Vibrational Emission Study of the CN and C2 in Nylon and ZnO/Nylon Polymer Using Laser-Induced Breakdown Spectroscopy (LIBS)

The laser-induced breakdown spectroscopy (LIBS) technique was performed on polymers to study the neutral and ionic emission lines along with the CN violet system (B2Σ+ to X2Σ+) and the C2 Swan system (d3 Пg–a3 Пu). For the laser-based emission analyses, the plasma was produced by focusing the laser beam of a Q-switched Nd: YAG laser (2ω) at an optical wavelength of 532 nm, 5 ns pulse width, and a repetition frequency of 10 Hz. The integration time of the detection system was fixed at 1–10 ms while the target sample was positioned in air ambiance. Two organic polymers were investigated in this work: nylon and nylon doped with ZnO. The molecular optical emission study of nylon and doped nylon polymer sample reveals CN and C2 molecular structures present in the polymer. The vibrational emission analysis of CN and C2 bands gives information about the molecular structure of polymers and dynamics influencing the excitation structures of the molecules. Besides, it was further investigated that the intensity of the molecular optical emission structure strongly depends on the electron number density (cm−3), excitation temperature (eV), and laser irradiance (W/cm2). These results suggest that LIBS is a reliable diagnostic technique for the study of polymers regarding their molecular structure, identification, and compositional analysis.


Introduction
Nylon is a polyamide polymer that is frequently used to make a variety of different types of attire products, such as lithium batteries [1], 3-D printed nylon memory composites [2], asleep bags, rope, seat belts in vehicles, parachuting fabric, tubing pipe, sheets, and dental floss [3]. Unlike other organic or semi-synthetic structures, nylon fibers are completely synthetic. Laser-induced breakdown spectroscopy (LIBS) has been developed as a rapid, reliable, non-destructive, real-time, and in situ detection analytical technique for the spectroscopic study of different organic, inorganic and explosive textiles [4][5][6][7]. LIBS can also be coupled with further spectroscopic techniques, for instance Raman spectroscopy and photoluminescence spectroscopy, to enhance the capacities of LIBS for organic composite analysis [8]. In LIBS, a high-power laser beam is focused through the convex lens on the target surface, which causes laser-matter interaction. This interaction causes high-density micro-plasma containing electrons, ions, and neutral ingredients. The light emitted from this hot plasma shows the spectral identifications of the atomic as well as molecular optical emissions that appear in the sample, which can be used for chemical analysis. For the last couple of years, LIBS has been concerned with polymer composite analyses using Nylon 6/6 doped with ZnO (ZnO/nylon) can be employed as a photocatalyst for the photo degradation of alizarin red dye [30].
In the present work, the LIBS spectra of organic materials such as pure nylon and nylon doped with ZnO (ZnO/nylon) have been studied including the vibrational emission of the C 2 and CN molecular optical bands. The obtained molecular optical emission is established on the optimized experimental parameters such as the integration time, laser pulse energy, and laser pulse irradiance. Laser irradiance performs a significant role in the vibrational transitions observed in the molecular optical emissions. Furthermore, the intensity variation of vibrational transitions and trend of plasma parameters has been investigated by varying laser irradiance for pure and ZnO/nylon polymers. The analysis of vibrational emission of CN (hetero-nuclear diatomic) and C 2 (homo-nuclear diatomic) molecules in polymers has revealed that LIBS is a fast and consistent diagnostic technique for the study of polymers regarding their molecular structure.

