Über die Erzeugung und aufheizung von plasmen durch absorption intensiver laserstrahlen

Abstract

The conditions for producing laser-plasmas for spectrochemical analysis are discussed by means of a simplified model. This model is compared with experimental observations on plasmas, which had been generated by focussed radiation of lasers operated in the normal non- Q-switched as well as in the Q-switched mode. Several materials have been investigated (C, Al, Fe, Zn, W, and Pb) and measurements of the total amount of material being produced by the laser-beam, of the kinetic energies of the plasma particles expanding in vacuum and of the excitation temperature have been made. In addition to this time resolved observations have been made. Plasmas produced by normal mode laserpulses showed excitation temperatures of 4900°K (C) up to 7900°K (W), those produced by a series of giant pulses had temperatures up to 12,000°K (Fe). In the latter case average particle energies of about 3.5 eV have been measured under the same conditions. It has been argued, that laser produced plasmas having a high particle and electron density as well as a high temperature in the very beginning soon after expanding become cold non thermal plasmas characterized by a high electron density which is due to an insufficient rate of recombination. By this means subsequent pulses of a pulse series can never the less be absorbed to excite the plasma.