Excitation Characteristics Of Mg Alloy Under Laser Irradiation

Atomic emission spectrometry is one of the earliest spectrometry of optical analysis and laser induced breakdown spectroscopy is an important application of it,which use laser induced plasma as light source to analyze solid,liquid,gas,etc.Mg is a kind of future material with obvious advantages and disadvantages,recently researchers believe that laser will enhance its physical properties. Then analyze the characteristics of Mg plasma by LIBS will be necessary and that is the contents of this subject.The contents of this paper were divided into three parts:1. The principle,basic theories and phenomena of laser-induced plasma were presented.The character of spectrum, analytical method and parameter calculation of spectral lines were discussed.2. The second part is to analyze the characteristics of plasma under different laser energy and calculate the parameters.A series of plasma characteristic spectral lines of Mg alloy were obtained under nanosecond laser shock produced by a pulsed Nd:YAG laser(1064nm,maximum energy 500mJ),which was taken under standard atmospheric pressure and at room temperature.Results indicated that the evolutionary rates of spectral lines were different, and the laser energy was enough to ionization Mg alloy under this experimental condition by the spectral lines of MgⅠ,MgⅡ.The electron temperature of Mg plasma were calculated by the measured relative Emission-line intensity(MgⅠ383.2nm,MgⅠ470.3nm,MgⅠ518.4nm).The experimental results showed that the secondary excitation Mg atoms could be got under this experimental condition.The electron temperature of Mg plasma decreased with the laser energy reduced while the laser energy was in the range of 200 ~ 500 mJ.When the laser energy was in the range of350 mJ ~ 500 mJ,the electron temperature changed rapidly. When the laser energy was in the range of 200mJ~350mJ, the electron temperature changed slowly and tended towards stability.It is found that when the laser energy was 300 mJ, the plasma temperature raised suddenly,which could not accord with the trend because of plasma shielding. When the laser energy was300 mJ, the relative intensity of spectral lines was reduced which was lower than that of 350 mJ and 250 mJ.And it is against the variation trend of the relative intensity of spectral linesincreases with the increase of laser energy, which prove plasma shielding phenomenon had occurred and plasma with high power laser separate the coupling between laser and material.The plasma temperature was increased significantly, which is not consistent with the trend.When the plasma shielding phenomenon happened, laser energy was absorbed by the plasma, resulting in the rise of plasma temperature.3. The third part is to analyze the time evolution process of laser-induced Mg plasma. A decelerator was added on the basis of the experiment equipment of fist part.The experimental results showed that plasma was still in the forming stage while the delay time was in100ns;when the delay time was in the 250 ns,the electron temperature reduced rapidly and changed obviously while the delay time in 50 ns.When the delay time was in the range from250 ns to 500 ns,the electron temperature changed slowly.It is believed that the composite radiation of plasma increased and the probability of collision between electron and atoms and molecules decreased。The life time of Mg plasma is about 50μs after the spectrum while the delay time was in the range from 1 to 50 μs were analyzed.