Introduction

Multifarious applications in various fields like Archeology [1], Nuclear industry [2, 3], Semiconductor industry [4] and Bio-medical uses [5], have already proven the capability of Laser induced breakdown spectroscopy (LIBS) as a promising technique for qualitative as well as quantitative analysis. In this methodology, owing to the breakdown of the sample by laser, a plasma is created depending upon the irradiance threshold, which is in turn dependent upon both laser and sample characteristics. The characteristics of both ablation and hydrodynamics of plasma depend upon complex laser-matter interaction [6], for which many times numerical methods are employed for modeling [7]. The formed plasma is not static, instead, it expands at a super-sonic speed, and after a persistence time of microseconds, it quenches. Along with the temporal evolution of the plasma, the associated plasma parameters like plasma electron temperature (Te), and plasma electron density (Ne) exhibit a characteristic trend. To use LIBS for analytical purposes, it is therefore very important to understand the physics of associated plasma parameters.

In India’s three-stage power program, utilization of Thorium (Th) stands as the ultimate objective within the nuclear industry. This is primarily attributed to Th’s vast abundance within the country [8]. In Indian Molten Salt Reactor (MSR), the proposed fuel consists of UF4 and ThF4 in the matrix of LiF [9]. There is a scarcity of studies in the field of LIBS application specifically focused on Th. Sarkar et al. has demonstrated that with the application of partial least square regression (PLSR) in the analysis of Th based waste glass, Th determination can be done with ~ 5% precision [10]. Judge et al. have shown the comparison of actinides like U and Th detection capability using both powders and pressed pellets by LIBS [11]. However, being a high Z element, the emission spectra of Th is highly complicated. According to a report made by Redman et al. in 2014, the region of 350 nm to 1175 nm, contains 19,874 Th lines [12]. Due to the inherent complexity, it is very important to optimize experimental conditions to minimize the effect of spectral broadening and continuum reduction among other conditions, which may affect the analytical outcome of a Th analysis by LIBS. Hence having a proper understanding of the evolution of the plasma parameters is of prime importance. For theoretical modeling of plasma and checking the applicability of different associated theories, the presence of Local Thermodynamic Equilibrium (LTE) is an important assumption, the assertion of which needs to be focused upon.

In this study, along with the study of the temporal evolution of plasma parameters already mentioned, the criteria for holding LTE was checked temporally. For ascertaining the effect of laser properties on LTE, experiments were carried out in different irradiance conditions after optimization of the experimental conditions. The line broadening and shifting in laser-induced plasma (LIP) due to Stark broadening was also studied with both atomic and ionic lines and the difference in their behavior was rationalized. Also, to ascertain the effect of atmospheric conditions on the LIP parameters, Th LIP’s emission spectra were recorded in three different ambient air conditions, namely, Air, Argon (Ar), and Helium(He) and the corresponding changes in intensity pattern and lifetime were studied. In summary, this paper extensively explores the parametric study of Th LIP. The forthcoming MSR power program will employ Th based complex fuel systems. For analytical quantification and related studies by LIBS, a proper understanding of associated plasma physics is necessary. Specially, selection of emission lines, optimal experimental conditions, and the identifying holding region of LTE is of prime interest. Also, for proper quantification, understanding the temporal variation of line broadening is crucial. Therefore, this study of complete understanding of Th – LIP, paves a footstone for a better analytical comprehension of Th plasma characteristics by LIBS.

Experimental

LIBS system and measurements

The LIBS study employed a Q-switched Nd: YAG laser (Brilliant B, Quantel) with a pulse width of ~ 7 ns and a repetition rate of 10 Hz. The laser was operated in 2nd harmonics of 532 nm and focused utilizing a plano-convex lens with broadband AR coating (Thorlabs, LA4380, f = 100 mm), while light collection from plasma involved a biconvex lens with a focal length of 35 mm. The gathered light was directed to the Czerny-Turner spectrometer (SR750 Shamrock, Andor) via an optical fiber bundle (D = 200 μm, SR-OPT-8024, ANDOR) through an f-matching setup. For the analysis, 2400 groves/mm grating was used. The spectral recording was performed using an ICCD detector (1024 × 1024 pixels, iStar DH334T-18 F-03, Andor). Wavelength calibration relied on a Hg-Ar lamp certified by NIST (HG-1, Ocean Optics, USA). For intensity calibration, a Deuterium-Halogen lamp (DH-2000-BAL, Ocean Optics) was used. To measure the laser pulse energy, a handheld pyroelectric meter (Ophire Photonics, Israel) was employed. Sample holder integrated with computer-controlled XYZ-translational stage facilitated changing the surface position for each measurement. The LIBS setup’s schematic diagram is presented in Fig. 1. Spectral analysis algorithm ‘ANUSAP’, was developed in-house, using LabVIEW 2013 (National Instruments).

