Solid materials with different structure containing C and N were analyzed by laser-induced breakdown spectroscopy (LIBS). Comparing the emission molecular species in different atmosphere (air and argon), it can be determined that whether the molecular species are directly vaporized from sample or generated through dissociation or the interaction between plasma and air molecules. The results showed that the characteristic of C2 bands emission is similar with that of neutral atomic carbon emission CI in different atmosphere (air and argon). While the characteristic of CN bands emission is more complicated and it has great relationship with the existence of CN radicals, the interaction between plasma and air ambient, and the recombination of excited partials.
© 2011 OSA
In laser-induced breakdown spectroscopy (LIBS), plasma formed by a focused laser pulse can be used to determine the chemical species that constitute the sample [1,2]. Due to its versatility, this technique has been applied in many applications within variant disciplines. The development of LIBS portable systems for field applications is constantly increasing in recent years [3–5]. On-line and real-time analyses are also possible . However, extending this technique for analyzing non-metal elements still remains a challenging task. Only a few spectral lines of these elements are suitable with their detection and quantification. Most strong emission lines occur either in vacuum ultra-violet (VUV) or in near infra-red (NIR) spectral region. On the other hand, atmospheric gas is mainly composed of nitrogen and oxygen elements. Indeed, the process of laser ablation is complex and produced plasmas are characterized by different atomic, molecular, and cluster-like chemical species which are ablated directly from the irradiated target or produced by probable chemical reactions in gas-phase . Emission bands from molecules containing C2 in the Swan system and CN in the violet system can be identified at the same time from the laser-induced plasma spectroscopy with the atomic emission [8,9].
Recently, more researchers are concerned on molecular spectral signature. Sattman et al. identified polymers using C2 emission and C/H ratio , while Anzano et al. used C2 Swan system, CN band, carbon, hydrogen, oxygen, and nitrogen emission intensity ratios to identify plastics and found that the most important ratios were C2/C and H/C for the classification of polymer materials . Grégoire et al. used LIBS for polymer identification and they found that the C2 signal plays a crucial role in improving the separation of different polymers using line ratios, PCA, or PLS . Lucena et al. also pointed that molecular emission constitutes an important source of information for identification purposes and they used signal intensities of atomic, ionic, and molecular emission of different species in plasma to identify explosives with different organic compounds . Baudelet et al. used femtosecond LIBS to analyze Escherichia coli and obtained strong CN molecular bands with the direct ablation of native CN molecular bonds . It provided a valuable spectral signature to identify bacterium. They also used UV-LIBS to study different kinetic behaviors of CN bonds, which is vaporized from sample or recombined with ambient air for the analysis of organic samples, demonstrating the importance of controlling of laser fluence for the detection of native CN . St-Onge et al. pointed out that C2 is released from graphite and organic compounds containing aromatic rings, indicating that CN is formed from the reaction of C2 and atmospheric nitrogen . They also devised experiments to determine whether C2 emission could be related to the presence of double bonds in general . Dinescu et al. used excimer laser to ablate the graphite in nitrogen atmosphere, and their results showed that C2 and CN bands intensities have significant relationship with laser fluence, N2 pressure, and the position in plasma plume .
Most of the above work focused on organic or biological material identification using molecular emission from CN and C2. Compared to the analysis of metallic samples, the LIBS plasma formation process of non-metal elements is complex and the analysis of these elements under atmospheric pressure is much more sensitive to the interaction between plasma and ambient air, since such interaction leads to interfering emission for non-metal elements detection such as O, N, or CN, resulting from the dissociation of air molecular or the recombination between plasma and air .
Actually, non-metallic elements are presented in actual detection samples with different structure, such as the carbon in coal and fly ash, the nitrogen in coal and fertilizer [19–21]. It indicated that, even for the same element, the structures are different in the different materials. Our works are focused on characteristics of the sample structure and surrounding environment which are two important factors affecting LIBS spectrum. The experiments performed both in air and in argon atmospheres have been compared to evaluate the contribution of air in the spectrum. Several solid materials with different structure containing carbon and nitrogen were subjected to LIBS analysis in order to explore the influence of molecular structure and atmosphere on the characteristic of CN and C2 molecular band emission. Of special interest to us is the origin analysis of the molecular emission from CN and C2 and its correlation with atomic carbon line.
