Energy Transfer in Ion– and Laser–Solid Interactions

Abstract

While the fundamentals of ion beam interaction with solids had been studied as early as the 1930s, its utility in the modification of materials was not fully recognized until the 60’s and 70’s. About the same time, the fabrication of high–power lasers permitted their application in the processing of materials, especially the use of short-pulsed lasers. Both techniques are nowadays widely used in a great variety of applications. The electromagnetic radiation (or photons, from a quantum mechanical point of view) from lasers interact with the electrons of the materials, transferring energy to them within femtoseconds. Energetic ions also transfer part of their energy to the electrons of the solid, but they can also interact directly with the nuclei in elastic collisions. The primary energy transferred involved in these precesses is not thermal and some assumptions must be made before treating the problem as a thermal one. Furthermore, these processes take place in very short periods of time and are localized in the nanometer range. This means that the system can hardly satisfy the condition of thermodynamic equilibrium. Despite the complexity of these processes, many of the effects on the materials can be understood by using simple classical concepts contained in the heat equation. During the last decades, different aspects of the ion–solid interaction have been incorporated in the calculation of the temperature evolution in the so–called thermal spike. This implementation has been possible in part, by the development of fast computers, but also by the availability of ultra short laser pulses that have given a great amount of information about the dynamics of electronic processes Elsayed-Ali et al. (1987); Schoenlein et al. (1987); Sun et al. (1994)). From a thermal point of view, these processes are very similar either for ions or for lasers pulses. The results obtained in one case can be applied most of the times to the other. For many of these experimental phenomena, the estimation of the temperature is only the first step and supplementary diffusion or stress equations must be solved, consistent with the spatial temperature evolution in order to describe them. From another point of view, nano–structures are nowadays of great interest in technology. Nano–structured materials have opened the possibility to fabricate smaller, more efficient and faster devices. Thus, the fabrication and characterization of new nano–structured materials has become very important and the use of ion beams and short laser pulses have proved to be quite appropriate tools for that purpose (Klaumunzer (2006); Meldrum et al. (2001); Takeda & Kishimoto (2003)). Thus, their modeling and understanding is very important. 4