Materials and Methods
Two organic polymers were utilized in this study, namely: nylon, and ZnO/nylon. The molecular formula along with the 2D and 3D molecular structure of the nylon organic polymer is presented in Figure 1. Nylon (6/6) and ZnO was obtained from Sigma-Aldrich (USA). The composite ZnO/Nylon polymer film was prepared using solution method technique.
Polymers 2022, 14, x FOR PEER REVIEW 3 of 14 and inorganic properties of the elements along with their comparative chemical composition. Nylon 6/6 is useful as a potential substantial in several areas for diverse applications such as in lithium-ion batteries, antibacterial agents, and cement to enhance the properties [26][27][28]. Nylon 6/6 has extraordinary thermal stability, hydrophilicity and excessive tensile power [29]. Nylon 6/6 doped with ZnO (ZnO/nylon) can be employed as a photocatalyst for the photo degradation of alizarin red dye [30].
In the present work, the LIBS spectra of organic materials such as pure nylon and nylon doped with ZnO (ZnO/nylon) have been studied including the vibrational emission of the C2 and CN molecular optical bands. The obtained molecular optical emission is established on the optimized experimental parameters such as the integration time, laser pulse energy, and laser pulse irradiance. Laser irradiance performs a significant role in the vibrational transitions observed in the molecular optical emissions. Furthermore, the intensity variation of vibrational transitions and trend of plasma parameters has been investigated by varying laser irradiance for pure and ZnO/nylon polymers. The analysis of vibrational emission of CN (hetero-nuclear diatomic) and C2 (homo-nuclear diatomic) molecules in polymers has revealed that LIBS is a fast and consistent diagnostic technique for the study of polymers regarding their molecular structure.

Materials and Methods
Two organic polymers were utilized in this study, namely: nylon, and ZnO/nylon. The molecular formula along with the 2D and 3D molecular structure of the nylon organic polymer is presented in Figure 1. Nylon (6/6) and ZnO was obtained from Sigma-Aldrich (USA). The composite ZnO/Nylon polymer film was prepared using solution method technique.

Experimental Details
The details of the experimental system utilized to accomplish the optical emission of any sample have been described somewhere else [31][32][33]. In brief, a second harmonic (2ω), Q-switched Nd:YAG laser (Quantel (Brilliant-B), France), was used; it had a 532 nm wavelength and operated at a repetition frequency of 10 Hz with a 5 ns pulse width, and was

Experimental Details
The details of the experimental system utilized to accomplish the optical emission of any sample have been described somewhere else [31][32][33]. In brief, a second harmonic (2ω), Q-switched Nd:YAG laser (Quantel (Brilliant-B), France), was used; it had a 532 nm wavelength and operated at a repetition frequency of 10 Hz with a 5 ns pulse width, and was capable of delivering the pulse energy of about 200 mJ. The LIBS experiments were performed at room temperature in an airy ambiance. The laser beam was converged onto the sample surface, which was positioned in the air at atmospheric pressure using a convex (quartz) lens with a focal length of 10 cm. The samples were kept on a rotating circular stage with a rotation speed of~10 rpm to enable a clean surface for each fresh laser shot. The optical emission spectra were recorded using an optical fiber attached to a charged coupled devices (CCDs) array spectrometer with an optical wavelength range from 200 nm to 720 nm. An average of ten laser shots were employed to cleanse the surface of the target sample. The spectra were accomplished at an average of 20 laser pulse shots at different spots on the target surface. The averaged optical emission spectrum was then utilized to acquire the molecular optical emission of the polymer samples, which takes into account the target sample inhomogeneity and lowers the statistical inaccuracies.

