Detection of laser-produced tin plasma emission lines in atmospheric environment by optical emission spectroscopy technique

A spectroscopic study on laser-produced tin plasma utilizing the optical emission spectroscopy (OES) technique is presented. Plasma is produced from a solid tin target irradiated with pulsed laser in room environment. Electron temperature is determined at different laser peak powers from the ratio of line intensities, while electron density is deduced from Saha-Boltzmann equation. A limited number of suitable tin lines are detected, and the effect of the laser peak power on the intensity of emission lines is discussed. Electron temperatures are measured in the range of 0.36 eV–0.44 eV with electron densities of the order 1017 cm–3 as the laser peak power is varied from 11 MW to 22 MW.


Introduction
Pulsed-laser induced plasma of solids is the subject of investigation in many fields of applied research such as laser plasma sources to x-ray lasers, inertial confinement fusion, and laboratory astrophysics [1][2][3]. A pulsed laser source is employed to vaporize and excite the analyte forming plasma [4]. The optical emission from the relaxation of excited species within the plasma yields information regarding the composition of the material under test [5][6][7].
The plasma and its characteristics (electron density, electron temperature, spatial and temporal behavior) depend on the target's thermophysical properties and laser beam parameters, such as laser pulse, temporal duration and shape, laser wavelength and energy [8][9][10]. Plasma descriptions start by trying to characterize the properties of the assembly of atoms, molecules, electrons, and ions rather than the individual species. If thermodynamic equilibrium exists, the plasma properties such as the description of the speed of the particles and the relative populations of energy level can be described through the concept of the temperature [11][12][13][14][15].
The electron temperature is an equally important plasma parameter which can be spectroscopically determined in a variety of ways: from the ratio of integrated line intensities, from the ratio of line intensity to underlying continuum, and from the shape of the continuum spectrum [16,17]. The diagnostic techniques employed for the determination of electron density includes plasma spectroscopy, Langmiur probe, microwave and laser interferometry, and Thomson scattering [18,19].

Photonic Sensors 290
Spectroscopy technique is the simplest as far as instrumentation is concerned [20].
In this work, the analysis of nanosecond Nd:YAG laser-produced plasma of solid tin target in air is presented. The effect of laser peak power on the intensities of spectral lines is also reported. A study on plasma parameters, such as the electron temperature (T e ), electron density (n e ), and their dependencies on laser peak power, is also reported. Electron density is determined by Saha-Boltzmann equation. Electron temperature is calculated from relative intensity ratio method using Sn I (333.06 nm) and Sn I (607.326 nm) lines.

Experiment
The optical emission spectra of tin plasma are recorded using the experimental setup of laser-induced breakdown spectroscopy (LIBS) shown in Fig. 1. It consists of pulsed Nd: YAG laser of 1064 nm wavelength, 9 ns duration, 1 Hz pulse repetition frequency, and peak power varying from 11 MW to 22 MW. The laser beam is focused on the surface of the irradiated sample located at the focal length of a converging lens (f = 10 cm). An optical fiber holding photodetector is adjusted at 45° with beam direction at 5 cm distance from the sample where plasma is generated. The emission from the tin plasma plume is recorded using Ocean Optics HR 4000 CG-UV-NIR spectrum analyzer in the spectral range of 320 nm-750 nm.

Results and discussions
The optical emission spectrum of laser-produced tin plasma in the range of 320 nm to 750 nm is shown in Fig. 2  The intensities of Sn lines at 326.233 nm, 333.06 nm, and 607.326 nm are measured at different laser peak powers. Figure 3 shows the influence of the laser peak power on the spectral line intensities. It is found that emission intensity of the spectral lines increases with an increase in the laser peak power from 22 MW to 88 MW. This is due to the absorption of laser photon by the plasma, and at the same time the plasma is relatively transparent to the laser beam so the ablation of the target increases [22].
The increasing of target ablation produces an increase in plasma height and plasma emission. At higher values of laser peak powers, plasma shielding effect is observed, i.e., the plasma becomes opaque to the laser beam which shields the target so the lines intensities decrease. Under the assumption that the plasma is in local thermodynamic equilibrium (LTE), the lower limit of the electron density is given by McWhirter criterion [23]: 12  where m e is the electron mass, k B is the Boltzmann constant, h is Planck's constant, E ion is the ionization potential of the neutral species in its ground state, ji I is the intensity of the spectral line of the transition from level j to i, ji  is the wavelength, ji A is the transition probability, g j is the statistical weight, and j g is the energy value of higher level Figure 4 shows the electron density of laserinduced tin plasma at different laser peak powers. It can be observed that the electron density grows as the laser peak power increases. The reason is that when a solid tin sample is irradiated by Nd: YAG laser, a collision-induced process occurs, hence free electrons in the focal volume are accelerated by the electric field of the laser and gain energy by colliding with neutral atoms. When the electrons have gained amount of energy, they can ionize atoms by collision, and this causes the electron density to grow with the laser peak power. The electron density dramatically decreases at high laser peak powers, which is due to the plasma shielding as discussed previously.
where I, , g, A, and E are the total intensity, wavelength, statistical weight, absorption oscillator strength, and excitation energy of one of the lines, respectively. Primed quantities are those for the second line. These values for the two lines considered are taken from tables of the NIST [21]. The electron temperature is calculated using (3) from the ratio of the intensities of tin lines (333.06 nm and 607.326 nm). Figure 5 shows that the electron temperature (T e ) increases with increasing laser peak power. The electron temperature is strongly dependent on the laser peak power as the latter is the source of evaporation, atomization, and ionization of the target when focused on.

Conclusions
A tin target is irradiated by a 1064 nm Q-switched Nd: YAG laser to produce transient and elongated plasma. Measurements of electron density and temperature are carried out by the optical emission spectroscopy (OES) technique. Line intensity ratios of the successive ionization stages of the tin are used for the determination of electron temperature, and Stark broadened profile of first ionized tin species is used for the electron density measurements. The dependencies of electron density and electron temperature on different experimental parameters like distance from the target surface are studied. The electron temperature in the range of 0.36 eV-0.44 eV is obtained for the tin plasma from the Sn I line intensities while electron densities of the order of 10 17 cm 3 are observed.