For this study, certified reference material (CRM) ThO2 prepared by Radiochemistry Division, BARC in association with Nuclear Fuel Complex, India was used [13]. The CRM-ThO2-B was chosen as this had the lowest impurity concentrations. For preparation of the pellet, 30 bar of pressure was applied on the sample for 10 min, and a 10 mm diameter pellet was obtained. Afterward, the pellet was sintered at 800oC in an inert atmosphere for 4 h.

Fig. 1
figure 1

LIBS instrumental setup used for the analysis

Full size image

Emission lines selection for analysis

As already mentioned, being a high Z element, Th possesses numerous numbers of energy levels, making the emission spectra highly complex. Therefore, before choosing the region of analysis, careful observation must be done to ensure fulfillment of factors like abundant line intensity, spectral purity, and good signal-to-background ratio among others [14, 15]. Considering all the factors, the region of 405–411 nm was determined to be suitable for analysis as it encompasses both Th(I) and Th(II) lines, crucial for this analysis. Moreover, to ensure accurate identification of the Th peaks, Fe emission lines from a NIST-certified SRM sample of stainless steel (SRM-1295) were recorded in the region of 406.359 nm to 410.980 nm. Subsequently, the wavelength calibration was recalibrated. The intensity calibration was also carried out using a Deuterium-halogen lamp. The Th emission spectrum of the region of interest is shown in Fig. 2 and intense lines are identified. Due to extremely complex nature of the emission spectrum of Th, some of the emission lines, due to the limited instrumental resolutions were observed as multiplets. In the present study only spectrally pure emission lines, or marginally interfered emission lines were used. The details of the emission lines used for analysis are tabulated in Table 1.

Fig. 2
figure 2

Th LIBS emission spectra in the region of interest

Full size image
Table 1 Details of Th emission lines used for analysis [16]
Full size table

Optimization of experimental parameters

For appropriately assessing the associated plasma parameters like Te, Ne, or spectral broadening, optimization of experimental parameters to generate LIP like the number of laser shots, acquisition time delay (td) and acquisition time width (tw) are of great prominence [17, 18]. In a LIBS experiment, spectra can be recorded in two modes of analysis; digging mode and scanning mode [19, 20] In digging mode, an optimized number of laser shots are used to ablate a spot on the sample surface to record the spectrum, whereas in the scanning mode, for each laser shot a fresh surface is ablated. Due to scarcity of pure Th standard sample and requirement for analysing the sample multiple times, in this study, digging mode of analysis has been used. As abundantly reported in literature, to tackle the surface texture inhomogeneity, it is a common procedure in LIBS to take multiple number of laser shot accumulation for recording a single spectrum [21,22,23]. The performance of LIBS improves as a function of the square root of number of shots accumulation [24] thereby nullifying the effect of shot to shot variation. Therefore, in this study, to find the optimum number of shots, the near to maximum number of shots has been considered, after which the fluctuation in intensity becomes larger than the 10% level. The fluctuation in intensity associated with the Th (II) 409.475 nm for a varying number of laser shots in digging mode was plotted in Fig. 3. It is evident from the plot that after 40–50 shots, the fluctuation of intensity becomes > 10% indicative of significant crater influence, which will introduce error in the analytical result. So, as an optimum, 40 laser shots were used for the rest of the studies.

Fig. 3
figure 3

Fluctuation of intensity in tern of %RSD of the Th(II) 409.475 nm peak with varying number of shots

Full size image

In LIP, the effective collisions between heavier species and free electrons are instrumental in establishing equilibrium majorly by collisional excitation/de-excitation or ionization/recombination pathways, while radiative pathways act as minor ones. The radiative energy emerges through the process of electron-ion recombination, representing non-quantized energy along with having the contribution of free-free transition [25, 26]. This is responsible for creating the high continuum at early acquisition delay, and decays gradually with time. Hence, selecting an optimized delay time for recording the spectra is crucial in LIBS for ensuring accurate analysis. To comprehend the variation in signal intensity of Th (I) and Th (II) lines with td, we examine two Th atomic lines namely, 406.341 nm and 407.550 nm, along with two Th ionic lines namely, 406.920 nm and 409.475 nm. Each line intensity was normalized using (0,1) normalization.

