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WSN 153(2) (2021) 192-204 EISSN 2392-2192

Pulsed laser induced potassium oxide plasma analyzed by optical emission spectrum technique

Madyan A. Khalaf Department of Physics, College of Sciences, Mustansiriyah University, Baghdad, Iraq E-mail address: [email protected]

ABSTRACT In this work, a spectroscopic research on laser-produced potassium oxide plasma using optical emission spectroscopy (OES) technology is provided. Laser-induced K2O plasma produced at various laser energy has also been defined by visible emission spectroscopy. Plasma is created by a solid potassium oxide object radiated by a pulsed laser in a room environment. A Nd:YAG laser pulse 9 ns in duration with wavelength of (532) nm and a focal length of (10) cm in the range of energy (300-700) mJ is used to generate plasma from a planar K2O sample. The electron temperature (Te) was calculated in ratio line intensities method, while the electron density (ne) was calculated using Saha-Boltzmann equation, also another plasma parameters was calculated such as plasma (fp), Debye length (λD) and Debye number (ND). The increase in electron temperature and density was observed as a function of laser energies. Measurement of the production of the electron temperature in the rang (0.996-1.202) eV, While the electron density is in the rang (1.8×1016-8.8×1016) cm-3.

Keywords: Laser - Induced Plasma (LIP), Nd:YAG laser, Optical Emission Spectroscopic, potassium oxide plasma

1. INTRODUCTION

Laser-induced plasma spectroscopy (LISP) is an effective and versatile method for qualitative and quantitative elementary experiments in a broad variety of purposes from materials science to medicines [1]. The ablation cycle utilizing large pulses length laser (> 1

( Received 02 January 2021; Accepted 19 January 2021; Date of Publication 20 January 2021 ) World Scientific News 153(2) (2021) 192-204

ns) is split across several steps. Throughout the first step, the laser light interacts with the solid resulting throughout accelerated ionization of the goal surface through plasma within a limited time span relative to the length of the pulse. During the second level, the laser light is efficiently consumed by the plasma that spreads isothermally. Throughout the third level, at the conclusion of the laser pulse, the ensuing plasma plume extends nearly adiabatically into a medium which can contain vacuum or background air, even without applying field [2]. Laser generated K2O plasma is perhaps the most successful candidate during the next stage ultraviolet ultraviolet (EUV) light source used in semiconductor lithography industry to manufacture microchips with a 32 nm or smaller feature scale [3]. Plasma and its properties (electron density, electron temperature, spatial and temporal behavior) depends on the thermophysical characteristics of the aim and the laser beam variables, such as laser pulse, time period and form, laser wavelength and strength [4]. Plasma definitions begin by attempting to describe the characteristics of particles, gases, electrons and ions groups rather than the individual organisms. If there is a thermodynamic equilibrium, the plasma characteristics, such as the definition of the velocity of the particles and the relative concentrations of the level of energy, could be defined by the principle of temperature [5]. P Awareness of free electron number densities, ions, atoms and their temperatures is important for the classification of the laser-induced plasma (LIP) that is critical for modelling and diagnostic targets. Above that the parameters are generally calculated indirectly by means of optical emission tests, relying on the premise that local thermodynamic equilibrium plasma (LTE) is present [6]. Optical emission spectroscopy (OES) has as of late pulled in a great deal of consideration for portrayal dependent on the LIPS. The ratio method is one of the most common techniques for the optical emission spectrum. It is used in calculating the electron temperature, while the Boltzmann plot method is one of the best methods for calculating the electron density [7]. In this experiment the ratio method is used to calculate the electron temperature. It is a common method for calculating the electron temperature at which the intensity of a two of atomic or ion spectral lines at the same ionization stage can be calculated. In the local thermodynamic equilibrium (LTE), the plasma temperature is calculated from the equation [8]:

−(E1−E2) 푇 = I λ A g ………………………….…. (1) k ln( 1 1 2 2) I2λ2A1g1 where (I1 and I2) is the intensity, g is the statistical weight, A is the transition probability, λ is the wavelength and (E1 and E2) is the energy of excited state in eV and k is the Boltzmann constant. Electron density describes the number of free electrons per unit volume. Saha- Boltzmann equation utilizes spectral lines of the same element and successive ionization stages. The Saha- Boltzmann equation is given as [9]:

(E1−E2−Xz) I1 21 3/2 kT ne = ∗ 6.04 × 10 (T) e …………………..…………. (2) I2 where:

