Wall loss of atomic nitrogen determined by ionization threshold mass spectrometry

Abstract

In the afterglow of an inductively coupled N 2 plasma relative N atom densities are measured by ionization threshold mass spectrometry (ITMS) 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 N 2 rot of N 2 is determined with the optical emission spectroscopy. For T N 2 rot a procedure is presented to evaluate the spectrum of the transition v' = 0 → v" = 2 of the second positive system (C 3 Π u → B 3 Π g ) of N 2 . 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.

Measurement and modeling of neutral, radical, and ion densities in H2-N2-Ar plasmas

“…T rot from H 2 and N 2 were in very good agreement with each other within the experimental uncertainty (see Ref. 43 ). T rot is, therefore, taken as an estimate of the gas temperature T g and is applied to calculate the gas densities according to an ideal gas law:…”

“…The experimental technique applied here is described in detail in Ref. 43 . For the measurement of relative radical densities the plasma monitor in the neutral modus is used operating with a fixed electron energy E cath .…”

“…For further details see Ref. 43 . The signal S is computed from S = N/(m cycle × t dwell ) (N -counting events, m cycle -number of pulses, t dwell = 0.05 ms -dwell time for N ) and has the unit counts per second (cps).…”

“…The determination of the used E cath for N is explained in detail in Ref. 43 . For the radicals hydrogen H on mass channel 1 amu/e (deuterium D on 2 amu/e), NH on mass channel 15 amu/e (ND on 16 amu/e), and NH 2 on mass channel 16 amu/e (ND 2 on 18 amu/e), the electron energies E cath (H + ) = 15 eV, E cath (NH + ) = 16 eV, and E cath (NH + 2 ) = 14 eV are used, respectively 54 .…”

“…A mean value of T rot = 460 ± 50 K is obtained. We want to note that in a former study 43 the rotational temperature of molecular nitrogen was evaluated from the spectrum of the transition v' = 0 → v" = 2 of the second positive system (C 3 Π u → B 3 Π g ) of N 2 . T rot from H 2 and N 2 were in very good agreement with each other within the experimental uncertainty (see Ref.…”