Characterizations Of Pulsed-laser Ablation Of Ti-Al Alloy And SiC Target

The interactions of pulsed laser beam with solid materials have attracted increasing interest in the last decades for its potential applications in different areas including surface modification, nanoparticles and clusters production, pulsed laser deposition, chemical analysis, etc. In the past years, laser ablations of materials have been widely studied, whereas, it is rather difficult to completely reveal such events, because of the complex interactions of the particles, and the inhomogeneity evolution of the plasma. Moreover, the characteristics of the plasma should be highly depend on the experimental parameters, such as the laser pulse, the intensity of the laser beam, the pressure of the background gas, and the spatial position of the lens relative to the target, etc. Therefore, it is absolutely necessary to study the spatial and temporal evolution of plasma parameters in different conditions to get a deeper insight into the fundamental mechanisms of the laser ablation process, and also provide important effects on the practical application of laser ablation.In this dissertation, optical emission lines of the plasmas generated on Ti-Al alloy and SiC crystal targets by Nd:YAG high-power laser beam were used to investigate the temporal and the spatial evolution of the electron temperature and density of the plasmas. Under the assumption of local thermodynamic equilibrium, the electron temperature and density are determined by using the Boltzmann plot method and the Stark-broadened line profile of singly ionized ion, respectively.The temporal evolution of optical emission spectroscopy in plasma produced by a1064nm pulsed laser irradiating on Ti-Al alloy targets were measured in air and vacuum conditions. When the laser power density used at10.04GW/cm2, at the delay time of160ns, Ne calculated in air environment is about3.3times higher than that measured in vacuum condition, even at600ns delay time, the plasma remains practically constant, Ne in air is also about1.8times higher than that in vacuum. It is interesting to see that in vacuum, the time scales (-900ns) of the line at368.52nm from Ti ions are much shorter than those (-4μs) measured in air environment. These results clearly reveal that the plasma characterized by hotter and denser with long-lifetime can be easily induced in air environment. The comparative studies give detailed insights into the influences of laser power density on the evolution of the oxygen lines at background gas. The relationship of the integrated intensity of O(I)777.19nm,777.42nm, and777.64nm lines and the laser power density can be expressed as:O(Ⅰ)-Aexp(Ⅰ/12), corresponding to the exponential relation of Haughes’s model. It is understandable that air plasma ignited near the target surface can be considered as a cascade avalanche process. The effect of lens to sample distance (LTSD) on plasma evolution have been investigated over Ti-Al alloy targets by532nm laser ablation with intensity～6.5GW/cm2in ambient medium and vacuum conditions. It is worth noting that electron temperature and density increase rapidly with the increase of LTSD from short distance to the focal length. Moreover, the plasma parameters by laser ablation in ambient medium will further increase too when the LTSD extends the focal length, while for vacuum condition, the plasma parameters substantially decrease.The influence of the laser wavelength on the spatial distribution of the plasma has been studied. The plasma was produced by ablating SiC targets with laser pulses from Nd:YAG laser operating at wavelengths of532nm and1064nm. Under these experimental conditions, the electron temperature and density decrease rapidly with the increasing of the distance to the target surface in a short distance range of3mm. At longer distance (>3mm) from the target surface, the plasma parameters remain nearly constant produced by1064nm laser ablation. At the same region, the532nm laser will case the plasma become more reactive. Based on the stronger interactions of higher energy particles with ambient gas during the process of532nm laser ablation, a detail discussion of the tail region fluctuates has been addressed. The function of the time delay between the C(Ⅱ)426.7nm emission line of intensity maximum and the distance from the target surface was used to investigate the characteristics of dynamics distribution in the plume. The expansion dynamics with the initial velocity of36km/s generated by532nm laser ablation can be interpreted by drag force model. For1064nm laser ablation, the time delay recorded shows a linear dependence at short distance region (<7mm), and the expansion velocity calculated from the slope of the linear fit relation in the free expansion region is about25km/s. It was deduced that the spatial location of the substrate in the instable tail region of the plasma, will have detrimental effect on the quality of the SiC films prepared using PLD technique.We also presented the effect of lens to sample distance (LTSD) on plasma evolution by ablating SiC targets using532nm laser pulses in vacuum, which is consistent with the description of Ti-Al alloy targets by the532nm laser irradiating, as the increase of LTSD from short distance to the focal length. At longer distance, the intensities of C ions (426.7nm) and Si ions (634.71nm) gradually decrease with the increase of the LTSD from the focal length, and the intensity of Si atoms (633.19nm) is not significantly changed in the same process. In this work, when the LTSD located at107mm and115mm, the temporal evolution of plasma has been investigated, respectively. The nature and characteristics of the plasma will strongly depend on the changing of LTSD for delay time less than200ns after the ignition of the plasma. However, for delay time longer than300ns, the effect of LTSD on plasma evolution tends to be less important. If one can properly adjust the lens to sample distance, it could significantly improve the analytical capabilities of laser induced breakdown spectroscopy (LIBS).