Energy Level Offset Analysis Of Lead Atom In Laser Plasma

Since the first laser was invented by T. H. Maiman, a physicist of USA in 1960, laser technique has shown its vital force and has been introduced to almost every field of natural science. In recent years, the rapid development of the high energy ultra short laser provide people with extreme experimental conditions such as unprecedented power density (>1020 W/cm2 ), field intensity (>1011 V/cm ) and time precision (10-15 s) etc which will lead to a lot of created study and breakthrough. The achievement of the study of"laser-material interaction"is an important symbol of the development of laser technique. Laser-material interaction is one of the most important tasks of physics not only in theoretic fields but also in practical application such as laser processing, preparation of materials, manufacture of military weapons and so on.Laser ablation material induced plasma is an important result of"laser-material interaction". When the power density of the incident laser pulse exceeds the damage threshold of the material, the surface of the irradiated material instantaneously reaches a temperature higher than the material's vaporization temperature due to photon and multi-photon absorption and other absorption mechanisms. Thus the material will be melted, vaporized, even ionized within several nanoseconds, causing explosion of the surface and formation of the dense plasma. Metallic lead has the highest cosmic abundance among the elements heavier than barium. Date on stark parameters and transition probabilities are needed not for atomic structure research, but also for astrophysics plasma modeling and analytical techniques for plasma diagnosis. In fact, these results are not only essential for understanding the laser material interaction process and clarifying the local behavior of the expanding plasma, but also enabling us to optimize the condition of PLD.Spectral diagnosis was used to detect the laser induced Pb plasma in this paper. First, the optical emission spectra of the Pb plasma in air at the distance d=0.3 mm from the target were recorded and analyzed. Temporal and spatial evolvement traits of spectral lines were investigated. The electron densities were determined using Stark broadening parameters of Pbâ… 373.995 nm lines. By analyzing the Lorentzian fitting of the spectral lines of lead atom, dates on Stark width and line shift of lead atom were measured at different delay time and laser energies, and, the evolvement traits of spectral lines Stark width and line shift of lead atom as a function of delay time and laser energy were analyzed from three aspects. The experimental data of Stark widths and line shifts were analyzed using the regularity of the Stark parameters' dependence on effective ionization potential, and an inverse experimental result was found compared with the theoretical calculation and analyzed by quadratic stark effect. The atomic energy level offset in plasma surroundings was explored by analyzing the line shift. Second, the temporal optical emission spectra of the Pb plasma in air at the different position from the target surface were recorded and analyzed. Temporal evolvements of ten spectral lines wavelength in the center of lead atom at different position to the target, and, it is find that the upper energy level of the spectral lines with hypsochromic shift in plasma surrounding have the same electron configuration, and, the hypsochromic shift of spectral lines were investigated. In addition, the evolvements of center wavelength about ten lead atomic spectral lines at different delay time as a function of position to the target were given, and, it is indicated that the center wavelength of the spectral lines decreases with the position to he target increase, and that was illuminate using Doppler effect which arise from the relative motion between the radiant atom and the detector. In a word the main work and results of this paper are as follows:(1)From theoretical analysis and Gaussian fit, as well as Lorentz fit of the line spectra of Pb plasma, it is confirmed that the line broadening results mainly from Stark broadening mechanism. The electron densities at various delay time and laser energies have been measured using the experimental well know Stark profile of the Pb I 373.995 nm line, which decrease with the time delay and increase with the laser energy enhancement. The formulation of the electronic density evolvement traits along with the delay time was gained that: y = A₁e^-x/T₁+A₂e^-x/T₂+Y₀.(2) The Stark widths and the line shifts of the lead atomic spectral lines emitting from the same upper level or having the same lower energy level were obtained and analyzed. The result is: W_357.274nm> W_257.727nm,d_{357.274 nm} > d_257.727nm,W_{373.995 nm} > W_266.316nm,d_{373.995 nm} > d_266.316nm;W_{357.274 nm} > W_373.995nm,d_{357.274 nm} > d_373.995nm,W_{257.727 nm} > W_266.318nm,d_257.727 nm > d_266.318nm, W_{3 63. 958nm} < W_368.348nm,d_{363. 958nm} < d_368.348nm. The experimental result was explained using the regularity of the Stark parameters' dependence on effective ionization potential, and, it is find that the W and d of the spectral line at 363.958 nm are smaller than that of 368.348 nm line, and, similar result was also reported in other article but not satisfied with the theoretic analysis. The abnormal result was explained from the aspects of self-absorption in plasma and quadratic stark effect.(3) The line shift of spectral lines in plasma surrounding is caused by the Stark effect primarily, so the energy level offset can be obtained by analyzing the corresponding line shift. By analyzing the experimental data, the relations of the corresponding energy level offset of the 6 lead atomic spectral lines are got, which are:ΔE_{e(6p^{2 1}D2 )} <ΔE_{e(6p^{2 3}p2)},ΔE_{e(6p7s ¹P1⁰ )} >ΔE_{e(6p7s ³P1⁰)},ΔE_{e(6p7s ³P1⁰ )} <ΔE₍e(6p7s ³P₀⁰). Besides,The evolvement trend of the Stark width and shift with the delay time and laser energy was explained in the aspect of W c changing in plasma plume expanding, quadratic Stark-effect and the shielding effect of the free electron.(4) In the second experiment, the electron densities were also measured using the experimental well know Stark profile of the Pb I 373.995 nm line, and, the spatial evolvements of the electronic density at the different delay time (300 ns,500 ns,800 ns,1000 ns) as a function of the position to the target were obtained. The temporal and spatial evolutions of the integrated intensities of four lead atomic spectral lines(357.274 nm,363.958 nm,368.348 nm,373.995 nm) in the wavelength range from 350 to 380 nm in air with a laser energy of 20 mJ were given. As the spectral intensity of a specific transition is proportional to the number density of the atom in the upper excited state, it can be concluded that lead atoms are excited constantly during the plasma expansion.(5) The temporal evolvements of the center wavelength of 10 lead atomic spectral lines (357.274 nm,363.958 nm,368.348 nm,373.995 nm,257.727 nm,266.316 nm,261.418 nm,280.199 nm,282.320 nm,287.332 nm) at the different position (0.1 mm,0.3 mm,0.5 mm,0.7 mm,0.9 mm,1.4 mm,1.9 mm,2.4 mm) to the target were obtained. It is discovered that the upper transition energy level of four lead atomic spectral lines with hypsochromic shift in the plasma surrounding have the same electron configuration, and, the hypsochromic shift of spectral lines in plasma were illuminated using quadratic stark effect, for the example of Pbâ… 287.332 nm.(6) The spatial evolvements of center wavelength about 10 lead atomic spectral lines at the delay time 300 ns,500 ns,800 ns,1000 ns were detected, and , it is indicated that the center wavelength of the spectral lines decreases with the position to he target increase, and that can be illuminated using Doppler effect. Expanding velocity of the plasma in different direction as a function of distance to the target surface was calculated by using the dynamic modal of the plasma expending, and it provide a reasonable explanation to the spatial evolvements of the center wavelength of the spectral lines.