efficiency of copper at 266 nm measured by Salle et al., who found 0.23 atom/photon (37 000 μm3/mJ) with experimental conditions close to ours [7]. For LIBS analysis, it is useful to work in the energy range where the crater volume is approximately linear. Indeed, considering that the LIBS signal fluctuations are mostly due to ablated mass variations [37, 13,14], then those fluctuations could be easily corrected by a simple shot-to-shot energy measurement. In this regime, uncontrolled effects of the plasma on the ablation are minimized. It is then much more controllable for analytical purposes. And the knowledge of the ablation efficiency in the linear range enables to predict the ablated mass induced by the analysis, which can be a useful data both for chemical measurements or in order to provide inputs for ablation models. The observation of an approximately linear regime also means that in the 5–40 GW/cm² range, the fraction of laser energy used for the ablation, and the fraction used for plasma excitation, remain constant. This result is in agreement with a possible self-regulating regime during laser ablation and plasma coupling with the laser pulse tail. This regime was theoretically investigated by several authors [38–40], and it was highlighted in the recent paper by Autrique et al., from both experimental and theoretical considerations [12]. The validity of this selfregulation regime then deserves to be further investigated by additional experiments, which are beyond the scope of this paper and might be addressed in the future. Hence, to test this hypothesis in this irradiance range for which ablation conditions are similar, the plasma dimensions, electron density and temperature, and emission, should be measured as a function of time, to see if plasma conditions also remain similar. 4. Conclusion

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Pulse energy (mJ) Fig. 8. Crater diameter vs. pulse energy for 10 accumulated laser shots, in air at 1 bar and in the vacuum. Error bars represent ±2 standard deviations.

In this paper, we used a 266 nm laser to perform ablation of a pure copper sample, in air at 1 bar and in the vacuum. We measured the laser ablation crater morphology and dimensions over a wide irradiance range (0.35–96 GW/cm²), with the objective of determining an interaction regime enabling to maximally control the laser ablation. At low pulse energy, the crater morphology is similar to the laser beam profile. The crater volume, depth and diameter are totally correlated to the pulse energy and there is no difference between the results in air at 1 bar and in the vacuum. In this energy range, laser ablation is driven by the laser beam characteristics at the sample surface. And the fraction of laser energy used for laser ablation and for plasma heating is

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thought to be approximately constant, which is consistent with recent modeling results on a possible self-regulating regime during nanosecond laser ablation [12]. It is interesting to note that this model extends up to 1–2 GW/cm². Our experimental results suggest that this regime might exist up to much higher irradiances. Then, this enables to define and measure the ablation efficiency of copper at 266 nm. In our experimental conditions we found 0.15 ± 0.03 atom/photon. At high pulse energy, results obtained in the vacuum follow the same trend as at low pulse energy. Conversely, in air at 1 bar, a saturation of the crater volume and depth is observed. In this energy range, plasma shielding and modification of laser beam propagation occur. This strong laser–plasma interaction results in a much less controlled laser ablation, since the fraction of laser energy used for laser ablation depends on the irradiance. In conclusion, this work enabled us to determine an irradiance range, between 5 and 40 GW/cm², for which laser ablation can be well controlled by the user. However, LIBS is not only laser ablation, and photons emitted by the hot plasma shall be detected. From an analytical point of view, the final objective could be to obtain the most luminous plasma while keeping a maximum control of the measurement processes by the laser, whose parameters are the only ones that the user can fix. Then, our future work will address the search for the optimum balance between laser ablation and plasma emission processes for analytical purposes. Acknowledgements One of the authors (J. Picard) thanks the Burgundy region for financial support. C. Quéré (CEA Saclay) is acknowledged for assistance during the profilometry measurements, and J. Varlet (CEA Saclay), for assistance during the SEM measurements. References [1] A. Miziolek, V. Palleschi, I. Schechter, Laser-Induced Breakdown Spectroscopy, Cambridge Univ Press, 2006. [2] D. Hahn, N. Omenetto, Laser-induced breakdown spectroscopy (LIBS), part I: review of basic diagnostics and plasma–particle interactions: still-challenging issues within the analytical plasma community, Appl. Spectrosc. 64 (2010) 335A–366A. [3] D. Hahn, N. Omenetto, Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields, Appl. Spectrosc. 66 (2012) 347–419. [4] E. Tognoni, V. Palleschi, M. Corsi, G. Cristoforetti, Quantitative micro-analysis by laser-induced breakdown spectroscopy: a review of the experimental approaches, Spectrochim. Acta Part B 57 (2002) 1115–1130. [5] G. Cristoforetti, G. Lorenzetti, P. Benedetti, E. Tognoni, S. Legnaioli, V. Palleschi, Effect of laser parameters on plasma shielding in single and double pulse configurations during the ablation of an aluminium target, J. Phys. D. Appl. Phys. 42 (2009). [6] L. Cabalin, J. Laserna, Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation, Spectrochim. Acta Part B 53 (1998) 723–730. [7] B. Salle, C. Chaleard, V. Detalle, J. Lacour, P. Mauchien, C. Nouvellon, A. Semerok, Laser ablation efficiency of metal samples with UV laser nanosecond pulses, Appl. Surf. Sci. 138 (1999) 302–305. [8] A. Gojani, J. Yoh, New ablation experiment aimed at metal expulsion at the hydrodynamic regime, Appl. Surf. Sci. 255 (2009) 9268–9272. [9] T. Labutin, A. Popov, V. Lednev, N. Zorov, Correlation between properties of a solid sample and laser-induced plasma parameters, Spectrochim. Acta Part B 64 (2009) 938–949. [10] D. Bleiner, Z.Y. Chen, D. Autrique, A. Bogaerts, Role of laser-induced melting and vaporization of metals during ICP-MS and LIBS analysis, investigated with computer simulations and experiments, J. Anal. At. Spectrom. 21 (2006) 910–921. [11] G. Clair, D. L'Hermite, 1D modelling of nanosecond laser ablation of copper samples in argon at P = 1 atm with a wavelength of 532 nm, J. Appl. Phys. 110 (2011). [12] D. Autrique, G. Clair, D. L'Hermite, V. Alexiades, A. Bogaerts, B. Rethfeld, The role of mass removal mechanisms in the onset of ns-laser induced plasma formation, J. Appl. Phys. 114 (2013).

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