Spectroscopic study of a pulsed argon rf ICP discharge: stepwise excitation in the afterglow and its application in optical spectroscopy

Abstract

It is shown experimentally in a noble-gas, pulsed rf ICP at low pressure (∼20 mTorr) that stepwise excitation may cause a maximum in the time dependence of the optical emission during the power-free stage following the rf pulse. During this stage, fast electrons are created by chemi-ionization of metastable atoms and collisions of second kind between metastables and slow electrons. These fast electrons have energies much greater than the overall average electron energy, which is typically ∼0.1 eV for the power-free stage. If the flux of fast electrons exceeds the ambipolar flux of ions to the chamber walls, a dramatic increase in the near-wall potential drop occurs, from a few tenths of a volt to several volts, and a corresponding increase in the near-wall electric fields. In this case, a fraction of the fast electrons are reflected by the near-wall potential drop and accumulate in the plasma volume, leading to an increase in density as a function of time. Eventually, the fast electrons have sufficient density to produce significant excitation of the metastable states, increasing with time and creating a maximum in the optical emission. In the opposite case, when the flux of fast electrons is less than the flux of ions to the wall, stepwise excitation in the afterglow is usually negligible. In this case, a maximum can be caused by recombination, but generally, only at higher gas pressures. These effects should be taken into account in spectroscopy of post-discharge plasmas of various types and may be useful for measurements of relative cross-sections for stepwise excitation of atoms and molecules by monoenergetic electrons. The latter is possible because the collisional energy relaxation time of the fast electrons at these pressures is on the order of milliseconds, leading to the existence of a near monoenergetic energy spectrum for a significant time.