Pressure Effects in Laser-Induced Plasmas of Trinitrotoluene and Pyrene by Laser-Induced Breakdown Spectroscopy (LIBS) Toma´s Delgado, Jose´ M. Vadillo, J. Javier Laserna* Department of Analytical Chemistry, University of Ma´laga, 29071 Ma´laga, Spain
The influence of the ambient atmosphere on the dynamics of plasma expansion, besides the interaction between excited plasma and gas molecules, has been studied for specific organic aromatic compounds. To analyze the influence of air on the formation pathways of atomic and molecular species inside the plasma plume, the spectral emissions in laser-induced breakdown spectroscopy (LIBS) of 2,4,6-trinitrotoluene (TNT) and pyrene were compared at different pressure environments, from high vacuum to atmospheric pressure. Pelletized samples of the compounds were introduced in a vacuum chamber for excitation with the fourth harmonic output of an Nd : YAG laser (266 nm). The optical emission signal was collected with an optical fiber connected to a spectrograph fitted with a intensified charge-coupled device detector. Results from LIBS spectra indicate that changes in pressure level affect the kinetics of the characteristic excited species and their spatial distribution inside the plasma plume. Index Headings: Laser-induced plasmas; 2,4,6-trinitrotoluene; TNT; Laser ablation; Explosive compounds.
INTRODUCTION The analysis of organics by laser-induced breakdown spectroscopy (LIBS) has certain drawbacks compared with samples of an inorganic nature. The interference effects involving recombination of species and their interaction with the atmosphere must be taken into account. A large body of knowledge has been coined since the early years of LIBS concerning the influence of pressure and the nature of atmosphere in the dynamics of generation and evolution of plasma using emission spectroscopy. Time-resolved studies have been applied historically for nominal nanosecond LIBS, and a strong dependence on the ambient pressure was deduced from measurements of the emission intensity as a function of delay times. Although much research has been focused on metallic or ceramic samples,1–3 probably due to the extraordinary capability of LIBS to solve real industrial problems, LIBS may be found in almost any combination of matrices and elements.4 The dynamics of carbon or carbon-containing species also has been subject of study,5,6 probably due to the driving force of understanding the processes behind the deposition of diamond-like carbon or carbon nitrides by pulsed laser deposition. In recent years, a growing interest in extracting as much molecular information as possible from LIBS Received 28 May 2013; accepted 3 September 2013. * Author to whom correspondence should be sent. E-mail: laserna@ uma.es. DOI: 10.1366/13-07164
Volume 68, Number 1, 2014
experiments of organics has emerged.7,8 Due to the presence of H, O, and N in the surrounding atmosphere as well as on the analyzed compounds, temporally resolved studies have been used to identify native species coming from an organic sample or as a result of reactions between the ambient air and the excited material in the plasma. The different origins of emitters resulted in different kinetic behaviors. Recently, other studies concerning the influence of ambient pressure on the kinetics of excited species in laser-induced plasmas from ablation of carbons and organic compounds are emerging.9–13 Laser-induced plasmas constitute a spatially inhomogeneous and transient system, so temporally and spatially resolved spectroscopy stands as an efficient tool to carry out this type of investigations to ascertain the plasma expansion dynamics and to study the physical properties of its emission. As noted, pressure is a crucial factor in the generation of plasma because it has a deciding effect on many areas of study of plasmas, from its reactivity and the rate of collisions inside the plume to the different expansion velocities, including the morphology of the plume and the spectral characteristics of the emitted light. The key of the alterations undergone by laser-induced plasmas is the interaction between the plasma plume and the surrounding gas. In the case of ambient air, the excitation of molecules of gas in the vicinity of irradiated organic samples by collision with excited species from ablated material inside the plasma leads to dissociation of O2 and N2 molecules from the air and subsequent chemical reactions. These reactions lead to the generation of new organic diatomic species in the plasma, such as –CN radicals.8 The rate of collisions in the system depends directly on the density of species in the plume, and therefore on the surrounding pressure. Atomic as well as molecular species produced after ablation due to interaction between plasma and ambient gas also can contribute to the total intensities of specific emission lines and bands.8,13 The interactions described, based on nanosecond laser pulse widths, may be significantly different when the laser pulse width shortens to the femtosecond regimen. In this work, the main aim was to understand the way that air pressure affects the kinetics of different excited species’ characteristics of organic aromatic compounds inside the generated plasma. A series of temporally and spatially resolved studies were performed to explore various plasma dynamics phenomena, including emission characteristics. The high-energetic nitro aromatic compound 2,4,6-trinitrotoluene (TNT), and pyrene, a polycyclic aromatic hydrocarbon in which oxygen or
0003-7028/14/6801-0033/0 Q 2014 Society for Applied Spectroscopy
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FIG. 1. Scheme of the experimental setup.
nitrogen is absent, were selected as model compounds for the comparative analysis because of their undeniable analytical interest and their suitable physical properties for plasma formation and emission signal acquisition.