∗ I2λ2 I2 = ………..…………………….. (3) g2A2

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XZ is the ionization energy in eV, g2 is the statistical weight of transition from level (2) to level (1), (λ2 is the corresponding wavelength of transition from level (2) to level (1) and A2 is a transition probability of transition from level (2) to level (1).While the plasma frequency is calculated from the equation [10]:

2 푛푒 푒 푓푝 = ...... (4) 푚푒 휀표

This frequency, depending only on the plasma density. The plasma frequency is one of the most important of plasma parameters [7]. The Debye length is the fundamental characteristic of the behavior of plasma as it represents the distance in which the individual particle effects another charged particle that carries a reverse charge inside the medium of the plasma. Debye length ((λD) is directly proportional to the square root of the electron temperature and inversely to the electron density according to [11]:

ɛo KB Te λD = √ 2 = 7430*(Te /ne) …………………………… (5) ne e e where ne is the density of the electron, Te is the electron Temperature and e is the electron charge. The number of particles in the Debye sphere (ND) which is dependent on the electron density and electron temperature and it represents second condition for plasma existence ND>>>1 as follows [12] :

4 3 ND = 휋 λD ne ………………………… (6) 3

2. EXPERIMENT PART

2. 1. Sample preparation Use the pomegranate powder with purity 99.99% with the properties listed in Table (1). to prepare the target.

Table 1. Some properties of Tin.

Some general characteristics K2O

Density (g / cm3) 2.35 Atomic mass (gm / mol) 94.20 Melting point (°C) 740 Crystal structure Antifluorite cubic

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A weight of (3 grams) was pressed per tablet of the material powder shown in Figure (1), where the powder was placed inside a cylinder made of stainless steel and pressed by a hydraulic piston shown in Figure (2) with a compressive strength of approximately 6 tons and the powder was converted into a disk (pellets) of K2O material so that the thickness of the disk (3 mm and a diameter of 10 mm), which allows the opportunity to use both surfaces in the process of generating the spectrum. The use of the powder after pressing it is preferable in practice to reduce the size of the air gaps that exist within a single disk.

Figure 1. Shows the shape of the potassium oxide material powder.

2. 2. Generation of plasma

In this experiment, plasma was generated using pulsed laser on solid target K2O. The experimental arrangement of laser induced plasma spectroscopy (LIPS) as shown in Figure 3. The plasma was generated by a Q-switched pulsed laser Nd:YAG with a wavelength of (1604) nm and frequency of (6) Hz. The pulse laser energy was transferred using the Q-switch delay streak light using the laser controller and the energy meter was calculated. The laser beam centered on the object produces a 45° angle in which the laser beam evaporates and ionizes the object material, producing a plasma plume over the target point. The optical emission spectroscopy (OES) method was used to determine the electron temperatures, densities also plasma frequency, the length of Debye and Debye number were determined mathematically.

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Figure 2. Manual press for pellets preparation

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Figure 3. Schematic diagram of the experimental LIBS set-up

2. 3. Plasma spectrum measurement The spectrometer that is used must be fast, and same response time in every shot so, Surwit (S3000-UV-NIR) spectrometer show in Figure (4) was used in the setup to determine emission wavelengths and has a high goals, relying upon grinding utilized in it, and reacts to a wavelength between 200-900 nm.

Figure 4. The spectrometer with optical fiber used in the work.

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The spectrum of plasma with different value of energies, prepare by K2O with the laser pulse energy was varied from 500 to 800 mJ, each spectrum was obtained over a wavelength range of 300-700 nm. The results were discussed and then compared with the National Institute of Standards and Technology data (NIST database) then, evaluate the plasma parameters [13].

3. RESULTS AND DISCUSSION

In this paper, plasma is generated from the projection and concentration of pulsed laser beams on the solid surface of the potassium oxide material. The plasma resulting from the interaction of laser beams with the surface of the target material contains electrons and ions in an excited state in addition to neutral atoms as well as radiation. The process of plasma analysis is done by measuring and knowing its parameters of the electron temperature (Te) and the electron density (ne). The knowledge of the plasma temperature and density of the plasma species is important for understanding the atomic ionization and excitation processes occurring inside the plasma. The optical emission spectra K2O plasma was recorded by using optical emission spectroscopy technique with 532 nm Nd-YAG laser. Fig. (5) to Fig. (9) shows the spectroscopic patterns for laser induced on K2O component, which confined in the air in the spectral range (300-700) nm with E = (300 - 700) mJ. This result is consistent with [14, 15].

70000

60000 E = 700 mJ

50000

40000 Intensity (a.u.) 30000

20000

10000 300 400 500 600 700 800 900 Wavelength (nm)

Figure 5. Emission spectra induced by 532 nm laser, laser energies (700) mJ for K2O target in the air.

