1. Introduction

For the last two decades, laser-induced breakdown spectroscopy (LIBS) has garnered significant popularity as an analytical technology [1]. Its distinct advantages, including no sample preparation requirement, versatility across various sample types, remote sensing capability, and fast-speed analysis, have contributed to its widespread adoption [2,3]. Solid samples intended for quantitative and qualitative analysis of elements are typically situated within an ambient gas environment including but not limited to air, N₂, He, Ne, Ar, O₂ [4,5]. The generation of laser-induced plasma (LIP) follows laser-induced breakdown (LIB) and involves intricate processes. Following the laser’s ablation of the sample, interactions between the surrounding gas and plasma give rise to the formation of a plasma plume. The resultant LIBS signal is significantly influenced by the composition and pressure of the ambient gas [4,6]. Atmosphere control has emerged as a viable method for enhancing the detection sensitivity [7]. Given their lower breakdown threshold compared to air, rare gases offer an advantageous environment for amplifying the LIBS signal owing to their lower conductivity and specific heat properties [6]. Particularly, the performance of the LIBS signal in an argon environment has been notably high [5,8–11]. In addition, the gas mixtures have been employed to enhance the detection sensitivity, such as CO₂-N₂-He mixture [12], Ar-Ne and He-Ne mixtures [13], He-Ne-Ar mixture [7], Ar-N₂ mixture [14], He-Xe mixture [15], and various other combinations. While extensive research has focused on the interaction between rare gas mixture and plasma of sample, the exploration of interactions between rare gases in LIP, especially in the absence of any sample, remains relatively scarce in existing literature.

In recent years, the utilization of helium mixed with other rare gases has assumed a pivotal role in advancing optically pumped rare gas metastable laser (OPRGL). These lasers hold great promise as high-power sources, particularly when rare gas (e.g., Ne, Ar, Kr, Xe) pressure is considerably lower than that of buffer gas (He). To illustrate, let’s consider the case of the Ar laser. Following an electric discharge within the Ar-He mixture [16–20], metastable 1s₅ level Ar atoms are generated, serving as the gain media for the Ar laser. Subsequently, a pump laser operating at 811.5 nm is employed to excite metastable 1s₅ level Ar atoms to the 2p₉ level, He functions as the buffer gas, facilitating collisions with Ar atoms and accelerating the relaxation of Ar atoms from 2p₉ to 2p₁₀ levels. This process results in the generation of lasing at 912.3 nm (2p₁₀→1s₅). A significant milestone was achieved in 2012 when Han and Heaven successfully demonstrated a pulsed Kr laser within Kr-He mixture [21]. The following year, they showcased a diode-pumped Ar laser within Ar-He mixture [22]. In 2017, substantial progress was made, with Ar laser power reaching 3.8 W through the use of a pulsed direct current (DC) discharge [23]. OPRGL has not been limited to Ar laser [24–28] but has rather expanded its horizons to include Xe laser [29]. Furthermore, OPRGL can also be realized in microplasma by microwave discharge [30], and in plasma jet-type configurations [31]. However, it’s worth noting that as the gas pressure increases, the threshold of electric discharge breakdown (EDB) significantly rises, resulting in considerable challenges. Moreover, the electromagnetic interference (EMI) generated by capacitor during discharge can severely constrain the practical application of lasers [32]. In contrast, the threshold of laser-induced breakdown (LIB) decreases notably with increasing gas pressure. Importantly, the EMI associated with LIB is often less pronounced than that arising from EDB. Therefore, the process of preparing metastable rare gas atoms via laser-induced ionization holds promising prospects for development and has the potential to propel the advancement of OPRGL technology. However, the exploration of preparing metastable rare gas atoms by LIB is sparse, especially in terms of understanding the temporal evolution of atomic emission spectra within rare gas mixture. Therefore, it is imperative to initiate a comprehensive investigation into the LIB mechanism in rare gas-He mixture.