EXPERIMENTAL The experimental arrangement used in this work is shown in Fig. 1. A quadrupled Nd:YAG laser (266 nm, 12 mJ, 5 ns pulse width) was used to irradiate the organic samples in solid phase placed into a stainless steel vacuum chamber as described previously.14 The laser beam, after passing a high-energy variable attenuator (model M-935-10, Newport) was focused with a quartz plane-convex lens (focal length, 200 mm) 20 mm above the surface of the sample (lens-to-sample distance, 180 mm), and the radiation emitted from the plasma plume was collected through the lateral quartz view port of the chamber by a 7.62 cm (3 in) quartz plane-convex focal lens (150 mm). After being reflected with a ultravioletcoated mirror placed at 458 to the collection path, the emission from the whole plasma was tightly focused (0.53) in the extreme of a quartz optical fiber (600 lm in diameter) placed orthogonal to the plume expansion direction and connected to the entrance slit of a 500 mm focal-length imaging spectrograph, equipped with an intensified charge-coupled device as a detector. For space-resolved experiments, the plasma image was magnified (33) to allow the selective collection of light from specific regions of the plasma plume. Acquisition conditions were fixed at a reading window of 1 ms, with a delay of 50 ns. For time-resolved experiments, the acquisition window was reduced to 10 ns, with a delay window of 10 ns. A dispersive grating of 600 grooves/mm that let a spectral range of 40 nm and a reciprocal linear dispersion of 0.19 nm/pixel was used for time-integrated
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FIG. 2. Images of TNT and pyrene plasmas at different pressure levels. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
plasma and for time-resolved and spatial-resolved analysis. A full width half-maximum of 0.8 nm was attained in the Ha line under the cited conditions, in agreement with a recent study.15 The chamber pressure was monitored at real time by a single-gauge Pirani system that allows a range control from 104 to 1000 mbar. Specific pressure values were obtained by means of controlled air leaks through a needle valve. Pyrene was acquired from Sigma-Aldrich (98%, Reference 185 515), whereas TNT was obtained from the Spanish Ministerio de Defensa as small flakes. The bulk samples were prepared by compressing powders of pure compounds using a hydraulic press (15 min; 2 tons) to pellets of 10 mm in diameter and 2 mm in thickness. The samples then were fixed onto an aluminum holder and introduced in the analysis chamber by a load-lock device equipped with a three-dimensional micrometer for fine control of the sample position.
RESULTS AND DISCUSSION Figure 2 shows a sequence of time-integrated pictures (exposition time, 250 ms) from TNT and pyrene plasmas at different pressure values from 1000 to 103 mbar
FIG. 3. Normalized intensities of main emitting species depending on the air pressure level for (top) pyrene and (bottom) TNT. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
formed at a fluence value of 12.7 J/cm2. The lower dimensions of the TNT plasmas compared with the pyrene plumes is due to the lower consistency of the TNT pellets because its explosive nature impairs the use of excessive pressure in the hydraulic press. A gray rectangle shows the position and dimensions of the pellets in the sample holder, and a scale has been inset to observe the expansion processes taking place. In general, as pressure becomes lower, the plasma in both cases presents a larger volume due to the reduced density of the surrounding gas. The physical characteristics of the plasmas also change. The figures reveal that the brighter portion of the plasma is located near the surface. However, as the pressure decreases, two welldefined regions appear: one region close to the surface, with little modification in its dimensions; and a second region that expands in axial and radial directions, in accordance with a previous report.16 Figure 3 shows the variation in the intensity of different atomic and ionic lines and molecular bands with the
increase of air pressure in pyrene (Fig. 3, top) and TNT (Fig. 3, bottom). The brighter portion of the plasma close to the surface was collected on the fiber optic. The different species present in the spectra will be related to the elemental composition of the sampled compounds (e.g., H, C, N, and O in TNT; H and C in pyrene) or to the contribution of molecules and atoms of the surrounding atmosphere. The characteristics of the plasma in terms of electron temperature and density are not expected to be significantly different in the region close to the surface under identical excitation conditions. Signals for each species of interest were taken from the emission spectra recorded under the different experimental conditions and were normalized individually. Thus, it is possible to compare the evolution of each species as a function of pressure, despite the differences in spectral response of the detector or concentration. The data correspond to integration times of 1 ms. At atmospheric pressure, both spectra show intense signals from molecular bands related to the presence of C¼C bonds due to the breakage of the aromatic rings (C2 bands). The presence of CN bands can be due to (as in TNT) the recombination of released C species with N2 in the surrounding atmosphere. In this case, as the pressure decreases, there must be an extinction of CN due to the lower number of collisions with the surrounding atmosphere. In pyrene, an aromatic molecule without O and N atoms in its structure, the LIBS measurements at pressures below 0.01 mbar only provide emission lines from C, H, and a very weak contribution of C2. No evidence of CN bands is observed. As the pressure rises and the atmosphere gets richer in N2 and O2, emissions due to N(I) and O(I) increase, reaching a maximum at 1 mbar, coinciding with the largest volume and brightness of the expanding plume at this pressure value. The significant presence of CN emission from 0.1 mbar upward is due to different recombination reactions in the plasma plume immediately after the increase of nitrogen emission. The rise of the population of excited CN fragments in the plume occurs in parallel to the increasing abundance of C2. The fact that both molecular species follow an exponential decay fit suggests that the main routes of formation of such species are directly related. In the case of TNT, at high vacuum we can observe emission signals corresponding to native N and O atoms coming from the nitro substituents. Apart from this observation, the behavior of the emission species with the increase in air pressure is quite similar to that of pyrene, suggesting that the dynamics of expansion of plasma and the collision processes in the plume effect are mainly governed by the elemental composition of the molecule and the density and characteristics of the background gas. Although the molecular emissions kinetics in the laser ablation of both molecules follows a similar trend with pressure variation, the C2 net signal was much larger for pyrene than for TNT due to the direct dependence of this type of emission with the degree of aromaticity of the molecule.17,18 Time-resolved studies of the emitted species represent a valuable tool to determine differences between native species directly released from the molecule or generated after recombination with species from the
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FIG. 4. Time-resolved studies for CN. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2. Pressure was 10 mbar.
