Time evolution of the electron density and temperature in laser-produced plasmas from YBa2Cu3O7

Laser radiation at 1.06 μm from a pulsed Nd:YAG laser was focused onto a multielement YBa2Cu3O7 target in vacuum and the plasma thus generated was studied using time-resolved spectroscopic techniques. Line broadening of the Ba I emission line at 553.5 nm was monitored as a function of time elapsed after the incidence of a laser pulse on the target. Measured line profiles of barium species were used to infer the electron density and temperature, and the time evolution of these important plasma parameters has been worked out. PACS: 52.50Jm; 52.70 kz Pulsed laser ablation has become a very successful method for the deposition of high-quality superconducting YBa2Cu3O7 thin films [1–4]. Because of this and other technological applications, considerable efforts have been made to understand the dynamics of the laser-ablated material. The ablation process is accompanied by formation of a bright plasma plume expanding from the target. The laser-induced plasma is characterized by plasma temperature, density, particle expansion velocities, particle charge states and internal fields due to charge separation during plasma expansion. Optical methods are the most suitable for performing the deposition process control. However, the solution of the processing control problems depends on understanding the basic physics and chemistry associated with laser–target and particle–particle interaction occurring within the laserinduced plasma plume. The emission spectra obtained from a multicomponent YBa2Cu3O7 target show numerous emission lines and bands corresponding to neutral particles, ions and diatomic molecules. The dynamics of the laser-ablated YBa2Cu3O7 plume has been studied by various techniques such as optical emission spectroscopy [5–9], streak photography [10, 11], optical absorption spectroscopy [12–14], ∗ Present Address: Department of Physics, Sree Narayana College, Punalur 691 305, India ∗∗ E-mail: [email protected] optical time-of-flight measurements in the nanosecond timescale range [15–19], interferometry [20], and laser-induced fluorescence [21]. However, most of them have been aimed at identification of ablated species and on the evaluation of their velocity distributions. In our previous experiment it was found that the temporal distributions of various ionic, atomic and molecular species were strongly influenced by the spatial position of the plume from the target and the laser fluence used [15]. Besides the identification of the different species generated, most of the time-resolved measurements have focused on dynamics of ejected species from the surface. In the published studies, however, little attention has been paid to the fundamental parameters of the plasma such as electron temperature (Te), electron density (ne), and their evolution after the initiation of the laser plasma. Several diagnostic techniques are employed in the determination of ne and Te, including Thomson scattering [22], emission spectroscopy [23– 25], Langmuir probe [26, 27], and interferometry [20, 28]. In this paper we describe the temporal behaviour of the different aspects of the emission spectra of laser-generated plasma from YBa2Cu3O7. Also, the temporal dependence of the two vital parameters, electron temperature and electron density, is deduced. The Stark-broadening method is used for the determination of electron density, and the ratio of line intensities of the successive ionization states is used to elucidate the electron temperature. Using gated-integration techniques we report here the time variation of Stark broadening which directly gives the evolution of the internal electric field within the plasma.