In our prior research efforts [33,34], we successfully generated metastable Ar (1s₅) atoms for use in OPRGL through laser-induced ionization technique in Ar-He mixture. It is worth highlighting that the emission of Ar atomic emission spectrum plays a pivotal role in producing metastable 1s₅ level atoms. Through our analysis of the time-resolved Ar I 811.5 nm spectral line, we were able to ascertain that multiphoton excitation primarily contributed to what we termed “Regime1”. Furthermore, multiphoton ionization and electron impact ionization (avalanche ionization) contributed to Regime2. In this work, we have delved even deeper into the preparation of metastable Ar (1s5) atoms by examining the Ar I 763.5 nm spectral line. This investigation has allowed us to shed light on the intricate interplay of processes involving “excitation + radiation + collisional relaxation” process and “ion-electron recombination” process in the context of creating metastable Ar (1s₅) atoms. Moreover, we have scrutinized the temporal evolution of both the 763.5 nm and 811.5 nm spectral lines, revealing significant differences. Furthermore, we have extended our research to explore the initial plasma formation.

2. Experimental setup

The experimental schematic diagram is shown in Fig. 1. The second harmonic of an Nd: YAG laser (Spectral-Physics, GCR-230) serves as the irradiation source, delivering a pulse energy up to 170 mJ with a pulse width of ∼ 7.4 ns. The nanosecond laser operates at a repetition rate of 10 Hz. The 532 nm pump laser is focused into the gas cell through the end window via a 150 mm focal length lens, resulting in a peak power density of 96 GW/cm² at the focal spot. The gas cell for this experiment is filled with Ar-He mixture, specifically, 6.7 kPa of Ar (with an Ar atomic concentration of 1.62 × 10¹⁸cm^-3) and 60.0 kPa of He (with a He atomic concentration of 1.46 × 10¹⁹cm^-3). In addition, as a point of comparison, a pure Ar gas at 66.7 kPa (with an Ar atomic concentration of 1.62 × 10¹⁹cm^-3) is also introduced into the gas cell. To observe the emitted atomic spectrum, an observation window positioned on the side of the gas cell. The generated atomic emission spectrum is collected and transmitted to a spectrometer (Andor, SR-500i) by an optical fiber. The synchronization of the Nd: YAG laser and the intensified charge coupled device (ICCD) detector is realized by a time delay generator (Stanford Research System, DG645). By configuring the trigger time and the gate width of the ICCD detector, time-resolved atomic emission spectra can be acquired. It’s worth noting that the choice of the gate width is critical. A narrower gate width results in more precise measurements but may compromise the signal-to-noise ratio (SNR). Conversely, a wider gate width significantly improves SNR but may sacrifice the accuracy of time measurements, potentially leading to a loss of detail in the evolution process. In this experiment, the concentration of Ar atoms in Ar-He mixture is notably low, resulting in a relatively subdued intensity of Ar atomic emission spectrum. During the acquisition of time-resolved Ar atomic emission spectra, the time gate width of the ICCD detector remains constant at 6 ns. To capture the distinct stages in the temporal evolution of Ar atomic emission spectrum, various time delay intervals are carefully selected. In the time range spanning 0-162 ns, a 6 ns delay interval is employed. Subsequently, within the period of 162-2012ns, a more extended delay interval of 50 ns is chosen. Lastly, during the time range of 2012-10012 ns, an even longer delay interval of 200 ns is utilized.

 

Fig. 1. Schematic diagram of the experimental setup.

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3. Results and discussion

3.1 Atomic emission spectrum

The 532 nm pump laser was precisely focused onto the central region of the gas cell, yielding a peak power density of approximately 96 GW/cm² at the focal point. This resulted in a conspicuous ionization phenomenon observed in close proximity to the focus. As shown in Fig. 2, a series of atomic emission spectra were meticulously gathered using the spectrometer, with prior calibration of the spectrometer’s wavelength sensitivity. In Fig. 2(a), the atomic emission spectra of both pure Ar and Ar-He mixture were captured using an ICCD with a gate width of 1 µs and no delay applied. Upon close examination, it was noted that during the initial stage of ionization, the LIP within the pure Ar gas generated a distinct, continuous background radiation. This background radiation stems from two primary sources: the Bremsstrahlung radiation (free-free transition) and recombination radiation (free-bound transition) of electrons [35–37]. There exists a quasi-continuous energy level region proximate to the ionization limit of atoms. Electrons transitioning between this quasi-continuous region and discrete energy levels give rise to the quasi-continuous spectrum. Due to the finite resolution of the spectrometer, these background spectra manifest as continuous. Comparatively, the background radiation generated by the ionization of Ar-He mixture is notably feeble, while the background radiation resulting from the ionization of pure Ar gas is remarkably robust. This disparity arises from the fact that the first ionization energy of Ar atoms is 15.76 eV (127116.0 cm^-1), while the first ionization energy of He atoms is 24.59 eV (198336.0 cm^-1). This discrepancy significantly favors the ionization of Ar atoms, rendering them more susceptible to ionization. Consequently, the ionization degree of pure Ar gas is considerably higher. This higher degree of ionization results in a broader range of continuous electron transitions, consequently yielding a wider continuous spectral region and leading to the pronounced strength of the background radiation generated by the plasma. Conversely, in the Ar-He mixture, the predominance of He atoms renders them less prone to ionization, causing a relatively lower ionization degree and a weaker background radiation.