atmosphere or released from the sample. In principle, species generated by recombination should appear at longer times. CN represents an interesting species to follow, because it is present in the emission spectra of TNT and pyrene in a wide range of pressures. Because N is absent in pyrene, a time-resolved experiment was performed to follow the CN signal. Figure 4 shows the temporal evolution of CN (Dt ¼ 0) in the pyrene and TNT expanding plasma at 10 mbar. Gate widths of 10 ns and delay windows of 20 ns were used to obtain the temporal profile of the emission. Normalized intensities have been plotted for better visualization of the curves. The signals were recorded from the plasma region closer to the sample surface, where the ablation processes are primarily influenced by the composition of the ablated plume and the surrounding gas. Line intensities detected from different zones of the plasma at different delays from the laser pulse depend on the distribution and temporal evolution of the plasma temperature and electron density. As determination of these parameters falls out of the scope of the present work, it should be noted that the comparative results for the different compounds obtained here refer to the emission intensities and can only be qualitatively related to the number densities of the emitting species. The existence of an internal source of N in TNT is expected to modify the emission. As observed, the maximum intensity for CN emission occurs in TNT at earlier times than in pyrene. In contrast, the CN emission in pyrene lasts longer. The results for pyrene are in accordance with a two-body recombination process (C þ N ! CN), where the source of N is external, and the reaction takes a time to be completed. Some temporal differences may also be found between ionic, atomic, and molecular species. Detection of ionic emission lines demands very low pressures (,102 mbar) and prompt detection (short delay times) due to their ephemeral cycle of excitation– ionization–relaxation in the emission process. This fact is quite evident in Fig. 5, where the temporal evolution of C(I), C(II), and C2 (Dt ¼ 0) at 103 mbar of pyrene is shown as a function of plasma lifetime. Similar results were obtained for TNT. Time parameters (gate width and
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FIG. 5. Time-resolved study for C(I), C(II), and C2 on pyrene sample at 103 mbar. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
delay time) are shown as in Fig. 4: There is a sequence in the appearance of the different species, where the C(II) emission shows up early, followed by C(I) and C2 emissions. Even at low pressure conditions, the different dissociation and recombination pathways involved in the formation of diatomic species are significant to extend their lifetime way beyond the one for ionic and atomic species. To check the differences in the spectra as the plasma expands, the collection fiber optic was attached to a threedimensional micrometer stage to select the emission from specific zones of the plasma (next to sample surface area, plasma core, and peripheral zone). The distribution of species along the plume in the expansion direction was clearly observed using the orthogonal collection system. Figure 6 shows the results obtained for pyrene and TNT at three different distances from the sample surface (0, 2.5, and 5 mm) at 1 mbar in two selected regions of interest: 320–440 and 540–900 nm. Signals from atomic species (H, N, and O) were detected at higher intensity near the target surface, whereas the emissions from excited molecules (C2, CH, and CN) were placed along the entire plume. Measurements beyond 10 mm in length exhibited no trace of molecular emission bands. In principle, a plausible and nonspeculative explanation may consider that in the first mm of the plasma (regardless if it is TNT or pyrene), the high electron density and electronic temperature allows full bond breakage, explaining the larger presence of atomic species, whereas as the plasma expands, molecular species are formed by reaction and recombination processes. Such an assumption is somehow supported by theoretical papers where it is demonstrated the almost constant plasma temperature and front expansion during the first millimeter of the plasma as long as a certain base pressure has been reached.19
CONCLUSIONS The influence of pressure level on the dynamics of plasma expansion and on the interaction between excited plasma and gas molecules has been demonstrated for TNT and pyrene. Temporally and spatially
FIG. 6. Spatially resolved LIBS spectra of plasma plumes of (a) pyrene and (b) TNT at different distances along their expansion axis. To help in the visualization, the spectra are shown in two regions: 320–440 nm (left) and 540–900 nm (right). Each spectrum corresponds to the sum of 50 consecutive shots. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2. Pressure was 1 mbar.
resolved studies have been carried out as tool for characterization of the dynamics and evolution of emission species in the plasma. Results indicate that pressure level becomes a critical parameter in LIBS trials of these model organic compounds. Consequently, the efficient control of this variable can enhance to a large extent the features of generated emission signals used in the analysis and distinction of organics materials. Changes in density of the ambient atmosphere have a strong influence on the dynamics of plasma expansion and the subsequent optical spectra. The effect of air on LIBS signal began to be evident from 0.01 mbar upward, whereas at 1 mbar the plume reached the maximum volume and brightness level. This fact is in agreement with the corresponding emission signals, because at intermediate pressures, the spectra exhibit the highest intensity for atomic nitrogen and oxygen species, thus evidencing chemical activity between ablated material and excited air in the plume. As for molecular fragments, C2 emission is strongly linked to molecular structure and come mainly from molecular fragmentation, whereas –CN radicals are mostly produced by chemical reactions and recombina-
tion processes. In fact, spatially resolved studies of pyrene shown an appreciable CN emission in the core of plasma due to the interaction of native species with dissociated N2. The LIBS spectrum of pyrene in the highvacuum regime shows no evidence of CN emission. However, the presence of CN bands in TNT under these conditions suggests the production of excited molecular fragments by means of direct fragmentation and interaction between native species from the organic molecule. ACKNOWLEDGMENTS This research has been performed using funds from the Ministerio de Economı´ a y Competitividad (CTQ2011-24433) and from the Consejerı´ a de Innovacio´n, Ciencia y Empresa de la Junta de Andalucı´ a (Project P07-FQM-03308). TD thanks the Consejerı´ a de Innovacio´n for his research contract. 1. K. Kagawa, S. Yokoi. ‘‘Application of the N2 Laser to Laser Microprobe Spectrochemical Analysis’’. Spectrochim. Acta, Part B. 1982. 37(9): 789-795. 2. Y. Iida. ‘‘Effects of Atmosphere on Laser Vaporization and Excitation Processes of Solid Samples’’. Spectrochim. Acta, Part B. 1990. 45(12): 1353-1367.