Due to the high continuum at early delay times, the desired emission signals are hard to observe, therefore the background subtracted intensity of the lines of interest is low as evident from Fig. 4. On temporal succession as the continuum decays faster than the emission lines [27], the emission signals become more prominent and subsequently, the signal intensity increases to a maximum at 0.8-1 µs. Following this, a decreasing trend is observed in each case, attributed to ongoing plasma processes like recombination as well as radiative decay [28]. Consequently, it can be concluded that, for analytical purposes, an optimum td of 0.8-1 µs should be employed for most favorable output. It is noteworthy that the decay of the ionic lines (409.475 nm and 406.920 nm) occurs at a relatively faster than the atomic line. Among the decay pathways, ions and atoms follow similar processes like electron impact de-excitation and spontaneous emission among others. But, additionally ions have more pathways to decay like collisional recombination and radiative recombination, which can make the overall ionic decay processes faster, which is also evident from the following literatures [29, 30] where the authors have shown that the ionic species decay faster than the atomic ones. For each case of spectral recording, the gate width (tw) was kept at half of that of acquisition delay (td). This is undertaken to comprehend about transient evolution of plasma instead of a time-averaged picture in a fixed tw with a common timezone.

Fig. 4
figure 4

Variation in the temporal change of signal intensity of selected atomic and ionic lines

Full size image

Local thermodynamic equilibrium

To achieve complete thermodynamic equilibrium in a plasma, different conditions related to statistical thermodynamics need to be upheld. The primary governing conditions are Maxwell distribution (for describing electron energy distribution function), Boltzmann distribution (for describing atomic State distribution function), Saha-Eggert equation (for ascertaining the population of different ionization stages), and Planck’s distribution function (for determining photon energy density) and each one is designated by own characteristic temperature [31]. Under these conditions, every process occurring in plasma is exactly balanced by the reverse process according to the ‘Principle of detailed balance’, allowing the characterization of equilibrium condition by a single temperature [32].

But in the case of plasma originating from the laser-matter interaction condition, the radiative equilibrium condition does not hold as it has an inherent assumption of considering the plasma as a black body with no escaping radiation. Therefore, a deviation from Planck’s condition is certain in the real case. But, if the collisional processes occurring in plasma are much more abundant than the radiative processes, then it can be assumed that apart from Tυ (the characteristic temperature of Planck’s distribution law), all the other temperatures are same. This condition is referred to as Local Thermodynamic Equilibrium (LTE), where, owing to the transient and inhomogeneous nature of LIP, spatial and time-dependent gradients in the plasma should be minimal [33].

For ascertaining whether the laser-induced plasma is in LTE or not, the most important parameter to be checked is the ‘McWhirter Criteria’. The semi-classical model predicts that the essential condition of holding LTE is that, collisional transitions should be at least 10 times more abundant than the radiation transitions [34]. Mathematically, the condition can be expressed as:

$${N_e}(c{m^{ - 3}}) \geqslant 1.6 \times {10^{12}}{T^{0.5}}{(\Delta E)^3}$$
(1)

where, ΔE is the largest energy gap of transition, and T is temperature in K. In our study, by parametric variation of the factors affecting laser-plasma interaction like irradiance, ambient atmosphere, temporal behavior of the LTE assumption was checked and rationalized.

Plasma electron temperature (Te)

Different processes occurring in LIP like collisional or photoionization, collision excitation and de-excitation, three-body recombination etc. control the physics of plasma. These processes are closely associated with Te, which is the manifestation of the kinetic energy of both electrons and heavy particles present in plasma. To calculate the plasma electronic temperature, different methods are available like Boltzmann method [35], Saha-Boltzmann method [36], Synthetic spectra method [37], or Line to Continuum method [38]. Based on the assumption of LTE condition, the Boltzmann plot method is one of the most abundantly used techniques for determining Te [35, 39]. The equation used for calculating temperature from line intensity is:

$$\ln (\frac{{I\lambda }}{{gA}})=\ln (\frac{{Nhc}}{{4\pi U}}) - \frac{{{E_k}}}{{{k_B}{T_e}}}$$
(2)

where, I is the intensity of the emission line under analysis, λ is wavelength of the emission line, A is transition probability, g is degeneracy of the upper energy level, N is number density of upper energy level, U = partition function of the species, and Ek= upper energy level, In this method, \(\ln \left( {{{I\lambda } \mathord{\left/ {\vphantom {{I\lambda } {Ag}}} \right. \kern-0pt} {Ag}}} \right)\)is plotted against Ek, and from the slope, plasma temperature is calculated [40]. A representative Boltzmann plot example with Th ionic lines mentioned in Table 1 is shown in Fig. 5.