Results and Discussion
Emission Studies of the CN Violet and C 2 Swan System LIBS has been a widely utilized analytical tool for the analysis of synthetic-organic materials such as teflon, epoxy, polystyrene, and nylon. Overall, all organic materials show very analogous molecular or atomic emission spectra. However, various features such as molecular bonding dissociation and vibrational emission can be studied by varying laser irradiance, integration time, and laser power density. Figure 2a shows a schematic transition diagram for the spectra of nylon in the CN spectral region from 385 nm to 388 nm along with (0,0), (1,1), (2,2), (3,3), and (4,4) vibrational transitions at 3.2 eV excitation energy. The molecular optical emission of the CN violet system of a diatomic molecule relates to radiative transitions from B 2 Σ + to X 2 Σ + of the electronic state for ∆ν = 0 at 2.4 eV excitation energy. The transitions from the lowest vibrational states correspond to high probability and strong relative intensity as compared to the highest vibrational states. Figure 2b shows potential energy curves against internuclear distance for the two electronic states of the C 2 Swan band system also known as vibronic (400-700 nm), revealing the transitions between d 3 Π g and a 3 Π u (from the Plank equation; ∆E = hc/λ). Emitted wavelength λ of the C 2 molecular optical emission lies between the spectral region from 400 nm to 700 nm. C 2 Swan system originated due to a variation in wavefunction-symmetry as of the allowed electronic dipole transition between two 3 Π states from upper to lower state (g → u). The Swan band spectrum of C 2 corresponds ∆ν = +1, ∆ν = +2, ∆ν = 0, and ∆ν = −1 between excited electronic states to the ground electronic states with ν = 0-10, and ν" = 0-9 vibrational transitions. Where, ν" and ν are the vibrational states' quantum parameters [34] for the lower and upper vibrational stages of the electronic levels, respectively. capable of delivering the pulse energy of about 200 mJ. The LIBS experiments were performed at room temperature in an airy ambiance. The laser beam was converged onto the sample surface, which was positioned in the air at atmospheric pressure using a convex (quartz) lens with a focal length of 10 cm. The samples were kept on a rotating circular stage with a rotation speed of ~10 rpm to enable a clean surface for each fresh laser shot. The optical emission spectra were recorded using an optical fiber attached to a charged coupled devices (CCDs) array spectrometer with an optical wavelength range from 200 nm to 720 nm. An average of ten laser shots were employed to cleanse the surface of the target sample. The spectra were accomplished at an average of 20 laser pulse shots at different spots on the target surface. The averaged optical emission spectrum was then utilized to acquire the molecular optical emission of the polymer samples, which takes into account the target sample inhomogeneity and lowers the statistical inaccuracies.

Plasma Characterization
To describe the laser-induced plasma established on the optical emission spectra, specific conditions have to be fulfilled, i.e., the plasma must be optically thin and should follow the local thermodynamical equilibrium (LTE) order. As optically thick plasma would entail self-absorption and saturation in the emitted line profiles causes an irregular peak in the spectrum, it influences the computations for electron plasma density and excitation temperature. We have utilized the line intensity proportion procedure to confirm the condition of an optically thin laser plasma [35,36]: where Iki and Inm are the detected spectral line intensities at optical wavelength λnm and λki, respectively. Aki and Anm are the transition probabilities, and are the statistical weights of the upper levels, kB is the Boltzmann constant and T is the excitation temperature. We have employed the optical emission lines with nearly the equivalent or very adjacent energies to the upper levels to minimize the temperature influence. The experimentally observed ratio of the spectral line intensities and the ratio determined from the spectroscopic factors are in excellent agreement within a 5% error. For illustration, the experimentally calculated intensity ratio using Zn I at transitions 328.23 nm and 330.29 nm is 0.45, whereas the intensity ratio calculated from the spectroscopic parameters is 0.47. The error relative to the theoretical ratio is found within 5%, which is acceptable. Thus, the optical plasma can be assumed as optically thin.

Plasma Excitation Temperature
The excitation temperature was determined by applying the Boltzmann plot technique, having established that the optical plasma is optically thin. The spectroscopic atomic factors of the optically thin spectral lines of Ca I and Zn I applied to construct the Boltzmann plots were acquired from the NIST database [37]. The spectroscopic parameters of the persistent lines including wavelength (nm), upper-level energy (eV), electron configuration transition, statistical weight, and transition probability (s −1 ) are presented in Table 1. To draw the Boltzmann plots, these lines were carefully chosen as they lie in a thin optical region where the efficiency of the detector device stays nearly persistent, thus

