Femtosecond Laser-Induced Breakdown Spectroscopy Studies of Nitropyrazoles: Effect of Varying Nitro Groups E. Nageswara Rao, Sreedhar Sunku, S. Venugopal Rao* Advanced Centre of Research in High Energy Materials (ACRHEM) University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500046, Telangana, India
Abstract The technique of femtosecond laser-induced breakdown spectroscopy (FLIBS) was employed to investigate seven explosive molecules of Nitropyrazole in three different atmospheres of ambient air, nitrogen, and argon. FLIBS data illustrated the presence of molecular emissions of CN Violet bands, C2 Swan bands along with atomic emission lines of C, H, O and N. To understand the plasma dynamics, the decay times of molecular and atomic emissions were determined from time-resolved spectral data obtained in three atmospheres of air, argon, and nitrogen. The CN decay time was observed to be the longest in air compared to nitrogen and argon atmospheres for the molecules PY and 4-NPY. In the case of C2 emission, the decay time was observed to be the longest in argon compared to the air and nitrogen environments for the molecules PY, 4-NPY, and 1M-3,4,5-TNPY. The intensities of CN, C2, C, H, O and N emission lines and various molecular/atomic intensity ratios such as CN/C2, CNsum/C2sum, CN/C, CNsum/C, C2/C, C2sum/C, (C2+C)/CN, [C2sum+C]/CNsum, O/H, O/N, and N/H were also deduced from the LIB spectra obtained in argon atmosphere. A correlation between the observed decay times and molecular emission intensities with respect to (a) number of nitro groups (b) atmospheric nitrogen content and (c) oxygen balance of molecules has been investigated. The relationship between (a) LIB signal intensity (b) the molecular/atomic intensity ratios and the oxygen balance of these organic explosives is also explored. Key words: Femtosecond LIBS, Nitropyrazoles, Temporal dynamics, Nitro groups, Intensity Ratios.
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Introduction Laser-induced breakdown spectroscopy (LIBS), an emerging tool for multi-elemental analysis, has specific advantages compared to other techniques such as Inductive Coupling Plasma Mass Spectrometry, Atomic Absorption Spectroscopy and Atomic Emission Spectroscopy mainly due to its capability for standoff detection.1-6 LIBS is a versatile and popular technique to detect traces in any form of solids, liquids and gases.4-7 LIBS is an advantageous technique compared to other types of elemental analysis because of its fast response, highly sensitivity, real-time detection.4 This technique has been successfully employed for discrimination and/or identification of biological materials,8-9 plastics,10 pharmaceuticals,11,12 bacteria,13,14 and energetic materials/explosives.15-21 The detection of explosive residues for security screening, environmental decontamination, demining and other applications relevant to homeland security is an active area of research and in this regard LIBS technique has been proved to be effective. LIBS technique has been regularly tested for deployment in the field of explosives detection. The stand-off detection capability of explosives has been accomplished up to 200 m distance.14 However, there are a number of challenges to be addressed and remedied before it can be implemented for practical explosives detection.4,20-22 The plasma is formed as a result of focused laser pulses on the sample target and emits radiation (light). The light emanating from plasma consists of molecular and atomic elemental signatures present in the sample and is usually resolved through a spectrometer with gating capability. Several atomic and molecular emissions transpire in different time scales of the plasma evolution. A thorough understanding of the progression dynamics of atomic and molecular species in the plasma will provide better opportunities to further explore and enhance the capabilities of this technique. There have been a large number of LIBS studies using ns laser pulses as an excitation source. However, 2
