Study of fundamental processes affecting the structure of the cathode zone of nitrogen/titanium dc discharge

Abstract

Processes affecting the emission of the cathode zone plasma of a N2/Ti dc discharge were studied experimentally and theoretically. Light maxima occurring at the negative glow head (maximum II) are due to the electron-molecule excitation. Emission characteristics of this region, space distributions of lighting, dependence of the line intensity (I) on the gas pressure (p) and current density (jd) can be fully explained by the Monte Carlo (M-C) simulation of electron motion in this region. Emission characteristics of the space close to the cathode surface (maximum I) are complex, depending on the emitting species. Molecular lines are found to be emitted mostly due to the fast ion-N2 interactions. The results of the analytical modelling of this process confirm the experimentally observed almost linear dependences of the intensities of these lines on the pressure and current density. Lines of atomic nitrogen are found to be due to the fast ion-N2 interactions as well as to the fast ion-solid excitation, both accompanied by a dissociation. Particular attention was paid to the emission of sputtered material lighting in the space close to the cathode surface. This lighting results from the fast ion-sputtered atom interaction as well as from that of the sputtered atom with `hot' atoms of the cathode surface layer. The experimentally observed dependences of the sputtered atom line intensity on the gas pressure (I~1/p) and the discharge current density (I~jd × jd) are qualitatively explained with a complex model that takes into account the analytical assessment of the energy distribution of bombarding ions, the M-C simulation of the ion-solid interaction and the M-C simulation of the motion of atoms roaming in the plasma phase. The plasma of the space close to the cathode surface (maximum I) of low-pressure discharges (~0.1 Torr) is an effective light source for the optical emission spectroscopy of buffer gas and solid species.