The schematic diagram of the setup used in this experiment is shown in Fig. 1 . A Q-switch Nd:YAG laser (Beamtech Optronics, E-lite 200) was applied as a radiation source. It was operated at its second harmonic wavelength of 532 nm, with 1-10 Hz repetition rate and maximum energy of 100 mJ/pulse. The laser beam was passed through an optical attenuator to reduce the pulse energy to 72 mJ and then focused on the sample surface with a lens of 100-mm focal length. The emission from the plasma plume was collected by a collimating lens at a 45°angle to the laser pulse and transferred to the spectrometer (Avantes, AvaSpec-2048FT) using a 2 m-long all–silica optical fiber bundle. The fiber optic spectrometer has eight channels where all spectra are taken simultaneously with the spectral resolution of 0.05–0.13 nm in the 175-1075 nm wavelength region and 2048 pixel CCD detector. The laser source with external trigger mode was triggered by a pulse generator which would put a TTL output with a 10 μs width to trigger spectrometer simultaneously. The rate of 1 Hz was used in order to evacuate splash particles produced by previous laser shots on the surface of the sample. The samples were fixed on a rotation stage to ensure that a fresh surface was available for each laser shot. All samples were analyzed as pellets and the pellets were analyzed immediately after the preparation to avoid the effect of humidity from air. In order to get the best signal-to-noise ratio, a delay time between the output triggered signal for the laser and the beginning scan of the spectrometer was set as 1417 ns with a gate width of 2 ms (which is the minimum value of the spectrometer).
A set of solid materials with different structure containing carbon and nitrogen were subjected to LIBS experiment. The molecular formula and chemical structure for the compounds investigated are shown in Fig. 2 . Measurements were performed both in air atmosphere and in argon flow. The gas flow with a flux of 15 l/min was directed to the sample surface adjusted to exclude the ambient air completely from the plasma plume.
3. Results and discussion
The plasma emission spectrum is well dominated by neutral atomic carbon, C2, and CN molecular bands emission. The common neutral atomic carbon emission lines presented in the plume have been identified based on NIST (National Institute for Standards and Technology) databases , as shown in Table 1 , and the molecular bands studied in the experiment are summarized in Table 2 . Comparing the excitation energy of neutral atomic carbon and molecular emission, it can be seen that the atomic carbon emission line is a factor of 2-3 larger than those for the diatomic species C2 and CN. Hence, molecular bands emission CN and C2 may be directly formed by fragmentation of the sample which contains diatomic structure like single bond C-C, double bond C = C, or C-N bond. We can easily observe them from LIBS spectrum together with atomic emission for laser ablation of the solid materials containing C and N.
Figure 3 presents the LIBS spectra for the solid materials containing C and N in different ambient (air and argon flow) in the range of 462–518 nm, which include the C2 emission of Swan system (d3∏g → a3∏u) of the sequence ( = 0, +1). The C2 molecular emission can be easily detected for all samples both in air and in argon flow except for urea, as shown in Fig. 3 c, which does not contain single bond C-C or double bond C = C. Some workers state that the recombination of carbon atoms C + C + M ↔ C2 + M is an important process forming C2 , while other workers argue that at longer delays the most important C2 formation process become the reaction of C with CN and CH: CN + C ↔ C2 + H, and C + CH ↔ C2 + H. the formation of C2 . The researchers have concluded the different conclusions about C2 formation process, which are mainly depending on the experiment condition. But the common in all the above reaction is that the ablated species should contain enough carbon ions. It can be expected that ablation of urea under our conditions (air and argon flow) does not produce enough excited carbon ions so that we cannot observe the C2 emission from the plasma. From that we also can conclude that the diatomic structure (single bond C-C or double bond C = C) presented in the sample would contribute to the observation of C2 molecular emission bands. It can be found that the molecular bonds emission intensity is larger in argon ambient than that in air ambient, which is similar to the neutral atomic carbon emission line intensity. Figure 4 presents the LIBS spectra for coal in different atmosphere (air and argon flow) in the range of 192–249nm, which include the neutral atomic carbon emission CI 193 nm and CI 247.8 nm. The above LIBS spectral range for the other samples show very similar characteristic and are not presented here. The ambient gas plays a major role in the chemistry in the plasma. Generally, in the buffer gas, such as argon, the larger rates of the production of emitting species will occur which would attribute the high electron concentration leading the enhancement of plasma emission intensity in an argon atmosphere [26–28].