 

Fig. 2. Ar and He atomic emission spectra generated in LIP with Ar-He mixture (6.7 kPa + 60.0 kPa), pure Ar (66.7 kPa). (a) 0 µs delay. (b) 1 µs delay. It is important to note that the spectrometer used has been meticulously calibrated for wavelength sensitivity.

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Figure 2(b) shows the atomic emission spectra obtained using an ICCD with a gate width of 1 µs and acquisition delay of 1 µs. Notably, the continuous background radiation, while robust during the initial stage of ionization, rapidly diminishes. Within approximately 1 µs following ionization [38], this background radiation essentially vanishes, a trend clearly illustrated by the atomic emission spectrum of pure Ar gas. In addition, within the spectral range spanning 615-855 nm, the atomic emission spectrum of the Ar-He mixture reveals the presence of He I lines at 667.8 nm, 706.5 nm (the latter overlapping with the 706.7 nm Ar I line), along with the He II line at 656.0 nm. All other lines observed in this spectral range correspond to Ar I lines. This inference is substantiated by comparing these atomic emission lines with those of pure Ar gas. These observations strongly suggest that He atoms exhibit greater resistance to ionization compared to Ar atoms. Considering the concentration of He atoms is 1-2 orders of magnitude higher than that of Ar atoms, the fact that He lines appear weaker than numerous Ar lines reinforces the notion that He atoms are indeed challenging to ionize. Notably, the ionization of Ar atoms appears to induce the ionization of He atoms in this context.

The schematic diagram of the energy level transition of Ar atom is shown in Fig. 3. The Ar I lines observed in Fig. 2 are remarkably diverse. Following the recombination process between Ar ions and electrons, highly excited Ar atoms are produced. These atoms subsequently ascend to the 4P level through radiation channels and collisional relaxation channels, transitioning across the 2p₁ to 2p₁₀ levels. Upon reaching the 4S level (1s₂-1s₅), a series of Ar I lines emerge. The typical transition wavelengths are detailed in Table 1.

 

Fig. 3. The schematic diagram of Ar atom energy level transition.

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Table 1. Typical Ar atomic emission peaks and their corresponding energy levels

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3.2 Initial plasma formation

Regarding the 4P→4S transition highlighted in Fig. 3 and Table 1, it’s worth noting that the lower energy level of the spectral lines, specifically those at 696.5 nm, 706.7 nm, 747.1 nm, 763.5 nm, 801.5 nm, and 811.5 nm, corresponds to the metastable 1s₅ level. Among these lines, the intensities of 696.5 nm and 706.7 nm are quite similar and relatively weak, contributing to less than 6% of the population of metastable 1s₅ level. Notably, the interference of the He I 706.5 nm line with the Ar I 706.7 nm line is a significant concern. The 747.1 nm spectral line, on the other hand, is too weak to be reliably measured. In contrast, the Ar I 811.5 nm line stands out as the most prominent spectral line, contributing to approximately 60% of the population of the metastable 1s₅ level. Additionally, the corresponding contributions from the 763.5 nm and 801.5 nm spectral lines are both above 15%. The robust radiation from the 763.5 nm, 801.5 nm and 811.5 nm lines underscores their significance as primary sources of the metastable 1s₅ level atoms. In our previous work [33,34], we demonstrated the pivotal role of the Ar I 811.5 nm spectral line in elucidating the generation of metastable Ar (1s₅) atoms. Specifically, we found that the multiphoton excitation process contributed to the first regime (6 - 600 ns delay) of the time-resolved Ar atomic emission spectrum at 811.5 nm, while the multiphoton ionization process was responsible for the second regime (600 ns - 10 µs delay). However, it’s important to note that the spectrometer’s response efficiency to the 811.5 nm spectral line is significantly lower than that of the 763.5 nm line. In fact, prior to calibrating the wavelength sensitivity of the spectrometer in Fig. 2, the intensity of 763.5 nm line surpassed that of 811.5 nm line. In light of these considerations, for a more comprehensive exploration of the influence of other processes on the initial plasma formation and the preparation of metastable 1s₅ level atoms, the 763.5 nm spectral line emerges as a more suitable choice.