Plasma Characterization
To describe the laser-induced plasma established on the optical emission spectra, specific conditions have to be fulfilled, i.e., the plasma must be optically thin and should follow the local thermodynamical equilibrium (LTE) order. As optically thick plasma would entail self-absorption and saturation in the emitted line profiles causes an irregular peak in the spectrum, it influences the computations for electron plasma density and excitation temperature. We have utilized the line intensity proportion procedure to confirm the condition of an optically thin laser plasma [35,36]: where I ki and I nm are the detected spectral line intensities at optical wavelength λ nm and λ ki , respectively. A ki and A nm are the transition probabilities, g k and g n are the statistical weights of the upper levels, k B is the Boltzmann constant and T is the excitation temperature. We have employed the optical emission lines with nearly the equivalent or very adjacent energies to the upper levels to minimize the temperature influence. The experimentally observed ratio of the spectral line intensities and the ratio determined from the spectroscopic factors are in excellent agreement within a 5% error. For illustration, the experimentally calculated intensity ratio using Zn I at transitions 328.23 nm and 330.29 nm is 0.45, whereas the intensity ratio calculated from the spectroscopic parameters is 0.47. The error relative to the theoretical ratio is found within 5%, which is acceptable. Thus, the optical plasma can be assumed as optically thin.

Plasma Excitation Temperature
The excitation temperature was determined by applying the Boltzmann plot technique, having established that the optical plasma is optically thin. The spectroscopic atomic factors of the optically thin spectral lines of Ca I and Zn I applied to construct the Boltzmann plots were acquired from the NIST database [37]. The spectroscopic parameters of the persistent lines including wavelength (nm), upper-level energy (eV), electron configuration transition, statistical weight, and transition probability (s −1 ) are presented in Table 1. To draw the Boltzmann plots, these lines were carefully chosen as they lie in a thin optical region where the efficiency of the detector device stays nearly persistent, thus reducing the inaccuracies Polymers 2022, 14, 3686 7 of 13 associated with the measurements of the line intensity. The excitation temperatures have been determined using the Boltzmann plot procedure in which numerous spectral emission lines along with their relative line intensities are considered [38,39]: where, F = hcd 4π , d is the characteristic plasma length, I ki is the spectral line intensity due to the k → i transition, λ ki is the spectral wavelength, h is the Planks constant, c is the speed of light, A ki is the transition probability, g k is the upper-level statistical weight, E k is the upper-level transition energy. Similarly, k B , T e , N, and U(T) are the Boltzmann constant, excitation temperature, total number density, and the temperature-dependent partition function, respectively. A plot corresponding to the logarithmic term in this equation versus the upper-level energies (E k ) yields a straight line (y = mx +c) and its slope (m) is like 1/kT e . The Boltzmann plots corresponding to the Ca I, and Zn I lines for pure nylon and doped nylon, respectively, are shown in Figure 5

Electron Number Density
The full width at a half area (FWHA) is calculated using a hydrogen Hα spectral emission line profile at 656.28 nm as demonstrated in Figure 6, displaying the experimentally analyzed line profile (blue color) corresponding to FWHA yielding as (0.99 ± 0.05) nm. The

Electron Number Density
The full width at a half area (FWHA) is calculated using a hydrogen H α spectral emission line profile at 656.28 nm as demonstrated in Figure 6, displaying the experimentally analyzed line profile (blue color) corresponding to FWHA yielding as (0.99 ± 0.05) nm. The simplest formula for the scheming of electron number density using the H α line is given as [40,41]:

Electron Number Density
The full width at a half area (FWHA) is calculated using a hydrogen Hα spectral emission line profile at 656.28 nm as demonstrated in Figure 6, displaying the experimentally analyzed line profile (blue color) corresponding to FWHA yielding as (0.99 ± 0.05) nm. The simplest formula for the scheming of electron number density using the Hα line is given as [40,41]: Here, Ne is the plasma number density, and ωFWHM is the full width at the half area. The parameter ωFWHM is calculated through the relation, = − . The calculated electron number density using Equation (3) is (2.4 ± 0.3) × 10 cm −3 . The error bars show the uncertainty (∼5%) in electron density, which is principally owing to the error in the electron impact parameter, FWHM, and in the deconvolution of the line width to the instrumental width.  Here, N e is the plasma number density, and ω FWHM is the full width at the half area. The parameter ω FWHM is calculated through the relation, ω FW H A = δω 2 − δω 1 . The calculated electron number density using Equation (3) is (2.4 ± 0.3) × 10 17 cm −3 . The error bars show the uncertainty (∼5%) in electron density, which is principally owing to the error in the electron impact parameter, FWHM, and in the deconvolution of the line width to the instrumental width.