Fig. 5
figure 5

A representative Boltzmann plot using Th(II) lines

Full size image

In optically thick plasmas, Te and Ne variations across the plasma volume lead to the re-absorption of radiation emitted from the hotter inner regions, by the cooler outer atoms. This is known as self-absorption(SA) [41]. This effect must be nullified for Te and Ne analysis. For removing SA effect from the experimental Te result in the present study, emission line selection for Boltzmann plot was critically monitored. Among 28 Th lines chosen for Te analysis, only three are resonance transitions, which is only around 10%. The other emission lines with higher low bound energy level are least likely to show SA, making the analytical result of Te accurate with respect to SA. Similarly, for Ne calculation the emission lines used were also having much higher low-bound energy level, making the result accurate with respect to SA. Moreover, the ratio of integrated intensities sharing similar upper energy levels served as an indicator of the extent of self-absorption, in agreement with as reported in literature [42]. For this purpose, emission intensity ratio of Th (II) 409.893 nm and Th (II) 410.438 nm lines were compared, which showed the ratio was in line with the theoretically predicted value within 10%, indicating absence of SA.

Plasma electron density (Ne)

For the calculation of Ne, diverse methods are available which are mainly divided into two subsections namely- electrical and optical diagnostics methods [43]. Where the first category is based on probe-based techniques like Langmuir probes [44], the second one includes methods like Thomson scattering [45], Stark broadening method [46], or Saha-Boltzmann method [47]. In the present work, due to the unavailability of electron impact parameter of Th lines, Stark broadening method could not be applied. The possibility of measurement of electron density by use of Ηα or Hβ lines was also not possible due to the very weak signal and a higher background owing to the complexity of the matrix here. Therefore, the Saha-Boltzmann two-line equation has been used for calculating the Ne [47].

$${N_e}=\frac{{2 \times {I_{atom}}{A_{ion}}{g_{ion}}{\lambda _{atom}}}}{{{I_{ion}}{A_{atom}}{g_{atom}}{\lambda _{ion}}}}{(\frac{{2\pi m{k_B}{T_e}}}{{{h^2}}})^{1.5}}\exp (\frac{{{E_{atom}} - {E_{ion}} - {E_I}}}{{{k_B}{T_e}}})$$
(3)

where, Iion= intensity of the ionic line, Iatom= intensity of the atomic line, me= mass of electron, Ne = number density of electrons, EI= ionization potential of atom, Eion and Eatom are upper energy levels of ionic line and atom respectively. For our study, we have chosen two representative lines namely- an atomic line of Th(I) 406.341 nm and an ionic line of Th(II) 406.92 nm, and the temporal evolution of Ne was comprehended by varying td.

Results and discussion

Effect of irradiance

Irradiance change by varying laser energy

One of the important characteristics of laser that affects plasma is laser irradiance. The irradiance depends on both laser energy/power and focused area. For, ascertaining the effect of laser energy change on the temporal decay profile of Te and Ne, the energy of the laser pulse was varied and spectra were recorded in three different laser energy levels of 20 mJ, 40 mJ and 60 mJ. According to the specification of the laser employed for study, the beam is Gaussian in nature. For measuring the exact temporal profile of the laser beam, a 1 GHz oscilloscope (DSO7104A, Agilent) was used. After fitting of the average of five recorded laser pulses by Gaussian function, the FWHM was measured to be 7.13 ± 0.02 ns. The irradiance in each case was calculated and was found to fall within the range of 1010 W/cm2, which exhibits an upward trend on increasing laser energy.