In a manner akin to conventional LIBS, the formation of plasma can be dissected into two fundamental processes when viewed at a microscopic level: the creation of initial plasma and the subsequent avalanche ionization (cascade ionization) of plasma, which is constrained by the initial electron density reaching the critical threshold for avalanche ionization [39]. The formation of initial plasma primarily involves two key mechanisms: multiphoton absorption and inverse Bremsstrahlung absorption. When short-wavelength lasers, exemplified by ultraviolet lasers like 266 nm and 355 nm lasers, interact with matter, they possess high single-photon energy. In these cases, the formation of initial plasma is predominantly governed by multiphoton ionization. Contrastingly, when long-wavelength lasers interact with matter, electrons gain sufficient energy through the absorption of laser radiation during collisions with neutral particles. This energy is then converted into the translational energy of electrons. Subsequently, these accelerated electrons collide with atoms, leading to more profound ionization events. Long-wavelength lasers, typified by infrared lasers like the 1064 nm laser, primarily induce initial plasma through the inverse Bremsstrahlung process, also known as impact ionization [40]. The inverse Bremsstrahlung absorption coefficient can be expressed as:

In Eq. (1), where α _IB is the inverse bremsstrahlung absorption coefficient, λ is the laser wavelength, n _e is the electron number density, and T _e is the electron temperature. According to the inverse Bremsstrahlung absorption equation, it becomes evident that longer wavelengths result in stronger absorption. As a consequence, for lasers with longer wavelengths, the primary mechanism driving the formation of the initial plasma is predominantly the inverse Bremsstrahlung ionization process. The 532 nm laser falls between ultraviolet and infrared wavelengths, presenting an intriguing scenario where both multiphoton ionization and inverse Bremsstrahlung process are likely to play a role [41]. This dual potentiality underscores the complex nature of the plasma formation process, dependent not only on the laser wavelength but also on the specific energy interactions with the targeted material.

In order to further verify the influence of the inverse Bremsstrahlung process on the initial plasma formation, we deliberately reduced the pump energy of the 532 nm laser, and conducted a detailed analysis of the background radiation and Ar 763.5 nm spectral line within the Ar-He mixture at delays of 12 ns, 22 ns, 1000 ns, and 2000ns, respectively. The results, as shown in Fig. 4(a), are quite illuminating. At a 12 ns delay, as the pump power density was progressively reduced, a remarkable reduction in the background radiation became evident. When the pump power density reached the low level of 38 GW/cm², no discernible 763.5 nm spectral line signal was detected. A similar trend emerged when the delay was extended to 22 ns, with the 763.5 nm spectral line signal vanishing as the pump power density decreased to 30 GW/cm², as portrayed in Fig. 4(b). Taking a leap to a delay of 1000 ns, the 763.5 nm spectral line signal entirely disappeared when the pump power density was lowered to 19 GW/cm², as shown in Fig. 4(c). Moving further out to a delay of 2000ns, we observed that a pump power density of 28 GW/cm² was the threshold at which the 763.5 nm spectral line signal vanished, as evidenced in Fig. 4(d). In summary, when the laser excitation was carried out with low pump power density, no 763.5 nm signal was detected at 12 ns and 22 ns delay times. However, at longer delays, such as 1 µs and 2 µs, a conspicuous 763.5 nm signal remained detectable. Notably, there was no significant signal broadening, and background radiation was notably absent. Remarkably, the cross section of multiphoton ionization (requiring 7 photons) and multiphoton absorption (with 6 photons) are strikingly similar. This suggests that multiphoton excitation and the multiphoton ionization likely occur simultaneously. Consequently, under identical pump power density conditions, the 763.5 nm spectral line in both the first regime (e.g., at 12 ns and 22 ns delays) and the second regime (e.g., at 1 µs and 2 µs delays) are generated concurrently. However, under conditions of low pump power density, only the 763.5 nm spectral line in the second regime was detected, implying that the multiphoton excitation and the multiphoton ionization did not transpire in the early regime. Considering that the second regime is attributed to the “ion-electron recombination” process, it indirectly substantiates the pivotal role played by the inverse Bremsstrahlung process in the initial plasma formation.

