Processes Of Excitation And Radiation In Laser Plasma

Laser-induced breakdown spectroscopy (LIBS) was first recorded in 1963 by Maker et al . Recently, LIBS has achieved great interest as spectroscopic sources. The optical emission spectroscopy (OES) of LIBS has become a very useful tool for the fundamental researches into the interaction of laser beams with materials, advanced micromachining, thin-film deposition and exploring the metal contents in alloy and in soils . As an analytical technique, LIBS has proved its unique versatility, permitting fast contact-less analysis of almost any type of material and the possibility to adapt the technique to the special requirements of diverse practical analytical problems.One of the distinguishing features of LIBS is its capacity to carry out the characterisation of the laser induced plasmas (LIPs). The spectroscopy of the radiation transmitted by the LIPs may be used to obtain the characteristic physical parameters, for instance, the electron temperature, the electron density and opacity. As the spectral line and continuum emission of the plasmas reckon on these parameters, the interest of plasma characterisation in order to win improvements in the applications of LIBS is clear. The present work is to offer an experimental study that aims to investigate the influence of the distances between the lens and the target on the properties of the plasma. The research in this area has been minimal. In this study, the lead is selected as the target, the plasma character is obtained and analysed via change in the distances between the lead target and the lens.Spectral diagnosis was used to detect the laser induced Pb plasma in this paper. First, The electron density, the electron temperature and the intensity of the plasma were analysed when the plasma as the optically thin. Through the experiment we get, the maximal intensity appeared at 6.1 cm. However, the maximal electron density presented at 5.8 cm. Besides, the self-absorption was analysed when the plasma as the optically thick. Moreover, the self-absorption can be observed in all of the wave ranges when the distances are more than the focal length. This phenomenon can be understood by the air breakdown. In a word the main work and results of this paper are as follows: (1) The study on the emission mechanism of laser plasma and the law of its evolution with the delay time was done. The intensity of continuum spectra and the line emission spectra of the lead plasma increases at the early stages and decreases in the after stages. This is because the mechanism of the continuum spectra is bremsstrahlung and recombination emission. But the bremsstrahlung dominates only in high temperature. The emission mechanism of line spectrum is collision excitation, especially the excitation of high power electron.(2) 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 has been measured using the experimental well know Stark profile of the Pb I 373.995 nm line, the electron density decreases with the time delay, the highest electron density emerges at the distance of 5.8 cm.(3) The self-absorption phenomenon is observed when the plasma is considered as optically thick. Self-absorption is affected when the distances between the lens and the target is different. The phenomenon is more noticeable when the target is far away from the focus rather than close to the focus, self-absorption is prominent when the lead target is placed behind the focus. The reason must be that when the target is placed the behind the focus the laser will firstly breakdown the air, thus plasma N and O will take part in the collision.