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3. Y.I. Lee, K. Song, H.K. Cha, J.M. Lee, M.C. Park, G.H. Lee. ‘‘Influence of Atmosphere and Irradiation Wavelength on Copper Plasma Emission Induced by Excimer and Q-switched Nd:YAG Laser Ablation’’. Appl. Spectrosc. 1997. 51(7): 959-964. 4. A.W. Miziolek, V. Palleschi, I. Schechter. Laser Induced Breakdown Spectroscopy. New York, NY: Cambridge University Press, 2006. 5. S. Acquaviva, M.L. De Giorgi. ‘‘Study of Kinetics of Atomic Carbon During Laser Ablation of Graphite in Nitrogen by Time- and SpaceResolved Emission Spectroscopy’’. Appl. Surf. Sci. 2002. 186: 329334. 6. S. Acquaviva, M.L. De Giorgi. ‘‘Temporal and Spatial Analysis of Plasmas During Graphite Laser Ablation in Low-Pressure N2 00 ’’. Appl. Surf. Sci. 2002. 197-198: 21-26. 7. M. Baudelet, M. Boueri, J. Yu, S.S. Mao, V. Piscitelli, X. Mao, R.E. Russo. ‘‘Time-Resolved Ultraviolet Laser-Induced Breakdown Spectroscopy for Organic Material Analysis’’. Spectrochim. Acta, Part B. 2007. 62(12): 1329-1334. 8. W.Q. Lei, Q.L. Ma, V. Motto-Ros, X.S. Bai, L.J. Zheng, H.P. Zeng, J. Yu. ‘‘Effect of Ablation Photon Energy on the Distribution of Molecular Species in Laser-Induced Plasma from Polymer in Air’’. Spectrochim. Acta, Part B. 2012. 73: 7-12. 9. A. Kushwaha, R.K. Thareja. ‘‘Dynamics of Laser-Ablated Carbon Plasma: Formation of C2 and CN’’. Appl. Opt. 2008. 47: G65-G71. 10. E. Vors, C. Gallou, L. Salmon. ‘‘Laser-Induced Breakdown Spectroscopy of Carbon in Helium and Nitrogen at High Pressure’’. Spectrochim. Acta, Part B. 2008. 63(10): 1198-1204.
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11. S.J. Choi, J.J. Yoh. ‘‘Laser-Induced Plasma Peculiarity at Low Pressures from the Elemental Lifetime Perspective’’. Opt. Express. 2011. 19(23): 23097-23103. 12. L. Mercadier, J. Hermann, C. Grisolia, A. Semerok. ‘‘Plume Segregation Observed in Hydrogen and Deuterium Containing Plasmas Produced by Laser Ablation of Carbon Fiber Tiles from a Fusion Reactor’’. Spectrochim. Acta, Part B. 2010. 65(8): 715-720. 13. P. Lucena, A. Don˜a, L.M. Tobaria, J.J. Laserna. ‘‘New Challenges and Insights in the Detection and Spectral Identification of Organic Explosives by Laser Induced Breakdown Spectroscopy’’. Spectrochim. Acta, Part B. 2011. 66(1): 12-20. 14. J.F. Alca´ntara., J.M. Vadillo, J.J. Laserna. ‘‘Subthreshold Two-Pulse Time-Delayed Laser Ionization of Cu’’. Appl. Phys. A. 2008. 92: 963967. 15. C.G. Parigger. ‘‘Atomic and Molecular Emissions in Laser-Induced Breakdown Spectroscopy’’. Spectrochim. Acta, Part B. 2013. 79-80: 4-16. 16. N. Glumac, G. Elliott. ‘‘The Effect of Ambient Pressure on LaserInduced Plasmas in Air’’. Opt. Lasers Eng. 2007. 45(1): 27-35. 17. A. Portnov, S. Rosenwaks, I. Bar. ‘‘Emission Following LaserInduced Breakdown Spectroscopy of Organic Compounds in Ambient Air’’. Appl. Opt. 2003. 42(15): 2835-2842. 18. A. Portnov, S. Rosenwaks, I. Bar. ‘‘Identification of Organic Compounds in Ambient Air via Characteristic Emission Following Laser Ablation’’. J. Luminesc. 2003. 102-103: 408-413. 19. J.K. Antony, G.S. Jatana, N.J. Vasa, V.L.N. Sridhar Raja, A.S. Laxmiprasad. ‘‘Modeling of Laser Induced Breakdown Spectroscopy for Very Low-Pressure Conditions’’. Appl. Phys. A. 2010. 101: 161-165.