 

Fig. 4. The 763.5 nm atomic emission spectrum and background radiation variations with the pump power density. (a) 12 ns delay. (b) 22 ns delay. (c) 1000 ns delay. (d) 2000ns delay.

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Remarkably, even under conditions of high pump power density, at delays of 12 ns and 22 ns, the initial 763.5 nm signal remains conspicuously robust, surpassing the signal intensity observed at 1 µs delay mark. Notably, our experimental findings negate the possibility of the initial atomic spectral line emanating from the “ion-electron recombination” process. Upon the initiation of the initial plasma through the concomitant processes of the multiphoton ionization and the inverse Bremsstrahlung process, it is plausible that electrons persist in interacting with Ar atoms, thereby instigating both electron impact ionization and electron impact excitation. Therefore, the generation of the first regime can be attributed to the multiphoton excitation and electron impact excitation, while the emergence of the second regime is rooted in the multiphoton ionization and the inverse Bremsstrahlung process.

Following the initial plasma generation [42], the ionization process transitions to its second stage, often referred to as avalanche ionization [43,44], also known as cascade ionization, or electron cascade growth. During this stage, the initial plasma continues to absorb laser energy, causing a continuous increase in electron density until it reaches the critical threshold for avalanche ionization. The free electrons initially produced during multiphoton ionization or inverse Bremsstrahlung process persists in ionizing atoms through collisions. This continuous collision-driven ionization leads to the generation of additional free electrons, creating a self-sustaining cycle. The cascade effect can occur repeatedly, especially when the laser power is high, causing the free electron density to increase exponentially, thus leading to the formation of avalanche ionization. Even after the cessation of laser irradiation, electrons may continue to interact with atoms during the plasma expansion, resulting in impact ionization.

3.3 Temporal evolution of Ar I 763.5 nm line

The investigation into the generation mechanism of the Ar I 763.5 nm line involved analyzing the time-resolved Ar atomic emission spectrum within 0-10 µs range. In Ar-He mixture, the peak power density around the focal spot was determined to be 96 GW/cm². The time-resolved spectrum of the Ar I 763.5 nm is visually represented in Fig. 5(a), with the ICCD gate width set to 6 ns. The spectral line intensity exhibited a distinct pattern, reaching a local maximum (labelled as Regime1) within a short period, precisely at a delay time of 48 ns, and subsequently experienced a rapid decrease until the delay time of 138 ns. Subsequently, around the delay time of 612 ns, the 763.5 nm line intensity reached its maximum (labelled as Regime2), followed by a gradual decline. Analyzing the attenuation of the Ar I 763.5 nm line was crucial, and this was segmented and examined. Figure 5(b) illustrates the waveform of 532 nm pump laser and the time evolution of Ar line at 763.5 nm within the range of 0-1300 ns. During the process of laser irradiation, the intensity of the Ar I 763.5 nm line exhibited rapid growth, reaching the first maximum within ∼20 ns after the conclusion of the laser irradiation.

 

Fig. 5. The time-resolved Ar atomic emission spectrum at 763.5 nm in Ar-He mixture. (a) Evolution of the intensity of Ar line at 763.5 nm with delay time. (b) Comparison waveforms between the 532 nm pump laser and the Ar line at 763.5 nm.