there are very limited reports on LIBS studies with femtosecond pulses in general, and that of high energy materials in particular.23-29 Femtosecond LIBS (FLIBS) offers the advantages of low breakdown threshold, minimal atmospheric interference, possibility of delivering pulses to very long distances through filamentation etc..10,30 Zhang et al.31 have studied ablation and ionization for elemental determination and have successfully demonstrated that the matrix effects were significantly reduced (by 50%) in the fs case compared to ns case. De Lucia et al.25 studied a series of organic polymers and RDX using femtosecond LIBS technique under the influence of argon. A correlation between the molecular structure and plasma emission was established by means of the percentages of atomic species such as C, H, N, O and the bond types (C-C, C=C, C-N, and CN) in combination with the atomic/molecular emission intensities and decay times. Time-resolved emission spectra were collected to estimate the life times of both atomic and molecular species and the differences in decay times observed were accredited to the alterations in (a) molecular structure of the organic polymers/RDX and (b) chemical reactions occurring in the plasma. These differences, they strongly argue, could potentially be exploited in the improvement of explosive residues discrimination with FLIBS data.25 Therefore, we strongly feel that the complete understanding of time-resolved molecular emissions in LIBS data is crucial, though challenging, for several applications in general, and explosives detection in particular.25 The LIB spectra typically comprise several atomic peaks of C, H, N, O and some molecular peaks such as CN, C2, CH and OH. Specifically, several researchers pointed out CN, C2 emissions are an effective signature which can help to predict the compound. Most organic explosives too contain C, H, O and N in their composition. It is of great interest to detect and understand the formation of molecular species CN, C2 and atomic fragments C, H, O, N and these studies have become been prevalent over the last decade.32-37 St-Onge et al.38 3
demonstrated that the formation of C2 and C released directly from the target of the graphite, and CN was formed later on by the interaction of C2 with atmospheric nitrogen (N2). Nishimura et al.39 established that large carbon molecular formation had been initiated with the ion collision followed by the formation of C2. The formation of CN molecular species also results from the reactions between native carbon atoms and nitrogen present in ambient air.40 The surrounding atmosphere, therefore, plays a major role during the plasma evolution. In the case of atmospheric absence (for example, in vacuum or in the presence of an inert gas like argon) the molecular formation will be purely from native radicals and recombination with sample constituents present in the plasma. The origin and routes of molecule production have not yet been completely understood in view of the complex nature of laser-induced plasma chemistry. Three main routes envisaged from detailed studies performed by Lucena et al.21 are (i) reaction of C in the plume with the surrounding air (ii) direct vaporization of CN radicals from the sample (iii) recombination of C and N atoms from the compound in the plasma. Furthermore, they established that the fragmentation seemed the dominant pathway for the production of C2 in aromatic compounds and in molecules containing carbon–carbon double bonds. Harilal et al.41 studied the time and space resolved spectroscopic analysis ofC2 species in the laser induced plasma produced from a high purity graphite target used 1.06 µm nanosecond radiation. At low laser fluences C2 intensity exhibited only single peak structure while beyond a threshold laser fluence a twin peak distribution in time was observed. They argue that the faster velocity component at higher laser fluences occurred due to species generated from recombination processes while the delayed peak was credited to the dissociation of higher carbon clusters resulting in C2 molecular formation.
Dong et al.42
studied several solid materials containing C and N and have determined that whether the molecular species are directly vaporized from sample or generated through the dissociation or the interaction between the plasma and air molecules. Their studies asserted that the inert gas 4
could enhance the emission intensities which are directly vaporized from the sample such as C and emission of molecular species C2.
CN molecular emission in graphite when ablated in low pressure nitrogen or in ambient air was studied by Vivien et al.43 They proposed that C2 is emitted from the target surface or formed in its vicinity, while CN formed in the periphery of the carbon vapor plume, principally through the C2+N2 2CN reaction.
Zelinger et al.44 and Fuge et al.45
monitored the CN and C2 molecules used spatial and time resolved spectroscopy of graphite target in nitrogen atmosphere. Babushok et al.46 studied RDX using LIBS technique and analyzed the reaction processes in the RDX plasma through kinetic modeling. They observed that the main generation reactions of excited states were electron-impact processes.