The influence of the ambient atmosphere on the dynamics of plasma expansion, besides the interaction between excited plasma and gas molecules, has been studied for specific organic aromatic compounds. To analyze the influence of air on the formation pathways of atomic and molecular species inside the plasma plume, the spectral emissions in laser-induced breakdown spectroscopy (LIBS) of 2,4,6-trinitrotoluene (TNT) and pyrene were compared at different pressure environments, from high vacuum to atmospheric pressure. Pelletized samples of the compounds were introduced in a vacuum chamber for excitation with the fourth harmonic output of an Nd : YAG laser (266 nm). The optical emission signal was collected with an optical fiber connected to a spectrograph fitted with a intensified charge-coupled device detector. Results from LIBS spectra indicate that changes in pressure level affect the kinetics of the characteristic excited species and their spatial distribution inside the plasma plume. Index Headings: Laser-induced plasmas; 2,4,6-trinitrotoluene; TNT; Laser ablation; Explosive compounds.
INTRODUCTION The analysis of organics by laser-induced breakdown spectroscopy (LIBS) has certain drawbacks compared with samples of an inorganic nature. The interference effects involving recombination of species and their interaction with the atmosphere must be taken into account. A large body of knowledge has been coined since the early years of LIBS concerning the influence of pressure and the nature of atmosphere in the dynamics of generation and evolution of plasma using emission spectroscopy. Time-resolved studies have been applied historically for nominal nanosecond LIBS, and a strong dependence on the ambient pressure was deduced from measurements of the emission intensity as a function of delay times. Although much research has been focused on metallic or ceramic samples,1–3 probably due to the extraordinary capability of LIBS to solve real industrial problems, LIBS may be found in almost any combination of matrices and elements.4 The dynamics of carbon or carbon-containing species also has been subject of study,5,6 probably due to the driving force of understanding the processes behind the deposition of diamond-like carbon or carbon nitrides by pulsed laser deposition. In recent years, a growing interest in extracting as much molecular information as possible from LIBS Received 28 May 2013; accepted 3 September 2013. * Author to whom correspondence should be sent. E-mail: laserna@ uma.es. DOI: 10.1366/13-07164
Volume 68, Number 1, 2014
experiments of organics has emerged.7,8 Due to the presence of H, O, and N in the surrounding atmosphere as well as on the analyzed compounds, temporally resolved studies have been used to identify native species coming from an organic sample or as a result of reactions between the ambient air and the excited material in the plasma. The different origins of emitters resulted in different kinetic behaviors. Recently, other studies concerning the influence of ambient pressure on the kinetics of excited species in laser-induced plasmas from ablation of carbons and organic compounds are emerging.9–13 Laser-induced plasmas constitute a spatially inhomogeneous and transient system, so temporally and spatially resolved spectroscopy stands as an efficient tool to carry out this type of investigations to ascertain the plasma expansion dynamics and to study the physical properties of its emission. As noted, pressure is a crucial factor in the generation of plasma because it has a deciding effect on many areas of study of plasmas, from its reactivity and the rate of collisions inside the plume to the different expansion velocities, including the morphology of the plume and the spectral characteristics of the emitted light. The key of the alterations undergone by laser-induced plasmas is the interaction between the plasma plume and the surrounding gas. In the case of ambient air, the excitation of molecules of gas in the vicinity of irradiated organic samples by collision with excited species from ablated material inside the plasma leads to dissociation of O2 and N2 molecules from the air and subsequent chemical reactions. These reactions lead to the generation of new organic diatomic species in the plasma, such as –CN radicals.8 The rate of collisions in the system depends directly on the density of species in the plume, and therefore on the surrounding pressure. Atomic as well as molecular species produced after ablation due to interaction between plasma and ambient gas also can contribute to the total intensities of specific emission lines and bands.8,13 The interactions described, based on nanosecond laser pulse widths, may be significantly different when the laser pulse width shortens to the femtosecond regimen. In this work, the main aim was to understand the way that air pressure affects the kinetics of different excited species’ characteristics of organic aromatic compounds inside the generated plasma. A series of temporally and spatially resolved studies were performed to explore various plasma dynamics phenomena, including emission characteristics. The high-energetic nitro aromatic compound 2,4,6-trinitrotoluene (TNT), and pyrene, a polycyclic aromatic hydrocarbon in which oxygen or
0003-7028/14/6801-0033/0 Q 2014 Society for Applied Spectroscopy
APPLIED SPECTROSCOPY
33
FIG. 1. Scheme of the experimental setup.
nitrogen is absent, were selected as model compounds for the comparative analysis because of their undeniable analytical interest and their suitable physical properties for plasma formation and emission signal acquisition.