To investigate the temporal progression of the plasma parameters, the values of Te and Ne were calculated at each condition. From Fig. 6a and b temporal decay of both Te and Ne can be comprehended. With increasing irradiance, both Te and Ne were observed to increase at any particular td. This can be attributed to the fact that with increasing irradiance, the amount of mass ablation increases, leading to more collision and resulting in higher Te as well as an increment of Ne. For each irradiance case, the decay trend of Te and Ne was found to fit with a power law equation against td. The equation employed for fitting is given below, where, y = plasma parameter, a = constant and m = decay rate.

$$y=at_{d}^{m}$$
(4)

It is interesting to observe that the rate of decay parameter (given by the power value over td) remains almost constant irrespective of the irradiance used for Te and Ne. The results clearly demonstrate that with increasing irradiance, the value of Te and Ne increases, but the rate of decay for both Te and Ne remains unchanged. Additionally, it is observed that the average decay rate of Te (~-0.09) is much slower than the decay of Ne (~-0.78). The difference can be explained based on the adiabatic expansion model of the plasma in vacuum [48], where it was shown that the Ne ∝ td−3 and Te ∝ td−2, exhibiting a higher temporal decay rate for Ne over Te. However, the effect of ion recombination is more prominent on Te decay, compared to that on Ne decay. In the recombination process, the released thermal energy goes to free electrons, which further reduces the temporal dependence of Te to td−1 instead of td−2. In practical cases, experimental LIP shows deviation from the theoretical co-relation as reported in literatures [49, 50] and showed lower degree of dependence with td, due to change of expansion mechanism of a LIP in ambient atmospheric condition. But the smoother decrease of Te remains expected [28], which is also evident by trend reported in the literature report by Zorba et al. [51].

Fig. 6
figure 6

(a) and (b) Temporal decay of Te and Ne at different irradiations

Full size image
Fig. 7
figure 7

Comparison of decay of Ne and McWhirter criteria in temporal analysis for different irradiance

Full size image

As mentioned in Sect. 2.4, ∆E denotes the largest energy gap of transition. Between the two lines, Th(I) 406.341 nm has greater energy difference between corresponding energy levels, therefore for checking the applicability of LTE by McWhirter condition, this line was used. From Fig. 7, it is evident that at initial delay, the Ne is significantly higher, surpassing the McWhirter criteria threshold. However, as the plasma expands, the Ne gradually decreases, approaching the boundary condition where the effective collision rates decrease. It is interesting to observe that in each case, the time region where LTE condition is fulfilled by McWhirter criteria holds was fairly long. This observation can be rationalized by the relative gaps between the energy levels, which determine the dominance of either collisional processes or radiative processes. In the case of non-metals, larger energy gaps necessitate much higher Ne, which translates into the fact that LTE can be held for only early delay times. On the contrary, for metals like Th, the closely spaced energy gaps lead to the holding of LTE for a longer time interval [28]. For the remainder of the study, irradiance corresponding to the laser energy of 40 mJ was utilized.

Irradiance change by changing lens to sample distance

In order to analyze the impact of altering the focusing area on irradiance and its effect on the corresponding temporal behavior of Ne and Te, the spot radius of the laser beam was varied by changing the lens-to-sample distance.

For ensuring the sample is at exact focus position, ‘Reflective light technique’ has been used. Where the sample is exactly focused, the surface can be seen clearly by the back-reflected light. Also, to confirm further, at different lens-target positions, single laser shot was taken on the sample surface. Then, crater size was measured using SEM in each case and the tightest spot was taken to be at exact focusing length from the lens. For determining the experimental spot size, an Oxygen-Free-High-Purity (OFHP) Cu sample was used and a laser spot was created using a 40 mJ laser energy pulse. By using a Scanning Electron Microscope (SEM), the spot size measured was around 150 μm at exact focused position. Figure 8 shows the SEM image of Cu sample ablated at 40 mJ at exact focused condition. The sample was displaced both positively and negatively perpendicular to the incident laser beam by distances of 1 mm and 2 mm, designated as ± 1 and ± 2 respectively The focal length of the lens is 100 mm, while the diameter of the laser beam at the lens measures 9 mm. The spot radius in the defocused positions were measured at different lens-sample distances. The details of the change in irradiance are tabulated in Table 2.