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The 532 nm pump laser, with a single-photon energy of 2.33 eV (equivalent to 18793.2 cm^-1), plays a pivotal role in this study. For multiphoton ionization, the Ar atom necessitates the absorption of 7 photons (Ar + 7hυ → Ar⁺ + e^-), while the He atom requires 11 photons (He + 11hυ → He⁺ + e^-). In this work, except the multiphoton ionization of Ar atoms induced by the 532 nm laser, the inverse Bremsstrahlung process contributes to the initial plasma generation. Through multiphoton absorption processes, exciting Ar atoms directly to the 2p₆ level is a challenging task. However, it’s possible for them to be excited to an energy level higher than 2p₆ level. Subsequently, these excited Ar atoms accumulate on the 2p₆ level through a sequence of cascade relaxation processes, amplified spontaneous emissions (ASE), fluorescence radiations, or other processes [45,46]. This intricate interplay of processes leads to the rapid generation of the Ar atomic emission spectrum at 763.5 nm. As shown in Fig. 6, when the Ar atom absorbs 6 photons (equivalent to 112759.2 cm^-1), it’s probable that they are excited to the 3d[7/2]₄ level (112750.2 cm^-1) (Ar + 6hυ → Ar*(3d[7/2]₄)). Notably, a direct transition from 3d[7/2]₄ level to the 2p₆ level is forbidden. Therefore, Ar atoms on 3d[7/2]₄ level likely undergo rapid relaxation to the adjacent 3d[3/2]₂ level (112138.9 cm^-1) through collisions with He atoms (Ar*(3d[7/2]₄) + He → Ar*(3d[3/2]₂)). The fluorescence lifetime from the 3d[3/2]₂ level to the 2p₆ level is 400 ns [47], whereas the 2p₆→1s₅ transition has a shorter lifetime of 27-29 ns [48]. Upon taking the logarithm of the intensity of 763.5 nm spectral line from the delay time of 48 to 138 ns, a linear fitting is performed, as shown in Fig. 7(a). This fitting reveals a decay time constant of (347.2 ± 57.7) ns for Regime1 (Ar*(2p₆) → Ar*(1s₅) + hυ). This is attributed to the continuous replenishment of Ar atoms from the 3d[3/2]₂ level to the 2p₆ level (Ar*(3d[3/2]₂) → Ar*(2p₆) + hυ), along with electron impact excitation (Ar + e^- → Ar* + e^-). These processes collectively contribute to the observed decay time constant of 347.2 ns in Regime1.

 

Fig. 6. Schematic diagram of metastable Ar (1s₅) atoms preparation by laser-induced preionization.

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Fig. 7. The decay time constant of Ar 763.5 nm spectral line. (a) Linear fitting of the decay curve of Regime1 after taking the logarithm. (b) Linear fitting of the decay curve of Regime2 after taking the logarithm.

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After the initial plasma formation it continues to absorb laser energy, advancing into the second stage, known as avalanche ionization. This stage sees the emergence of a substantial number of electrons. As these electrons are accelerated and collide with both Ar and He atoms, it triggers more pronounced electron impact ionization (Ar + e^- → Ar⁺ + e^- + e^-, Ar* + e^- → Ar⁺ + e^- + e^-, He + e^- → He⁺ + e^- + e^-, He* + e^- → He⁺ + e^- + e^-). Consequently, the ionization of He atoms is induced by the ionization of Ar atoms. Once the laser irradiation ceases, electrons may continue to collide with atoms during the plasma expansion, resulting in impact ionization. During the collision process of electrons, influenced by factors such as collision angle and electron velocity, both electron impact ionization and electron impact excitation transpire simultaneously. In OPRGL, He atoms play a crucial role in expediting relaxation between the energy levels of Ar atoms [49,50]. When certain Ar atoms are populated on the excited states by multiphoton excitation and electron impact excitation, the result is the rapid generation of Ar atomic emission spectra after the radiation process. Therefore, the generation mechanism of Regime1 can be attributed to the process of “excitation + radiation + collisional relaxation”.

Highly excited Ar atoms will be produced through a gradual recombination process (Ar⁺ + e^- → Ar*). Following this step, as they traverse a series of radiation channels (Ar* → →Ar*(2p₆) + hυ) and undergo collisional relaxation channels (Ar* + He → →Ar*(2p₆)), a substantial number of Ar atoms accumulate at the 2p₆ level. This accumulation paves the way for the prolonged emission of 763.5 nm radiation (Ar*(2p₆) → Ar*(1s₅) + hυ). Subsequent to the recombination process, the regenerated atomic spectrum emerges after hundreds of nanoseconds, with its maximum being achieved approximately at 1 µs [44]. However, following the maximum at Regime2, as the electron density consistently diminishes, the intensity of 763.5 nm spectral line also begins to decrease. This reduction results in a decelerated rate of highly excited Ar atoms generation via the recombination process. The formation of Regime2 can be attributed to the “ion-electron recombination” process. When analyzing the decay curve of Regime2 and plotting its logarithm, a linear fitting is performed, as shown in Fig. 7(b). This fitting reveals a decay time constant of (1651.5 ± 22.9) ns, illustrating that Regime2 conforms to a single exponential decay model.