EXPERIMENTAL The experimental arrangement used in this work is shown in Fig. 1. A quadrupled Nd:YAG laser (266 nm, 12 mJ, 5 ns pulse width) was used to irradiate the organic samples in solid phase placed into a stainless steel vacuum chamber as described previously.14 The laser beam, after passing a high-energy variable attenuator (model M-935-10, Newport) was focused with a quartz plane-convex lens (focal length, 200 mm) 20 mm above the surface of the sample (lens-to-sample distance, 180 mm), and the radiation emitted from the plasma plume was collected through the lateral quartz view port of the chamber by a 7.62 cm (3 in) quartz plane-convex focal lens (150 mm). After being reflected with a ultravioletcoated mirror placed at 458 to the collection path, the emission from the whole plasma was tightly focused (0.53) in the extreme of a quartz optical fiber (600 lm in diameter) placed orthogonal to the plume expansion direction and connected to the entrance slit of a 500 mm focal-length imaging spectrograph, equipped with an intensified charge-coupled device as a detector. For space-resolved experiments, the plasma image was magnified (33) to allow the selective collection of light from specific regions of the plasma plume. Acquisition conditions were fixed at a reading window of 1 ms, with a delay of 50 ns. For time-resolved experiments, the acquisition window was reduced to 10 ns, with a delay window of 10 ns. A dispersive grating of 600 grooves/mm that let a spectral range of 40 nm and a reciprocal linear dispersion of 0.19 nm/pixel was used for time-integrated
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FIG. 2. Images of TNT and pyrene plasmas at different pressure levels. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
plasma and for time-resolved and spatial-resolved analysis. A full width half-maximum of 0.8 nm was attained in the Ha line under the cited conditions, in agreement with a recent study.15 The chamber pressure was monitored at real time by a single-gauge Pirani system that allows a range control from 104 to 1000 mbar. Specific pressure values were obtained by means of controlled air leaks through a needle valve. Pyrene was acquired from Sigma-Aldrich (98%, Reference 185 515), whereas TNT was obtained from the Spanish Ministerio de Defensa as small flakes. The bulk samples were prepared by compressing powders of pure compounds using a hydraulic press (15 min; 2 tons) to pellets of 10 mm in diameter and 2 mm in thickness. The samples then were fixed onto an aluminum holder and introduced in the analysis chamber by a load-lock device equipped with a three-dimensional micrometer for fine control of the sample position.
RESULTS AND DISCUSSION Figure 2 shows a sequence of time-integrated pictures (exposition time, 250 ms) from TNT and pyrene plasmas at different pressure values from 1000 to 103 mbar
FIG. 3. Normalized intensities of main emitting species depending on the air pressure level for (top) pyrene and (bottom) TNT. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
formed at a fluence value of 12.7 J/cm2. The lower dimensions of the TNT plasmas compared with the pyrene plumes is due to the lower consistency of the TNT pellets because its explosive nature impairs the use of excessive pressure in the hydraulic press. A gray rectangle shows the position and dimensions of the pellets in the sample holder, and a scale has been inset to observe the expansion processes taking place. In general, as pressure becomes lower, the plasma in both cases presents a larger volume due to the reduced density of the surrounding gas. The physical characteristics of the plasmas also change. The figures reveal that the brighter portion of the plasma is located near the surface. However, as the pressure decreases, two welldefined regions appear: one region close to the surface, with little modification in its dimensions; and a second region that expands in axial and radial directions, in accordance with a previous report.16 Figure 3 shows the variation in the intensity of different atomic and ionic lines and molecular bands with the
increase of air pressure in pyrene (Fig. 3, top) and TNT (Fig. 3, bottom). The brighter portion of the plasma close to the surface was collected on the fiber optic. The different species present in the spectra will be related to the elemental composition of the sampled compounds (e.g., H, C, N, and O in TNT; H and C in pyrene) or to the contribution of molecules and atoms of the surrounding atmosphere. The characteristics of the plasma in terms of electron temperature and density are not expected to be significantly different in the region close to the surface under identical excitation conditions. Signals for each species of interest were taken from the emission spectra recorded under the different experimental conditions and were normalized individually. Thus, it is possible to compare the evolution of each species as a function of pressure, despite the differences in spectral response of the detector or concentration. The data correspond to integration times of 1 ms. At atmospheric pressure, both spectra show intense signals from molecular bands related to the presence of C¼C bonds due to the breakage of the aromatic rings (C2 bands). The presence of CN bands can be due to (as in TNT) the recombination of released C species with N2 in the surrounding atmosphere. In this case, as the pressure decreases, there must be an extinction of CN due to the lower number of collisions with the surrounding atmosphere. In pyrene, an aromatic molecule without O and N atoms in its structure, the LIBS measurements at pressures below 0.01 mbar only provide emission lines from C, H, and a very weak contribution of C2. No evidence of CN bands is observed. As the pressure rises and the atmosphere gets richer in N2 and O2, emissions due to N(I) and O(I) increase, reaching a maximum at 1 mbar, coinciding with the largest volume and brightness of the expanding plume at this pressure value. The significant presence of CN emission from 0.1 mbar upward is due to different recombination reactions in the plasma plume immediately after the increase of nitrogen emission. The rise of the population of excited CN fragments in the plume occurs in parallel to the increasing abundance of C2. The fact that both molecular species follow an exponential decay fit suggests that the main routes of formation of such species are directly related. In the case of TNT, at high vacuum we can observe emission signals corresponding to native N and O atoms coming from the nitro substituents. Apart from this observation, the behavior of the emission species with the increase in air pressure is quite similar to that of pyrene, suggesting that the dynamics of expansion of plasma and the collision processes in the plume effect are mainly governed by the elemental composition of the molecule and the density and characteristics of the background gas. Although the molecular emissions kinetics in the laser ablation of both molecules follows a similar trend with pressure variation, the C2 net signal was much larger for pyrene than for TNT due to the direct dependence of this type of emission with the degree of aromaticity of the molecule.17,18 Time-resolved studies of the emitted species represent a valuable tool to determine differences between native species directly released from the molecule or generated after recombination with species from the
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FIG. 4. Time-resolved studies for CN. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2. Pressure was 10 mbar.
