An Experimental Investigation On The Emission Spectra Of Femtosecond Laser-induced Nickel Plasmas

The plasma generated by focusing a high power density pulse laser beam on the sample surface is known as Laser-induced plasma. It is extensively concerned on the experiment and theoretical research on the laser induced plasma dynamics. The laser induced plasma not only can provide an atomic light source, but also can play an important role in promoting the development of the fields as chemistry, atomic and molecular physics, medicine, biophysics, astrophysics, and plasma physics. The presence of the laser induced breakdown spectroscopy technique applied in the online trace analysis based on the emission spectrum of laser induced plasma and the development of the techniques as laser isotopic element separation, laser producing nanomaterial, laser treating and laser based on the laser induced plasma make it necessary to have further knowing and understanding the laser induced plasma dynamics.The interaction between the laser beam and the sample surface leads to the complexity of the forming process of laser induced plasma. So the physical and chemical properties of the sample surface, and buffer gas environment, and the laser characteristics (such as wavelength, pulse energy, and pulse width) have great influence on the forming dynamics of laser induced plasma. The electron temperature and density is the most important physics parameters in description of the property of plasma. Due to the unstationary of the laser induced plasma, the forming processes of the plasma is described by this two parameters. Although there are many experiment methods to study the plasma production dynamics, the method by measuring the time-resolved emission spectrum of the plasma is well known as the most efficient research approach. In spite of the extensive investigation on the nanosecond pulse laser induced plasma dynamics, the study on the fs pulse and especially the dual fs pulse laser induced plasma were just interested in recent years. Because of apparent difference of the interaction mechanism between the nanosecond and femtosecond pulse laser induced plasma, the investigation on the time evolution property of the emission spectrum will play an important role in understanding the fs pulse laser induced plasma dynamics.In our experiment, the single pulse and collinear geometry dual pulse fs laser-induced Ni plasma under atmospheric environment are produced respectively by using the femtosecond laser beam at 800nm wavelength and 30fs pulse duration. The time resolved emission spectrum of Ni atom in the femtosecond laser induced plasma were measured and the time evolution properties of the electron temperatures were obtained. Comparing with the experimental results from the single pulse fs laser configuration with equal total pulse energy, the enhancement of the Ni atomic spectral line intensities created by the collinear geometry double pulse femtosecond induced plasma were investigated. Finally the dependence of the enhancement factors of the spectral line intensities from the double-pulse experiment on the inter-pulse delayΔt was obtained.In chapter one, the scientific background and significance of the research in this paper is described firstly,then the research progress of the production dynamics as well as the experimental study on the emission spectra of laser-induced plasmas were summarized. In chapter two, the basic theories of laser-induced plasma are reviewed. The chapter three is focused on the experimental setup and the measurement method.In chapter four, the time-resolved emission spectrum of Ni atom in single pulse fs laser induced Ni plasma was measured in air at atmospheric pressure using a femtosecond laser with 30fs pulse width and 800nm wavelength. The electron temperature of fs-laser induced Ni plasmas and its temporal evolution were obtained through the measured relative intensities of the Ni atomic emission spectra lines. In additions, the temporal evolution of Stark broadening and Stark shift of Ni atomic spectra lines were also obtained. It is shown that the electron temperature is varied from 7500 to 4500K when the time delay is in the range from 110 to 610ns, which is different with the dynamic characteristic of nanosecond pulse laser induced plasmas.In chapter five, the enhancement of the Ni atomic spectral line intensities created by the collinear geometry double pulse femtosecond laser induced plasma at different inter-pulse delayΔt were measured with the total pulse energy identical to the single pulse configuration. The dependence of the enhancement factors of the spectral line intensities from the double-pulse experiment on the inter-pulse delayΔt were obtained. By the comparison the fitted and calculated time evolution properties of the plasma emission spectrum lifetime, electron temperature, and the half height width of the emission spectral line between the two experiment configurations. The main causes for the enhancement of the spectral line intensity in dual pulse experimental regime were analyzed. It was shown from the experiment results that the enhancement factor was experienced the processes from increasing at first to decreasing and again to increasing finally when the inter pulse delay was in the range from 0 to 310 ps. At the first stage from 0 to 50ps, the enhancement factor increases until reaching a maximum, then decreased to the turning point before increased again in the second stage from 50 to 310 ps , and finally remained at a fixed value (5~8) from 310 to 1000ps. The enhancement of the spectral line intensity was mainly due to the increase of the electron temperature in the dual pulse fs laser induced plasma. However the increase of the electron density also can lead to the enhancement.In the last part of this thesis, some suggestions on the future research are presented based on the analysis of our experiment research.