Experimental and theoretical investigation of the early stage of laser induced plasma produced by the interaction between a KrF excimer laser and a metallic titanium target

摘要

The interaction of laser beam with solid matter and the consequent plasma generation has been studied for many years. Nevertheless the great efforts to exploit the laser matter interaction for material processing and diagnostic purposes, many aspects still need to be elucidated and clarified. In particular the plasma induced by ultraviolet and visible nanosecond laser is successfully employed for thin film deposition of a wide range of classical and novel materials (PLD) and for in situ qualitative elemental analysis (LIBS). In both these applications is really important to understand the composition and the temporal evolution of species in the plasma. The classical approach in the study of laser-induced plasma (LIP) is based on the assumption of Local Thermodynamic Equilibrium (LTE). In LIP all the energy is delivered during the laser pulse (tenths of ns) and then the system evolves spontaneously for few microseconds. The most part of initial energy is converted in kinetic energy so that the LIP expands with supersonic velocities (10~7-10~5 cm s~(-1)). In these conditions the velocity of expansion can change plasma parameters in a shorter time respect to that necessary for the establishment of elementary processes balances. The knowledge of the deviations from LTE is really important to understand the constraints and the corrections on theory to be taken into account for practical applications. The aim of this work is the investigation by emission spectroscopy of the basic aspects of the plasma generated by the interaction between a KrF excimer laser and a metallic target of titanium at low pressure. In this work, experimental observation and theoretical model have been employed to discuss fundamental concepts of the LIP in order to understand the main processes that must be taken into account for the analysis of this kind of plasma.