Figure 5 shows the LIBS spectra for the solid materials containing C and N in different ambient (air and argon flow) in the range of 355–390 nm which include the CN molecular emission of violet system (B2∑+ → X2∑+) of the sequence ( = 0, + 1). As shown, the emission characteristic of molecular bands for CN in its violet system is very different from C2 in its Swan system and the generation mechanism of CN radicals in the plasma is more complicated. It has been pointed out that the formation of CN occurs through the four-center reaction C2 + N2 ↔ 2CN, where nitrogen comes from ambient air [14,16,29]. But recently, Ma et al. argue that the reaction of C and N2 appears to be responsible for the CN formation at the longer gate delays . The reaction C2 + N2 → 2CN is highly endothermic with activation energy 1.8 eV, and thus requires extreme heating that can be obtained either on the front of a forceful ablation shockwave or on the front of a plasma-shielding layer absorbing the laser beam energy. The graphite sample contains C but no N. The CN molecular band emission in air ambient can be easily identified, while is not detected in argon flow ambient, as shown in Fig. 5 a. While we use a long delay of 1417ns with very long integration for 2 ms, the plasma must be almost thermalized in our experiments. Hence, the formation of CN molecular bands emission for graphite in air ambient is completely corresponding to the reaction C + N2 ↔ CN + N induced by the interaction between plasma and ambient gas.
While for P-Aminobenzene sulfonic acid anhydrous, urea and coal samples, the CN emission can also be easily detected both in air and argon ambient, as shown in Figs. 5 b-d. However, the effect of atmosphere on the samples with different molecular structure is not the same, as the origin of the CN molecular bands emission is different in air ambient. The strongest emission line CN 388.3nm of the violet system was chosen to obtain the comparison of the line intensity ratios under air and argon flow condition for the samples which can be both detected by the CN molecular emission spectra in two atmospheres. The result is shown in Fig. 6 which demonstrates that the intensity ratios are very different, with the maximum of coal, followed by P-Aminobenzene sulfonic acid anhydrous and urea.
As discussed before, for urea sample, there are not enough excited carbon ions and the C2 molecular emission cannot be detected both in air and argon ambient, so that the CN emission completely comes from inherent CN radicals as intermolecular bonds for laser ablation of urea.
Therefore, the emission line CN 388.3 nm intensity is smaller in air ambient than that in argon ambient. It is similar to the characteristic of atomic carbon emission in different ambient (air and argon flow).While for P-Aminobenzene sulfonic acid anhydrous and coal samples, the formation of CN molecular emission is related to the CN radicals and the reaction C + N2 ↔ CN + N due to the existence of C-N bonds and the higher carbon clusters evaporated directly from the sample. The content of nitrogen in coal is lower than 1wt.%, resulting that the concentration of C-N bonds is very small. So the reaction of C and N2 will be responsible for the increasing CN concentration for the ablation of coal in air ambient. Therefore, the emission line CN 388.3nm intensity in air ambient is much larger than that in argon ambient and the line intensity ratio is greater than 1. Comparing with the coal sample, the structure of P-Aminobenzene sulfonic acid anhydrous is simple but with more C-N bonds due to higher content of nitrogen than that in coal so that the intensity of CN 388.3nm have little difference in air and argon ambient and the line intensity ratio is closer to 1.
In order to further investigate the formation mechanism of CN molecular emission, graphite which only contains C and ammonium dihydrogen phosphate (NH4H2PO4) which contains N were blended together. The mixture contains the element of C and N simultaneously, but it does not contain CN radicals as intermolecular bonds. The spectrum of CN emission of violet system in the range of 355–390nm for graphite, ammonium dihydrogen phosphate and the mixture of both in argon ambient is shown in Fig. 7 . The CN molecular spectrum emission can be identified only for laser ablation of the mixture material as the argon gas was used to exclude the ambient air completely from the plasma plume. It also can get the CN molecular emission for the sample which does not contain the C-N bonds. Graphite is an easily ionized matter and the enough carbon ions can be evaporated directly from the sample. This easily ionized element in the matrix would attribute to the first stages of plasma formation . Hence, the molecular emission CN are from the recombination between atomic C and N evaporated from the sample which does not contain C-N bonds.
Molecular spectrum emission of molecules (C2 in its Swan system and CN in its violet system) is presented in plasma produced during laser ablation of materials containing carbon and nitrogen with different structure. Through the comparative experiments performed under air and argon flow conditions, the formation mechanism of atomic carbon and molecular species is analyzed and compared. It indicated that the influence by surrounding environment on the atomic and molecular emission especially for the CN molecular bond is different. The inert gas (Ar) can enhance the emission intensities which are directly vaporized from the sample like atomic carbon and C2 molecular emission. The generation mechanism of CN molecular is relatively more complicated and the origin of CN molecular production for samples with different molecular structures is different. There are three main routes: (i) Reaction in plume of C with air surrounding leading to production of CN. (ii) Direct vaporization from the sample due to the existence of CN radicals as intermolecular bonds with low excitation energy. (iii) Recombination of C and N atoms from the compound in the plasma to produce CN. An understanding of underlying mechanism for atomic and molecular emission will provide the basis for the detection of nonmetallic elements in solid material by LIBS.
The authors are grateful for the financial support from National Natural Science Foundation of China (No. 50576029 and No. 51071069).
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