Collective behavior of laser-produced plasma from a multicomponent YBa 2 Cu 3 O 7 target in air

Abstract

The dynamics of diffusion of electrons and ions from the laser-produced plasma from a multielement superconducting material, namely YBa 2 Cu 3 O 7 , using a Q-switched Nd:YAG laser is investigated by time-resolved emissionspectroscopic techniques at various laser irradiances. It is observed that beyond a laser irradiance of 2.6 × 10 11 W cm −2 , the ejected plume collectively drifts away from the target with a sharp increase in velocity to 1.25 × 10 6 cm s −1 , which is twice its velocity observed at lower laser irradiances. This sudden drift apparently occurs as a result of the formation of a charged double layer at the external plume boundary. This diffusion is collective, that is, the electrons and ions inside the plume diffuse together simultaneously and hence it is similar to the ambipolar diffusion of charged particles in a discharge plasma.PACS: 52.50.Jm; 52.70.Kz Laser-produced plasmas, obtained when high-power laser beams interact with solids, have evoked a great deal of attention both in basic and applied fields of research [1][2][3][4][5][6][7]. The study of the transport properties in laser-produced plasmas can shed light on the different mechanisms involved in their formation and evolution in time and space. Laser plasmas generated in low ambient gas pressures have a wide range of applications as in materials analysis [8] and thin-film deposition of materials such as metals, ferroelectrics, semiconductors, and high-T c superconductors [9][10][11][12][13][14][15][16]. Recently fabrication of stoichiometric films of high-T c superconductors in air at atmospheric pressure have been reported [17,18]. Plasmas formed at atmospheric pressure have a high temperature and density, and the concentrations of the various species may become close to the stoichiometric proportions in the target. With YBa 2 Cu 3 O 7 as target, laser plasmas formed at atmospheric pressure exhibit time-dependent emission characteristics and opacity [19]. Resonance lines are found to be severely self-reversed and they show anomalous line profiles due to anisotropic resonance scattering [20].Laser-target interaction leads to rapid expansion and the velocities of ejected species generally depend on their masses, which vary for of atoms, molecules, or clusters. A slight deviation from charge neutrality at the external plume boundary may create large electric fields because of the formation of a charged double layer, which results in the acceleration of individual charged particles [21][22][23][24]. Such charged double layers can be formed in laser-produced plasmas with electron densities in the range 10 6 -10 21 cm −3 [21]. Double layers play a very important role in different contexts ranging from astrophysical processes to plasma confinement in fusion devices [25]. Here we report the collective diffusion of ions and electrons in the plasma plume as a whole due to the electric fields created by charged double layers of electrons and ions.