It was
also noticed that the evolution of C2 in the RDX plasma plume demonstrated double-peak behaviour. They expressed the possibility of explosives identification using unique ratios of atomic intensities. Ma et al.47 discussed a few possible reactions involved in the formation of C2 and CN molecular bands and also proposed that other chemical reactions contribute to formation of CN. They also concluded that the reaction of C with N2 was accountable for the increasing CN concentration at longer gate delays while the reaction of C with CO or CN was responsible for the increasing C2 concentration. Portnov et al.48-49 used the C2/CN and O/N intensity ratios and analyzed several organic compounds, aromatic nitro compounds and polycyclic aromatic hydrocarbons in ambient air. Park etal.50 has reported the time resolved optical emission studies and temporal properties of laser ablation of graphite in He, N2 and Ar background gases. Baudeletet al.51 performed detailed time resolved UV-LIBS studies to understand the mechanisms for detection and identification of native atomic or molecular species form the organic samples. Dagdigian et al.52 proposed a kinetic model to describe the
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formation of molecular emission and predicted the relative intensities of the atomic C/H/N/O emissions lines in the spectra.
Nitropyrazoles have been used as biologically active compounds including antibiotics or their analogues, agrochemicals, dyestuffs, phosphores, nonlinear optical materials and recently as energetic materials.53 Pyrazoles have been studied as the models of aromatic systems. In the present work we performed FLIBS studies of seven different pyrazole samples which differ in the number of nitro groups in their structure. These nitropyrazole compounds were synthesized in house. Our aim was to investigate the differences in molecular and atomic spectral emissions with increasing the number of nitro groups in their structure. We observed that the LIBS spectrum was affected by two important factors (a) characteristics of the sample structure and (b) the surrounding environment.
Utilizing simple ratiometric
method (wherein atomic and/or molecular peak intensity ratios were used) we analyzed the LIBS data for possible discrimination of these molecules. For the ratiometric analysis the data was recorded in air and argon atmospheres to evaluate the contribution of the atmosphere in the spectrum. The intensities of molecular CN [where CN refers to the most intense peak at 388.32 nm and CNsum refers to sum of all the 12 peaks), C2 [where C2 refers to the most
intense peak at 516.52 and C2sum refers to sum of all the 9 peaks), atomic emission lines of C, H, O and N were considered for ratios. The various molecular/atomic intensity ratios such as CN/C2, CNsum/C2sum, CN/C, CNsum/C, C2/C, C2sum/C, (C2+C)/CN, (C2sum+C)/CNsum, O/H, O/N, and N/H were calculated in argon atmosphere and ambient air. The formation of molecular species (especially CN and C2) has been observed to be effected by the surrounding atmosphere. Therefore, we have recorded the LIB spectra of all the samples in argon atmosphere to avoid the contribution from the atmospheric interaction. In the present work,
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time resolved spectroscopic investigation of the plasma induced by fs laser pulses has also been carried out to understand the nanosecond dynamics of various molecular species.
2. Experimental Setup The use of ultrafast lasers with pulse lengths in the tens of femtoseconds (fs) has, until recently, been limited to laboratory experiments.54 The introduction of self mode-locking in Ti:sapphire based lasers in 1991 brought simplicity of use and commercialization of ultrashort laser pulse technology such that 10 fs laser pulses can be routinely used in laser applications.55 Furthermore, the use of fs pulses in combustion diagnostics has been successfully demonstrated very recently by two different groups.56-57 Complete details of the experiments with fs pulses were reported in our previous works.24,29,57 FLIBS studies have been performed with Ti: Sapphire laser based system. The pulse duration was ~40 fs (measured at the sample) with a maximum energy of ~2.5 mJ (1 kHz, 800 nm). In all the FLIBS experiments the input laser pulses had a bandwidth of ~26 nm (FWHM). The fs laser pulses were focused on the target sample with 80 mm plano-convex lens. The estimated bean diameter at focus was ~12±3 m corresponding to a peak irradiance of ~2.5 TW/cm2. Typical pulse energies used in our experiments was ~1 mJ. The fs LIB spectra were recorded in air, argon and nitrogen atmospheres. An optical fiber was coupled to the spectrometer (Andor Technologies, Mechelle 5000). The spectrally resolved lines were detected by an intensified charged coupled device (ICCD) (Andor I star, DH 734). The ICCD camera was operated in the gated mode. The spectral resolution measured (FWHM) at 577 nm was ~0.11 nm using a 10 m slit. The spectral resolving power of the spectrometer R was ~5000. The gate delay and gate width were adjusted such that the LIB spectra were obtained with different gate widths and gate delays.