Table 2 Comparison of irradiance at different lens to sample distance
Full size table
Fig. 8
figure 8

SEM image of OFHP Cu at exact focused position

Full size image

The temporal decay of Te and Ne for varying lens-to-sample distance is shown in Fig. 9a & b. It is evident that as irradiance decreases, both Te and Ne decreased. Notably a significant observation reveals that compared to upward movement over the same distance traveled, the values of Te and Ne during the downward transition are higher. For example, the Te value in 3µs for movement of + 1 mm is only 4636 K, whereas for − 1 mm movement, it is 5266 K. This phenomenon can be attributed to the fact that, when the sample is moved downwards, an air plasma, initially formed at the exact focal point above the Th sample surface. This pre-ablation of air contributes as an additional source of providing ions and electrons for the LIP of Th. In turn, the combined effect of both leads to the elevation of both Te and Ne. Thus, the values of Te and Ne at + 1 and + 2 mm positions are real values at reduced irradiance, which do not have any external assistance.

Fig. 9
figure 9

Temporal decay of (a) Te and (b) Ne at different lens to sample distance

Full size image

Figure 10a-c shows the trend of decrease of Ne at different focus conditions with respective McWhirter boundary. Under the focused condition, in the entire window of observation, the plasma remains above the criteria threshold. However, with a 1 mm displacement, only the earlier part of the plasma follows LTE criteria, and thereafter due to the decline in the Ne, the condition was not in accordance. Due to more decrement in number density in 2 mm movement condition, at no temporal condition, LTE was followed. These results display that the value of irradiance plays a pivotal role in deciding the applicability region of a specific plasma model.

Fig. 10
figure 10

Comparison of decay of Ne and McWhirter criteria in temporal analysis under different lens-to-sample distance condition

Full size image

Th emission line profile

In order to calculate the plasma parameters correctly, the appearance of the emission profile as a well-resolved spectra plays an instrumental role. One of the key parameters associated with the emission profile is width, which is measured by FWHM [52]. In an ideal scenario, the spectral line should appear as a line plot, but with various broadening mechanisms such as Natural broadening, Doppler broadening, and Stark broadening, the linewidth increases [53]. In LIP, where there is a high Ne, the dominant mechanism is Stark broadening. The local electric field generated by ions and electrons is capable of shifting the frequency of emissions as well as broadening the line profiles. Specifically, for Hydrogen and similar atoms, the linear stark effect tends to dominate, while the broadening of other species is primarily governed by the quadratic stark effect [54].

For comprehending the effect of td on the broadening, Fig. 11a is presented here, depicting the evolutions of Th(I) 405.925 nm and Th(II) 409.475 nm. It is observed that, in each case, at earlier td, the broadening is significant because of the very high Ne. With increasing time, the Ne decreases, and consequently, the FWHM also decreases. The FWHM calculations were carried out after background subtraction of the spectra. Additionally, at early td, there is a shift in the peak maxima for both of the lines. In the case of the atomic line, a red shift was observed, whereas the ionic line showed a blue shift. The quadratic stark effect which takes account of both electron and ion broadening, can explain both shift as well as the broadening of the peaks [55]. It is noteworthy to mention that, in each temporal condition, the width of the ionic line is greater than the atomic line. This observation can be explained by Griem’s semi-empirical formula, which compares the effective principle quantum number (n*) of the higher level of transition to that of the line width [56]. Upon calculation, it was found that the n* values for the Th(I) and Th(II) lines under consideration are 2.34 and 2.45 respectively. Since the line broadening by electron impact is directly proportional to n*, the FWHM of the ionic line in each condition is greater than that of the atomic line. The temporal decay pattern of FWHM is shown in Fig. 11b which also shows a decrease following a power law.

Fig. 11
figure 11

(a) Stark shift of peak position of Th(I) 405.925 nm and Th(II) 409.475 nm lines, (b) Temporal decay of FWHM trend for the same lines