atmosphere or released from the sample. In principle, species generated by recombination should appear at longer times. CN represents an interesting species to follow, because it is present in the emission spectra of TNT and pyrene in a wide range of pressures. Because N is absent in pyrene, a time-resolved experiment was performed to follow the CN signal. Figure 4 shows the temporal evolution of CN (Dt ¼ 0) in the pyrene and TNT expanding plasma at 10 mbar. Gate widths of 10 ns and delay windows of 20 ns were used to obtain the temporal profile of the emission. Normalized intensities have been plotted for better visualization of the curves. The signals were recorded from the plasma region closer to the sample surface, where the ablation processes are primarily influenced by the composition of the ablated plume and the surrounding gas. Line intensities detected from different zones of the plasma at different delays from the laser pulse depend on the distribution and temporal evolution of the plasma temperature and electron density. As determination of these parameters falls out of the scope of the present work, it should be noted that the comparative results for the different compounds obtained here refer to the emission intensities and can only be qualitatively related to the number densities of the emitting species. The existence of an internal source of N in TNT is expected to modify the emission. As observed, the maximum intensity for CN emission occurs in TNT at earlier times than in pyrene. In contrast, the CN emission in pyrene lasts longer. The results for pyrene are in accordance with a two-body recombination process (C þ N ! CN), where the source of N is external, and the reaction takes a time to be completed. Some temporal differences may also be found between ionic, atomic, and molecular species. Detection of ionic emission lines demands very low pressures (,102 mbar) and prompt detection (short delay times) due to their ephemeral cycle of excitation– ionization–relaxation in the emission process. This fact is quite evident in Fig. 5, where the temporal evolution of C(I), C(II), and C2 (Dt ¼ 0) at 103 mbar of pyrene is shown as a function of plasma lifetime. Similar results were obtained for TNT. Time parameters (gate width and
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FIG. 5. Time-resolved study for C(I), C(II), and C2 on pyrene sample at 103 mbar. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2.
delay time) are shown as in Fig. 4: There is a sequence in the appearance of the different species, where the C(II) emission shows up early, followed by C(I) and C2 emissions. Even at low pressure conditions, the different dissociation and recombination pathways involved in the formation of diatomic species are significant to extend their lifetime way beyond the one for ionic and atomic species. To check the differences in the spectra as the plasma expands, the collection fiber optic was attached to a threedimensional micrometer stage to select the emission from specific zones of the plasma (next to sample surface area, plasma core, and peripheral zone). The distribution of species along the plume in the expansion direction was clearly observed using the orthogonal collection system. Figure 6 shows the results obtained for pyrene and TNT at three different distances from the sample surface (0, 2.5, and 5 mm) at 1 mbar in two selected regions of interest: 320–440 and 540–900 nm. Signals from atomic species (H, N, and O) were detected at higher intensity near the target surface, whereas the emissions from excited molecules (C2, CH, and CN) were placed along the entire plume. Measurements beyond 10 mm in length exhibited no trace of molecular emission bands. In principle, a plausible and nonspeculative explanation may consider that in the first mm of the plasma (regardless if it is TNT or pyrene), the high electron density and electronic temperature allows full bond breakage, explaining the larger presence of atomic species, whereas as the plasma expands, molecular species are formed by reaction and recombination processes. Such an assumption is somehow supported by theoretical papers where it is demonstrated the almost constant plasma temperature and front expansion during the first millimeter of the plasma as long as a certain base pressure has been reached.19
CONCLUSIONS The influence of pressure level on the dynamics of plasma expansion and on the interaction between excited plasma and gas molecules has been demonstrated for TNT and pyrene. Temporally and spatially
FIG. 6. Spatially resolved LIBS spectra of plasma plumes of (a) pyrene and (b) TNT at different distances along their expansion axis. To help in the visualization, the spectra are shown in two regions: 320–440 nm (left) and 540–900 nm (right). Each spectrum corresponds to the sum of 50 consecutive shots. Experimental conditions were k 266 nm; 3.5 mJ/pulse; and 12.7 J/cm2. Pressure was 1 mbar.
resolved studies have been carried out as tool for characterization of the dynamics and evolution of emission species in the plasma. Results indicate that pressure level becomes a critical parameter in LIBS trials of these model organic compounds. Consequently, the efficient control of this variable can enhance to a large extent the features of generated emission signals used in the analysis and distinction of organics materials. Changes in density of the ambient atmosphere have a strong influence on the dynamics of plasma expansion and the subsequent optical spectra. The effect of air on LIBS signal began to be evident from 0.01 mbar upward, whereas at 1 mbar the plume reached the maximum volume and brightness level. This fact is in agreement with the corresponding emission signals, because at intermediate pressures, the spectra exhibit the highest intensity for atomic nitrogen and oxygen species, thus evidencing chemical activity between ablated material and excited air in the plume. As for molecular fragments, C2 emission is strongly linked to molecular structure and come mainly from molecular fragmentation, whereas –CN radicals are mostly produced by chemical reactions and recombina-
tion processes. In fact, spatially resolved studies of pyrene shown an appreciable CN emission in the core of plasma due to the interaction of native species with dissociated N2. The LIBS spectrum of pyrene in the highvacuum regime shows no evidence of CN emission. However, the presence of CN bands in TNT under these conditions suggests the production of excited molecular fragments by means of direct fragmentation and interaction between native species from the organic molecule. ACKNOWLEDGMENTS This research has been performed using funds from the Ministerio de Economı´ a y Competitividad (CTQ2011-24433) and from the Consejerı´ a de Innovacio´n, Ciencia y Empresa de la Junta de Andalucı´ a (Project P07-FQM-03308). TD thanks the Consejerı´ a de Innovacio´n for his research contract. 1. K. Kagawa, S. Yokoi. ‘‘Application of the N2 Laser to Laser Microprobe Spectrochemical Analysis’’. Spectrochim. Acta, Part B. 1982. 37(9): 789-795. 2. Y. Iida. ‘‘Effects of Atmosphere on Laser Vaporization and Excitation Processes of Solid Samples’’. Spectrochim. Acta, Part B. 1990. 45(12): 1353-1367.