Local Thermodynamical Equilibrium (LTE) Condition
The ionic and the excitation temperatures have come to be identical to the electron temperature for the optical plasma to be in local thermodynamical equilibrium (LTE). McWhirter advised a basis for a trivial limit of the electron number density to confirm the plasma to be in LTE. The above criteria was validated by calculating the smaller limit of the electron plasma density using the following relation [42,43]: N e cm −3 ≥ 1.6 × 10 12 (T (K) (∆E nm (eV)) 3 (4) where, ∆E(eV) is the highest transition energy between the upper to a lower ground level (n → m), and T is the excitation temperature in kelvin (K). In this work, the calculated electron number density using this relationship is on the order of~10 14 cm −3 . This value of the plasma number density is lower at three orders of magnitude than that determined from Equation (3). Therefore, it can be concluded that plasma is satisfying the LTE state. The validity of the LTE condition has also been confirmed by estimating the diffusion length using the Cristoforetti et al. criterion for an inhomogeneous plasma to be in LTE. According to this condition, the characteristic variation length "d" is significantly greater Polymers 2022, 14, 3686 9 of 13 than the diffusion length (10λ < d). The diffusion length was estimated employing the following relation [44,45].
where ∆E and k B T are determined in electron volt (eV), N e is the electron density measured in (cm −3 ), M A denotes the atomic mass of a specie, f 12 represents the oscillator strength taken from the NIST database [37], G is the gaunt factor carefully chosen by Cristoforetti et al. [45], d is the plasma diameter (typically a few mm) and also known as characteristic variation length. In the present work, we have calculated the diffusion length by using the emission line of neutral zinc (Zn I). The calculated diffusion length, λ ∼ = 1.87 × 10 −3 mm is much below the characteristic variational length of the optical plasma (10λ < d), which confirms that the plasma follows the LTE condition.

Effect of Laser Irradiance on the Plasma Parameters
The effect of electron plasma number density and temperature as a function of laser irradiance was investigated, as shown in Figure 7a,b. The dependency of plasma parameters on the laser irradiance was studied by varying it from 10 to 30 GW/cm 2 . The electron number density for both the samples (pure and doped nylon polymer) is determined through the FWHA of the H α line profile at 656.28 nm and plotted as a function of laser irradiance as shown in Figure 7a. The figure shows that the plasma electron number density increases exponentially from 2.4 × 10 17 cm −3 to 2.8 × 10 17 cm −3 and 2.49 × 10 17 cm −3 to 2.9 × 10 17 cm −3 for pure nylon and co-doped nylon, respectively, at the same laser irradiance from 10-30 GW/cm 2 . Similarly, excitation temperature was determined using the same samples and 532 nm wavelength at various laser irradiances ranging from 10 to 30 GW/cm 2 . Figure 7b demonstrates the variance in electron temperature as a function of laser irradiance from 0.65 eV to 0.76 eV and 0.64 eV to 0.77 eV for pure nylon and co-doped nylon, respectively. The electron temperature is showing almost linear growth concerning laser irradiance due to the sufficient laser absorption onto the target, revealing the inverse bremsstrahlung (IB) effect.