The samples were translated manually in X-Y
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directions. In the present case we had performed manual movement of the sample since (a) the sample quantity available was small and we could not make large sized pellets (a maximum of ~12 mm diameter was only possible) (b) we had limitations (in speed) with our translation stages and (c) 1 kHz pulses were incident on the samples (d) we observed that the pellets were not strong enough to withstand the laser pulses and particles were ejected from the surface forming a smoky region around the sample while translating using a motorized stage. Two separate sets of data were collected for one particular sample and we observed that the data/analysis were consistent implying that the manual sample movement was reliable. A collection lens system unit was placed to collect the light originating from the plasma and the light passed through a fiber optic cable and then transferred to gated ICCD spectrometer. The samples were prepared as 2-3 mm thick pellets. For preparing the pellets the sample powder was grounded in an agate motor and the resulting powder was pelletized using a hydraulic press under a base pressure of 4-6 tonnes.
Seven nitro group compounds, which differ in the number of nitro groups and possessing different number of C-C and C-N bonds, were used in the present study and they are a) pyrazole [PY, C3H4N2] b) 1-nitropyrazole [1-NPY, C3H3N3O2] c) 3-nitropyrazole [3NPY,C3H3N3O2] d) 4-nitropyrazole [4-NPY, C3H3N3O2] e) 1,3-dinitropyrazole [1,3-DNPY, C3H3N3O2] f) 3,4-dinitropyrazole [3,4-DNPY, C3H2N4O4] g) 1-methyl-3,4,5-trinitropyrazole [1M-3,4,5-TN-PY, C4H3N5O6]. Pyrazole has no nitro group and has two C-C bonds and two C-N bonds. 1-NPY, 3-NPY and 4-NPY have one nitro group, two C-C bonds and three C-N bonds. In the case of dinitropyrazoles, the number of nitro groups are 2, the number of C-C bonds are 2 while the number of C-N bonds are three for 1,3-DNPY and four for 3,4-DNPY. Finally, 1-M-3,4,5-TN-PY has largest number of nitro groups (three) compared to all the pyrazoles with the number of C-N bonds being six and C-C bonds being two. Pyrazoles were 8
nitrated with nitric acid-sulfuric acid, nitric acid-acetic anhydride, nitric acid-trifluoroacetic anhydride. The presence of nitro group in the pyrazole ring considerably enlarges the possibility of functionalization of various types of pyrazole derivatives. The methods to synthesize nitropyrazoles are diverse and depend upon the nature of substituent groups in the pyrazole ring, the electron density distribution in it, nitration mixtures, nitration conditions, etc. The synthesis of these compounds was performed to be able to predict the heat of explosion, density, detonation performance, stability and sensitivity. The molecular formulae and the chemical structure of these samples are summarized in table I.
3. Results and Discussion 3.1 Spectral features obtained with pyrazole samples
Figure 1 shows a typical spectrum of the sample PY recorded in argon and ambient air. We have observed several atomic (C, H, N and O) and molecular (CN and C2) spectral signatures in the spectral range of 200–900 nm.