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3. Y.I. Lee, K. Song, H.K. Cha, J.M. Lee, M.C. Park, G.H. Lee. ‘‘Influence of Atmosphere and Irradiation Wavelength on Copper Plasma Emission Induced by Excimer and Q-switched Nd:YAG Laser Ablation’’. Appl. Spectrosc. 1997. 51(7): 959-964. 4. A.W. Miziolek, V. Palleschi, I. Schechter. Laser Induced Breakdown Spectroscopy. New York, NY: Cambridge University Press, 2006. 5. S. Acquaviva, M.L. De Giorgi. ‘‘Study of Kinetics of Atomic Carbon During Laser Ablation of Graphite in Nitrogen by Time- and SpaceResolved Emission Spectroscopy’’. Appl. Surf. Sci. 2002. 186: 329334. 6. S. Acquaviva, M.L. De Giorgi. ‘‘Temporal and Spatial Analysis of Plasmas During Graphite Laser Ablation in Low-Pressure N2 00 ’’. Appl. Surf. Sci. 2002. 197-198: 21-26. 7. M. Baudelet, M. Boueri, J. Yu, S.S. Mao, V. Piscitelli, X. Mao, R.E. Russo. ‘‘Time-Resolved Ultraviolet Laser-Induced Breakdown Spectroscopy for Organic Material Analysis’’. Spectrochim. Acta, Part B. 2007. 62(12): 1329-1334. 8. W.Q. Lei, Q.L. Ma, V. Motto-Ros, X.S. Bai, L.J. Zheng, H.P. Zeng, J. Yu. ‘‘Effect of Ablation Photon Energy on the Distribution of Molecular Species in Laser-Induced Plasma from Polymer in Air’’. Spectrochim. Acta, Part B. 2012. 73: 7-12. 9. A. Kushwaha, R.K. Thareja. ‘‘Dynamics of Laser-Ablated Carbon Plasma: Formation of C2 and CN’’. Appl. Opt. 2008. 47: G65-G71. 10. E. Vors, C. Gallou, L. Salmon. ‘‘Laser-Induced Breakdown Spectroscopy of Carbon in Helium and Nitrogen at High Pressure’’. Spectrochim. Acta, Part B. 2008. 63(10): 1198-1204.
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11. S.J. Choi, J.J. Yoh. ‘‘Laser-Induced Plasma Peculiarity at Low Pressures from the Elemental Lifetime Perspective’’. Opt. Express. 2011. 19(23): 23097-23103. 12. L. Mercadier, J. Hermann, C. Grisolia, A. Semerok. ‘‘Plume Segregation Observed in Hydrogen and Deuterium Containing Plasmas Produced by Laser Ablation of Carbon Fiber Tiles from a Fusion Reactor’’. Spectrochim. Acta, Part B. 2010. 65(8): 715-720. 13. P. Lucena, A. Don˜a, L.M. Tobaria, J.J. Laserna. ‘‘New Challenges and Insights in the Detection and Spectral Identification of Organic Explosives by Laser Induced Breakdown Spectroscopy’’. Spectrochim. Acta, Part B. 2011. 66(1): 12-20. 14. J.F. Alca´ntara., J.M. Vadillo, J.J. Laserna. ‘‘Subthreshold Two-Pulse Time-Delayed Laser Ionization of Cu’’. Appl. Phys. A. 2008. 92: 963967. 15. C.G. Parigger. ‘‘Atomic and Molecular Emissions in Laser-Induced Breakdown Spectroscopy’’. Spectrochim. Acta, Part B. 2013. 79-80: 4-16. 16. N. Glumac, G. Elliott. ‘‘The Effect of Ambient Pressure on LaserInduced Plasmas in Air’’. Opt. Lasers Eng. 2007. 45(1): 27-35. 17. A. Portnov, S. Rosenwaks, I. Bar. ‘‘Emission Following LaserInduced Breakdown Spectroscopy of Organic Compounds in Ambient Air’’. Appl. Opt. 2003. 42(15): 2835-2842. 18. A. Portnov, S. Rosenwaks, I. Bar. ‘‘Identification of Organic Compounds in Ambient Air via Characteristic Emission Following Laser Ablation’’. J. Luminesc. 2003. 102-103: 408-413. 19. J.K. Antony, G.S. Jatana, N.J. Vasa, V.L.N. Sridhar Raja, A.S. Laxmiprasad. ‘‘Modeling of Laser Induced Breakdown Spectroscopy for Very Low-Pressure Conditions’’. Appl. Phys. A. 2010. 101: 161-165.