Effect of Laser Irradiance on CN and C2 Vibrational Emission
In this study, the intensities of the laser-produced optical plasma of the pure nylon and ZnO/nylon polymer were normalized by dividing the intensity of vibrational lines of CN and C2 using the intensity of the neutral carbon (C I) emission spectral line at 247.86 nm under time delay of 2 μs between the laser pulse to the preliminary of the acquisition system, 1 ms integration time and 532 nm laser wavelength. A similar normalization ap-

Effect of Laser Irradiance on CN and C 2 Vibrational Emission
In this study, the intensities of the laser-produced optical plasma of the pure nylon and ZnO/nylon polymer were normalized by dividing the intensity of vibrational lines of CN and C 2 using the intensity of the neutral carbon (C I) emission spectral line at 247.86 nm under time delay of 2 µs between the laser pulse to the preliminary of the acquisition system, 1 ms integration time and 532 nm laser wavelength. A similar normalization approach has also been investigated in previously reported literature [46,47]. Figure 8 shows the normalized intensity of CN at 388.34 nm and 516.42 nm due to (0,0) vibrational transition as a function of laser irradiance from 10-30 GW/cm 2 for pure nylon and doped nylon. As shown in the figure, the normalized intensities of the vibrational transitions are growing concerning laser irradiance. The maximum intensity of the CN and C 2 molecular band for (0,0) vibrational transition is observed at 30 GW/cm 2 irradiance for both the samples (nylon and doped nylon). At below 10 GW/cm 2 and above 30 GW/cm 2 , we observed that the signal vanishes for all the modes due to the low absorption of the energy onto the target surface and dissociation of the molecular band structures, respectively. The structural emission at CN: 388.34 nm and C 2 : 516.42 nm are the dominant molecular profiles throughout the whole scale of investigated irradiance from 10-30 GW/cm 2 . In an innovative sense, polymers can be distinguished from the other non-polymer materials corresponding to their vibrational structure. In the nylon sample, the vibrational emission of the CN Violet and C 2 Swan band due to the (0 → 0) vibrational transition is observed as strongly intense than the lower energy transitions, for instance (1 → 1), (2 → 2), and (3 → 3). Therefore, this factor is significantly very critical when exploring the LIBS spectra of polymers having organic molecular structures.

Conclusions
In the present work, molecular emissions, CN (B 2 Σ + to X 2 Σ + ) violet and the C2 (d 3 Пga 3 Пu) swan band system of nylon polymer plasma, at various irradiances from 10-30 GW/cm 2 , were noticed for the vibrational transitions Δν = −1,0,+1 and Δν = −1,0,+1,+2, respectively, using a laser-induced breakdown spectroscopy technique. The excitation temperature, plasma electron density, and line intensity as a function of laser irradiance were observed. Depending on the laser pulse irradiance, integration time, excitation temperature, and plasma number density, the characteristics of the CN and C2 emission spectra were found to change significantly. These analyses determine that the emission line inten-

Conclusions
In the present work, molecular emissions, CN (B 2 Σ + to X 2 Σ + ) violet and the C 2 (d 3 Π g -a 3 Π u ) swan band system of nylon polymer plasma, at various irradiances from 10-30 GW/cm 2 , were noticed for the vibrational transitions ∆ν = −1, 0, +1 and ∆ν = −1, 0, +1, +2, respectively, using a laser-induced breakdown spectroscopy technique. The excitation temperature, plasma electron density, and line intensity as a function of laser irradiance were observed. Depending on the laser pulse irradiance, integration time, excitation temperature, and plasma number density, the characteristics of the CN and C 2 emission spectra were found to change significantly. These analyses determine that the emission line intensities from CN and C 2 bands are very sensitive to laser irradiance, integration time and delay time among laser pulse shot and spectral acquisition. At low laser irradiances such as 10 GW/cm 2 , the emission bands due to CN and C 2 predominate while at higher irradiance the multiple neutral and ionized lines of various elements have been detected together with CN and C 2 molecular optical emissions. The excitation temperature is noticed to grow with rising laser irradiance and saturates at higher irradiance levels. The saturation of excitation temperature at higher laser irradiance is caused by exhaustion of excited-level population density of CN and C 2 molecules and by plasma shielding.