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70000

60000 E = 600 mJ

50000

40000

Intensity (a.u.) 30000

20000

10000 300 400 500 600 700 800 900

Wavelength (nm)

Figure 6. Emission spectra induced by 532 nm laser, laser energies (600) mJ for K2O target in the air.

70000

60000 E = 500 mJ

50000

40000

Intensity (a.u.) 30000

20000

10000 300 400 500 600 700 800 900 Wavelength (nm)

Figure 7. Emission spectra induced by 532 nm laser, laser energies (500) mJ for K2O target in the air.

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70000

60000 E = 400 mJ

50000

40000 Intensity Intensity (a. u.) 30000

20000

10000 300 400 500 600 700 800 900

Wavelength (nm)

Figure 8. Emission spectra induced by 532 nm laser, laser energies (400) mJ for K2O target in the air.

Figure 9. Emission spectra induced by 532 nm laser, laser energies (300) mJ for K2O target in the air.

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Tables (2) electron temperature (Te), electron density (ne), Debye length (λD), plasma frequency (fp) and Debye number (ND) for K2O targets at different laser pulse energies by the ratio method can be calculated through the intensity ratio of a pair of spectral lines of atom or ion of same ionization stage. Criteria plasma was achieved through the results of the plasma parameters (λD, fp and ND) it was shown that fp decrease with laser energy because it is proportional with ne, while λD and ND increase with it [16].

Table 2. Plasma parameters for K2O with different laser energies.

Laser energy Te (eV) n (cm-3) f (Hz) λ (cm) N (mJ) e p D d 300 0.996 1.86E+16 1.2E+12 5.9E-04 1.6E+07 400 1.054 3.05E+16 1.6E+12 4.7E-04 1.3E+07 500 1.113 4.79E+16 2.0E+12 3.9E-04 1.2E+07 600 1.159 6.62E+16 2.3E+12 3.3E-04 1.0E+07 700 1.202 8.80E+16 2.7E+12 3.0E-04 9.5E+06

The variances of (Te) and (ne) were determining the Ratio Method using two lines of (K2O I in this part) K2O shown in Figure (10) for different laser energies.

1,5 9,0

8,0 1,2

7,0 )

3 - 6,0 0,9

5,0 (cm^

16 16

^

(eV) e

4,0 10

T 0,6

* e

3,0 n

0,3 2,0 1,0

0,0 0,0 200 300 400 500 600 700 800 Laser energy (mJ)

Figure 10. Variation of Te and ne of plasma emitted from K2O target using laser with different energy

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The variances of (fp) and (λD) were determining the Ratio Method using two lines of (K2O I in this part) K2O shown in Figure (11) for different laser energies.

17,0 7,2 15,0 6,2 13,0

5,2 6

11,0 ^ (cm)

4,2 10

4 4

-

* * ^

9,0 d N

10 3,2 *

D 7,0

λ 2,2

1,2 5,0

0,2 3,0 200 300 400 500 600 700 800 900 Laser energy (mJ)

Figure 11. Variation of Nd and λD of plasma emitted from K2O target using laser with different energy

The values of Te were obtained from the Ratio method, from the analysis of recorded K2O I peaks for plasma induced on K2O component in the air using 532 nm laser, with different laser energies 300, 400, 500, 600 and 700 mJ. From the above it can be noted that the electron density and the electron temperature increases with the increase laser energy. The reason for this increases that the laser peak energy has a strong and important effect on the emission lines intensities, where the intensities of the spectral lines increase with increasing the laser peak energy because the mass ablation rate of the target also increases. The increase in laser energy will also increase its absorption in the plasma resulting in more ablation, which leads to increasing the number of excited atoms and hence the peaks of spectral line intensities of plasma emission.

4. CONCLUSIONS

A K2O target is irradiated by a 523 nm Q-switched Nd: YAG laser to produce K2O Plasma with different energies 500 - 800 mJ. Optical emission spectroscopic studies were performed to determine the dependencies of plasma parameters, such as electron density and electron temperature. The Plasma parameters were estimated in terms of their dependence on the laser energy. The results indicated that the values of (Te, ne and fp) were increased with increase of laser energy in the atmosphere while the values of (ND and λD) were decreased. We note that

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when energy increases the intensity emission lines of both K2O were clearly increased as well as the peaks became more sharper. All plasma parameters satisfy plasma conditions.

Acknowledgements

Authors would like to thank Plasma Laboratory, Department of Physics, College of Science, University of Mustansiriyah for their support and helping in finishing this work.

References

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