Temporal responses of spectral line intensities emitted by d.c. arc plasma with aerosol supply studied by power interruption technique

Abstract

The responses to the fast interruption of the arc current have been studied for the spectral line intensities of several elements emitted by U-shaped direct current (d.c.) arc plasma with aerosol supply. The arc was burned at atmospheric pressure. Solutions containing the investigated elements were introduced with a concentric glass nebulizer into the argon stream of 2.7 lmin -1 . The arc current of 6 A was interrupted in periods that lasted up to 200 μs. At the moment of power interruption, the spectral line intensities are either suddenly increased indicating Saha balance or suddenly decreased indicating Boltzmann balance. Plots of intensity vs. time are used to evaluate the mechanisms that maintain the populations of excited states in the stationary state and processes occurring in the plasma during the power-off period. Excited levels of the elements with high ionization energies, like Ar, H and O, are predominantly populated by three-body recombination process i.e. they are controlled by the Saha balance. Excited levels of elements with medium ionization energies are predominantly populated by collisions with electrons i.e. controlled by the Boltzmann balance. Some of the spectral lines of C, P and Hg with high-excited levels missed the sudden intensity change, a cooling jump, which could be interpreted by the presence of two opposite mechanisms, Saha and Boltzmann balances, that cancel out the effect. Spectral line responses of easily ionized elements were found to depend on the plasma region. In a 2-mm axis region, the intensities do not show a cooling jump, but increase slowly during the power-off period. At medium distances from the arc axis, the spectral lines show typical Boltzmann type response. Farther to the arc periphery, where the electron concentration is very low but emission is still high, the power interruption does not influence the intensities and this indicates that the excitation is not prevailed by electron collisions. © 2002 Elsevier Science B.V. All rights reserved.