Wall loss of atomic nitrogen determined by ionization threshold mass spectrometry

In the afterglow of an inductively coupled N₂ plasma, relative N atom densities are measured by ionization threshold mass spectrometry as a function of time in order to determine the wall loss time t_wN from the exponential decay curves. The procedure is performed with two mass spectrometers on different positions in the plasma chamber. t_wN is determined for various pressures, i.e., for 3.0, 5.0, 7.5, and 10 Pa. For this conditions also the internal plasma parameters electron density n_e and electron temperature T_e are determined with the Langmuir probe and the rotational temperature T_rot^N₂ of N₂ is determined with the optical emission spectroscopy. For T_rot^N₂, a procedure is presented to evaluate the spectrum of the transition υ^′=0→υ^″=2 of the second positive system (C³Π_u→B³Π_g) of N₂. With this method, a gas temperature of 610 K is determined. For both mass spectrometers, an increase of the wall loss times of atomic nitrogen with increasing pressure is observed. The wall loss time measured with the first mass spectrometer in the radial center of the cylindrical plasma vessel increases linearly from 0.31 ms for 3 Pa to 0.82 ms for 10 Pa. The wall loss time measured with the second mass spectrometer (further away from the discharge) is about 4 times higher. A model is applied to describe the measured t_wN. The main loss mechanism of atomic nitrogen for the considered pressure is diffusion to the wall. The surface loss probability β_N of atomic nitrogen on stainless steel was derived from t_wN and is found to be 1 for the present conditions. The difference in wall loss times measured with the mass spectrometers on different positions in the plasma chamber is attributed to the different diffusion lengths