CN Violet bands corresponding to
B2Σ+→X2Σ+transitions at 357–360 nm, 384–389 nm and 414–423 nm with values ofandrespectively. C2 Swan bands corresponding to D3Πg → a3Πu transitions at 460–475 nm, 510–520 nm and 550–565 nm with and , respectively, were also observed. The spectral range covering both CN and C2 peaks is illustrated clearly in figure 2. The complete assignment of each molecular peak and corresponding vibrational transitions are listed in table II. For detailed ratiometric analysis of 25 individual spectra were recorded for each sample in argon atmosphere and 4 spectra in air. All the spectra were obtained with a gate width of 800 ns and a gate delay of 100 ns. It is evident from the data presented in figure 1 that the peak intensity corresponding to CN band at 388.32 nm was stronger compared with C2peak intensity at 516.52 nm recorded in air whereas the C2 molecular peak 9
intensity was more prominent than CN peak in the spectra recorded in argon. Furthermore, C peak (247.88 nm) intensity was stronger in argon atmosphere compared to air atmosphere. The H peak (656.28 nm) intensity was stronger in argon atmosphere. The data clearly emphasizes the role of surrounding environment in the formation of atomic and molecular species such as C, CN, and C2. Figure 3 presents the FLIBS spectra of seven pyrazole samples recorded in argon atmosphere with similar experimental conditions. Several argon peaks were observed at 738.57 nm, 763.67 nm, 772.52 nm, 794.95 nm, 811.67 nm, 826.61 nm, 842.56 nm and 852.19 nm. The variations in molecular and atomic peak intensities for different pyrazole samples are evident from the data presented. C2 peak intensity at decreased from PY to 1M-3,4,5-TNPY and a similar behavior was observed for andtransitions. The H and O intensities were observed to be weak in all spectra. The data presented in figure 4 represents different atomic and molecular species intensities in the form of bar charts deduced from an average of 25 independent spectra for each sample in argon atmosphere. Here, CNsum represents the sum of twelve (12) peak intensities at 388.33 nm, 387.12 nm, 386.12 nm, 385.46 nm, 385.01 nm, 421.58 nm, 419.66 nm, 418.63 nm, 416.75 nm, 359.03 nm, 358.61 nm and 358.28 nm and C2sum represents the sum of nine (9) peak intensities at 516.52 nm, 512.88 nm, 563.48 nm, 558.51 nm, 554.01 nm, 473.66 nm, 471.47 nm, 469.70 nm and 468.48 nm.
The variation in spectral intensities was clearly observed in the
experimental data. Some of the important features summarized from this data are 1.
C2sum intensity decreased as the number of nitro groups increased from PY to 1M-
3,4,5-TNPY. C and N peak intensities increased from PY to 1M-3,4,5-TNPY. CNsum intensity decreased steadily from 1-NPY to1M-3,4,5-TNPY.
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2.
The CN intensity (at 388.32 nm) was observed to increase from PY to 1-NPY and it
was constant for the isomers (1-NPY,3-NPY and 4-NPY) followed by a decreasing trend for DNPY and TNPY. C2 band intensity at 516.52 nm decreased as the number of nitro groups increased. 3.
H peak (656.28 nm) intensity was nearly equal for PY to 4-NPY and later it increased
from dinitropyrazoles to 1M-3,4,5-TNPY (trinitropyrazole). O intensity (777.42 nm) increased from PY to 4-NPY but no trend was observed beyond that. 4.
The intensity ratios (atomic and molecular species) were analyzed with respect to the
C-C and C-N bonds. All the samples contain same number of C-C bonds but differ in number of C-N bonds. With increasing C-N bonds and constant number of C-C bonds the C2sum intensity was observed to decrease. Surprisingly, with increasing number of C-N bonds the CNsum intensity also decreased. This clearly suggests that formation of C2 and CN have dominant contributions other than from native bonds (C-C and C-N). In each of these molecules there is one C=N and the rest being C-N bonds. 5.
The samples 1-NPY, 3-NPY, and 4-NPY contain same number of nitro groups but 1-
NPY contains less C-N bonds (two) compared to other isomers (three). C2sum intensity decreased from 1-NPY to 3-NPY.
These three are structural isomers possessing same
molecular formula but have different physical and chemical properties. Similarly, 1,3-DNPY and 3,4-DNPY possess same number of nitro groups but differ in C-N bonds. However, we noticed a decreasing trend in the C2sum and CNsum intensities from 1,3-DNPY to 3,4-DNPY. 6.
C2sum was stronger in the pyrazole spectra when compared to CNsum where as in other
samples CNsum intensity was stronger compared to C2sum. The reason could be that in PY sample there is no nitro group and lower number of C-N bonds compared to other samples. 7.
C, N and O peaks also demonstrated an increment behavior with varying nitro groups
and C-N bonds. The increment in O and N intensity from PY to 1M-3,4,5-TNPY could be 11
due to the higher number of N and O atoms present in their composition. Our data clearly suggests that the molecular structure affected the formation of atomic and molecular radicals in the LIBS plasma. Relative efficiency correction (REC) was not performed for the data used for ratiometric analysis. Probably this could have resulted in a larger scattering in the data presented. The REC correction was erratic in our case for wavelengths
