Effects of easily ionisable elements on copper and zinc lines excited in a plasma pencil

Abstract

The influence of Easily Ionizable Elements (EIE) (chlorides of Na, K, Mg, Ca, Sr and Ba) on the measured spectral lines intensities of copper and zinc excited in a capacitively coupled tubular discharge, known as a plasma pencil, is described. A strong decrease was observed in the dependence on the concentration of the EIE. The decrease is not linear. Addition of NaCl, KCl or BaCl₂ causes a steep decrease of the copper or zinc signal for a relatively narrow range of 0.05–0.80 g l⁻¹. Other EIE, with relatively higher ionization energies, even caused an increase of the Cu I 324.754, 327.396, 510.554 and 578.213 nm and Zn I 213.856, 468.014, 472.216 and 481.053 nm signals. Higher EIE concentrations, up to 4 g l⁻¹, bring only moderate signal suppression or even a weak signal increase. Atoms of Mg, Ca and Sr likely undergo preferential partial ionization but less than Na, K and Ba so that Cu and Zn atoms do not have enough energy for their ionization but still have enough energy for excitation. More Cu and Zn excited atoms, instead of ions, are then available and stronger Cu and Zn atomic emission is measured. Atoms of Na, K and Ba are so much ionized that Cu and Zn atoms even loose energy for their excitation. A stronger decrease was found for the copper line than for the zinc line. An explanation can be done through inelastic collisions between Cu and the EIE excited atoms with very similar energetic levels. The influence of the EIE on the plasma properties was also monitored by calculation of excitation (2900 ± 280) K, rotational temperatures (1000 ± 160) K and electron number density of (4.6 ± 0.7) × 10¹⁹ m⁻³; but no significant change was observable.