A stark contrast in the temporal evolution between 763.5 nm and 811.5 nm spectral lines becomes evident when we examine our previous research [33]. In the time-resolved 811.5 nm line, Regime1 exhibits considerably greater strength compared to Regime2. However, the result is reversed when studying the temporal evolution of the 763.5 nm line, with Regime1 being weaker than Regime2. The primary driver behind this phenomenon lies in the distinct radiation channels and collisional relaxation channels responsible for generating 763.5 nm and 811.5 nm lines. During the stage of “excitation + radiation + collisional relaxation”, a portion of Ar atoms are excited to the 3d[7/2]₄ level. Notably, apart from the 2p₉ level, direct transitions between the 3d[7/2]₄ level and other fine energy levels within the Ar 4P state are forbidden. Consequently, the population of 2p₉ level accumulates more quickly than that of 2p₆ level, resulting in a swift radiation process to the metastable 1s₅ level. Further supporting this explanation is the time decay constant. In the time-resolved 811.5 nm line, the time decay constant of Regime1 is shorter, measuring at 223.7 ns, compared to the time-resolved 763.5 nm line, where it stands at 347.2 ns This discrepancy in decay constants serves as compelling evidence that underlines the previously mentioned conclusions.

During the stage of “ion-electron recombination”, following the recombination process, highly excited Ar atoms can populate both 2p₆ and 2p₉ levels through a variety of radiation channels and collisional relaxation channels. Given that the 2p₆ level is positioned at a higher energy state than the 2p₉ level, it is reasonable to expect that the population accumulation on the 2p₆ level will likely occur at a faster rate compared to the 2p₉ level. This distinction in population dynamics is further reinforced by the notable variation in the time decay constants between 763.5 nm (measuring 1651.5 ns) and 811.5 nm (measuring 2243.3 ns) spectral lines. Therefore, it becomes evident that the dissimilar radiation channels and collisional relaxation channels operating during these two stages stand as the primary factors influencing the temporal evolution of 763.5 nm and 811.5 nm spectral lines.

4. Conclusion

In Ar-He mixture, the production of metastable Ar (1s₅) atoms for OPRGL can be achieved using the LIB technique. The radiation of Ar 763.5 nm spectral line plays a crucial role in generating metastable 1s₅ level atoms. Through a comprehensive study of the time evolution of laser-induced ionization and recombination in Ar-He mixture, the interaction mechanism between laser, Ar atoms, He atoms and electrons in LIP has been deeply revealed. By lowering the pump energy of the 532 nm laser and investigating both the background radiation and Ar I 763.5 nm spectral line, it has been demonstrated that the inverse Bremsstrahlung process plays an important role in generating the initial plasma, concomitant with electron impact excitation. Throughout the temporal evolution of the 763.5 nm spectral line, two distinct regimes, Regime1 and Regime2, emerge. Regime1 primarily arises from a sequence of the “excitation + radiation + collisional relaxation” process. Initially, the plasma is generated through multiphoton ionization and inverse bremsstrahlung process, leading to the rapid generation of numerous electrons via avalanche ionization and plasma expansion. During the collision process of electrons with atoms, both electron impact ionization and electron impact excitation occur simultaneously. When Ar atoms are excited to higher energy states through multiphoton excitation and electron impact excitation, the 763.5 nm spectral line is emitted in a very short timespan. Regime2 is predominantly the result of the “ion-electron recombination” process. Highly excited Ar atoms are produced by the recombination of ion and electron. These excited atoms subsequently pass through a series of radiation channels and collisional relaxation channels, leading to a substantial accumulation of Ar atoms on the 2p₆ level. The radiation of 763.5 nm continues for an extended period in this regime. The variation in the temporal evolution between 763.5 nm and 811.5 nm spectral lines can be attributed primarily to the differential utilization of radiation channels and collisional relaxation channels in these two distinct processes. Given the low threshold of LIB under high gas pressure conditions, along with the minimal EMI associated with LIB, laser-induced preionization emerges as a promising solution to catalyze the future advancements in OPRGL.