Investigations on low temperature laser-generated plasmas F. CARIDI, L. TORRISI, D. MARGARONE, AND A. BORRIELLI Dipartimento di Fisica, Messina, Italy and INFN-LNS and INFN Sezione di Catania (Gr. Coll. Messina), Catania, Italy (RECEIVED 18 December 2007; ACCEPTED 25 March 2008) Abstract A nanosecond pulsed Nd-Yag laser, operating at an intensity of about 10 9 W/cm 2 , was employed to irradiate different metallic solid targets (Al, Cu, Ta, W, and Au) in vacuum. The measured ablation yield increases with the direct current (dc) electrical conductivity of the irradiated target. The produced plasma was characterized in terms of thermal and Coulomb interaction evaluating the ion temperature and the ion acceleration voltage developed in the non-equilibrium plasma core. The particles emission produced along the normal to the target surface was investigated measuring the neutral and the ion energy distributions and fitting the experimental data with the “Coulomb-Boltzmann-shifted” function. Results indicate that the mean energy of the distributions and the equivalent ion acceleration voltage of the non-equilibrium plasma increase with the free electron density of the irradiated element. Keywords: Electron density; Ion energy distribution; Laser-generated plasma INTRODUCTION Pulsed lasers with nanosecond pulse duration and intensities on the order of 10 10 W/cm 2 are largely employed in different fields, from microelectronics to laser ion sources, from cul- tural heritage to biomedical applications (Batani et al., 2007; Jungwirth 2005; Schaumann et al., 2005; Veiko et al., 2006). The laser-generated plasma, in fact, can be used to imprint surfaces, to generate ions at high energy and charge state, to clean the surface of old coins, and to deposit biocompatible thin films on medical prosthesis by means of the pulsed laser deposition technique (Bashir et al., 2007; Conde et al., 2004; Fernandez et al., 2005; Lorazo et al., 2006; Nelea et al., 2004; Thareja & Sharma 2006; Wieger et al., 2006; Wolowski et al., 2007). Although many applications of these lasers exist at present, further investigations are necessary in order to characterize the laser-produced plasmas in terms of ion and electron temperature, density, fractional ionization, angular distribution, and dependence of the plasma parameters on the nature of the laser irradiated target. The electron density of the plasma, in fact, determines the plasma evolution kinetics, the inverse bremsstrahlung laser absorption, the electron-ion interaction and ionization processes, the recombination and the charge separation effects which produce, in times on the order of a few nano- seconds, a high electric field inside the plasma. This field is responsible for the ion acceleration and for the ion energy distributions separation as a function of the charge state (Laska et al., 2007; Torrisi et al., 2006). The thermal processes occurring in the Knudsen layer, close to the target surface, can be understood evaluating the plasma temperature by assuming that the ionized gas is in a local thermal equilibrium (LTE) condition. The temperature can be measured directly knowing the energy distribution of the neutral species emitted from the plasma or with other methods. For infrared and visible laser irradiation, the free electron density of the target element and its dc electrical conduc- tivity, s dc , are important parameters that determine the final properties of the produced plasma, such as ablation yield, temperature, and electric field developed inside it. In this work, in order to measure the energy distri- butions for the various charge states and to evaluate their dependence on the free electron density of the irradiated elements, measurements were performed with a mass quadrupole spectrometer with electrostatic deflection. The free electrons of metallic targets are the first cause of infra- red and visible laser light absorption in the material, deter- mining the laser penetration depth, the thermal conductivity, the target reflectivity and, as it will be demonstrated, the equivalent ion temperature of the devel- oped non-equilibrium plasma. 265 Address correspondence and reprint requests to: Francesco Caridi, Dipartimento di Fisica, Ctr. Papardo 31, 98166 S. Agata, Messina, Italy. E-mail: fcaridi@unime.it Laser and Particle Beams (2008), 26, 265–271. Printed in the USA. Copyright # 2008 Cambridge University Press 0263-0346/08 $20.00 doi:10.1017/S0263034608000311