Figures (2) to (3) show the spectroscopy of plasma emission during LIBS in air and argon gases, respectively, under different vacuum pressures of AZO targets. The appeared emission peak lines were matched with the atomic and ionic standard lines of zinc and aluminum elements (Zn-I, Zn-II, Al-I and Al-II) that detected from the National Institute of Standard and Technology data (NIST) [17]. The variation of intensities for each pattern at different wavelengths is due to the different transitions probability and statistical weight of each transition [18]. The intensity of emitted lines for atomic species is higher than that for ionic ones due to the low ionized degree within plasma [19]. On the other hand, the emissions of Zn lines are higher than of Al lines due to their difference in content in the target.
The LIBS emission intensity lines increased in intensity directly with working pressure as a result of the enhancement of breakdown due spatial confinement effect of the surrounded gas molecules. The species confined within a limited space with high-density act as emitted sources during the de-excitation of their excited atoms by the laser [20]. According to Stark's principle, the lines become more breadth with increasing pulse laser energy, which suggests an increase in plasma density [21]. A significant broadening in atmospheric pressure has appeared [22].
Figure (4) illustrates the Lorentzian fitting for Zn-II 589.44 nm emission lines at different working pressure in air and Argon gas. It is clear that the line broadening increased with increasing the working pressure, especially at atmospheric pressure. When increasing pressure, the high-density of surrounding gas molecules restricted the plasma expansion cause to reflect back the shock wave and effectively compress the plasma plume by limiting the plasma plume expansion causing an inertial spatial confinement effect, thereby causing the ne to increase [1].
The electron temperature (Te) was determined according to Boltzmann-Plot in air and argon at different pressure as shown in Figures (5) and (6) respectively, using Al-I emission lines from NIST listed in Table (1) [17]. Te was determined from inverse the slope of linear relation between Ln (λji Iji/hcAji. gj) versus upper-level energy (Ej).
Table 1
Al-I emission lines from NIST used in calculations of electron temperature.
Wavelength (nm) | Aji. gj×107 | Lower energy level (eV) | upper energy level (eV) |
394.4005 | 9.98 | 0.00000 | 3.14272 |
396.1520 | 19.70 | 0.01389 | 3.14272 |
669.6015 | 0.40 | 3.14272 | 4.99382 |
The variation of Te and ne with the working pressure in air and Argon were shown in Figures (7) and (8) one can observed. It is clear that the electron temperature decreased with increasing surrounding pressure in both gases, while, ne has the opposite behavior. At lower pressure levels, the energetic electrons and other species can travel longer distances before colliding and lose some of their energy in different collision types. Therefore, kinetic energy becomes more dispersed as emissions with the increasing pressure of surrounding gas [5]. While, limiting the plasma plume expansion by the surrounded gas (in the air and Ar) with increasing pressure cause to confinement effect, thereby causing the ne to increase. Te in Ar is slightly lower than in air due to the higher cross-section of excitation collision for mono-atomic gases than the domain molecular gases in the air, which give a high probability of loss of energy and causes less dispersion of the plasma energy through fewer interactions with fast electrons [23].
Table (2) listed the plasma parameters of LIBS for the AZO target in the air and argon environments at different vacuum pressure using 300 mJ pulse laser energy. The plasma parameters satisfy the plasma criteria, with high plasma frequency, short Debye-length, and large Debye number [24]. Te decreased with increasing pressure as a result of increasing inelastic collisions. The ne increased with increasing the pressure from 0.08 to 760 Torr due to increasing the confinement effect [25]. Significant differences in plasma parameters have appeared in atmospheric pressure with the lowest electron temperature and the highest electron number density, which reflect the highest plasma frequency, lowest Debye length, and Debye number. Due to the significant effect of plasma pressure on the plasma number density, and its relation with plasma parameters, the plasma frequency values trend to the same behavior as ne as increased with increasing the pressure, and with higher values in Ar than air at 0.4 Torr and slightly lower in Ar than air at 0.08 Torr. λD and Nd have opposite behavior due to their inverse relation to ne.
Table 2
plasma parameters for LIBS from AZO targets in air and Ar at different vacuum pressure.
Gas | P (Torr) | Te (eV) | ne×1018 (cm− 3) | fp (Hz)×1012 | λD × 10− 6(cm) | Nd |
Air | 760 | 0.845 | 1.851 | 12.218 | 5.021 | 982 |
0.40 | 1.231 | 0.817 | 8.116 | 9.120 | 2595 |
0.20 | 1.254 | 0.795 | 8.007 | 9.330 | 2705 |
0.08 | 1.291 | 0.762 | 7.840 | 9.669 | 2886 |
Ar | 0.40 | 1.222 | 0.980 | 8.890 | 8.297 | 2345 |
0.20 | 1.244 | 0.762 | 7.840 | 9.490 | 2729 |
0.08 | 1.258 | 0.653 | 7.259 | 10.311 | 3000 |
Figures (9) shows the variation of Larmur radius (rL) of electrons gyration caused by the applied external field with working pressure in air and argon. The Larmur radius in a constant magnetic field is directly related to electron velocity (which is related to their temperature), so rL is reduced with increasing working pressure. The validity of magnetic confinement is confirmed by evaluating magnetic confinement factor (β), where thermal beta is defined as ration of plasma pressure to magnetic pressure. Laser induced plasma under vacuum has β < 1, indicate the efficient of magnetic confinement at low vacuum, where the magnetic pressure is higher as compared to plasma pressure [26]. β increased with increasing pressure i.e reducing the magnetic confinement efficiency, especially at atmospheric pressure with β > 1. Small variation in rL and β with less values in Ar than air. Reducing the confinement efficiency at atmospheric pressure due to the high number of collisions. The variation of vacuum level seems of higher effective on β values in Ar than Air.
Figure (10) shows the variation of emission intensity of Zn-II (589.44 nm) with working pressures in air and Argon. The LIBS intensities increase with increasing vacuum pressures from 0.08 to 0.4 Torr in the two types of gases and significantly increased in atmospheric air. The results indicate that the optimum pressure for the LIBS technique is at 0.4 Torr vacuum pressure from the AZO target. Despite the high increase in the emission intensity of the spectral lines at atmospheric pressure, it is not preferred because of the large broadening in the spectral lines, which leads to a decrease in the ability to distinguish between adjacent lines. On the other hand, the use of pure Argon gas which has distinctive spectral lines (especially if the wavelength range of the spectral study area does not contain lines of high intensities for the gas used) is better than using air, which contains different types of gases and impurities that affect the detection process using LIBS technology.