Full size image

Effect of atmosphere

The presence of different ambient atmospheres has a pronounced effect on the plasma hydrodynamics as well as on the characteristics emission from the plasma. Experiments were carried out in three different atmospheric conditions, namely Air, Ar, and He. Ambient atmosphere was created by purging different gases through a nozzle on the sample surface at 3 lpm rate to analyze the corresponding effects in plasma parameters. Figure 12 shows the intensity of Th spectral profile under different atmospheres at a td of 1.25 µs. It can be seen that in Ar atmosphere, the spectral intensity is maximum, whereas in He, it is lowest. Also background continuum intensity shows the same trend. The main factors associated with altering the signal intensity are the ionization energy of the ambient gas and density [57]. For a gas with lower ionization potential, the threshold of plasma formation is lower and therefore the plasma is produced more efficiently and therefore the lines are of higher intensity. The trend of emission intensity follows the order of Ar > Air > He which is consistent with the ionization potential order of He > Air > Ar. Also, the higher dense plasma in Ar atmospheric condition has a higher tendency of absorbing a substantial fraction of the laser energy, thereby improving laser-plasma interaction. This leads to an increase in the emission intensity and in turn plasma parameters like Te and Ne increment were observed. The laser absorption mainly takes place by electron-neutral or electron-ion inverse bremsstrahlung processes, and a cascade like increment in Ne with time is observed [58]. Mathematically,\({{d \in } \mathord{\left/ {\vphantom {{d \in } {dt}}} \right. \kern-0pt} {dt}} \propto M\). Ar having higher mass, evidently provides better cascade conditions compared to the others.

Th emission spectras are also recorded while varying the td under different atmospheric conditions, as presented in Fig. 13. It is evident that Ar atmosphere results in maximum emission lifetime, whereas He provides the shortest. Because of the lower mass of He, the expansion of the plasma plume is very fast due to the less momentum drag of the background gas. Also, with the presence of lighter surrounding atoms in case of He atom, the energy transfer by collisional processes is more effective than in other conditions, leading to a shorter LIP lifetime. Upon comparison of Fig. 13, it is shown that in He atmosphere, the signal persists for only ~ 3 µs, whereas in the case of Air and Ar, it lasts up to 22 µs and 25 µs respectively.

Fig. 12
figure 12

Comparison of Th LIP emission intensity at different atmosphere

Full size image
Fig. 13
figure 13

Temporal change in Th LIP emission profile in changing atmosphere (a) in Ar, (b) in Air and (c) in He atmosphere

Full size image

The effect of atmosphere on the temporal behaviour of plasma parameters is also examined and the obtained results are represented in Fig. 14a & b. It is apparent that in each temporal condition, the Te as well as the Ne decrease following a power law similar to the case of air atmosphere. However, the decay coefficient (b value) varies with changes in the atmosphere. The rate of decay follows the order Ar < Air < He. This variation can be attributed to both the plasma emission lifetime as described previously (shortest lifetime in case of He), and also on the differences in thermal conductivity among the atmospheres. The thermal conductivity of He is much greater than air or Ar. At 300 K, the conductivity of He, N2(O2) and Ar is 155.7, 26(26.5) and 17.7 mW/m. K respectively. Clearly, the heat conduction mechanism is more prevalent in He than the others, which leads to a faster cooling rate of the plasma. Given that the thermal conductivity of Air and Ar atmosphere are comparable, the rate of decay of Ne,Te were found to be similar in both cases. The emission lifetime of Ar and air LIP is also comparable as shown previously.

Fig. 14
figure 14

Temporal decay trend of (a) Te and (b) Ne respectively in different atmospheres

Full size image

Conclusion

Spectroscopic investigations of the Th LIP, generated using the second harmonic of an Nd: YAG laser, have been conducted in this study. The study demonstrated the significance of understanding the temporal evolution of plasma parameters such as Te and Ne in LIBS analysis. For the purpose of optimization of experimental parameters, the temporal pattern of signal and signal/background ratio was calculated and related. In the course of evolution, the faster decay of the ionic lines compared to the atomic lines was observed. To account for the broadening of the spectral lines, the presence of stark shift was ascertained for both ionic and atomic Th lines, and the temporal evolution of stark broadening was scrutinized. In the case of the atomic line, a red shift due to stark shift was observed, whereas the ionic line showed a blue shift. With Boltzmann plot, Te and Ne were calculated respectively, and their decay trend was comprehended for different experimental conditions. The temporal patterns of both Te and Ne follow power law decay. It was observed that the average decay rate of Te is much slower than the decay of Ne. It is interesting to observe that the rate of decay parameter remains almost constant irrespective of the irradiance used in LIP generation. In each condition, the basic criteria of fulfillment for LTE was checked and rationalized. With three different ambient atmosphere conditions, the change in the emission spectral pattern was observed and also the different observation window in each case was described. The trend of emission intensity decay follows the order of Ar > Air > He which is consistent with the ionization potential order of He > Air > Ar. In summary, the comprehensive parametric study of Th LIP conducted in this work provides an excellent fundamental understanding of Th - LIP using ns-Laser pulse.