Nitrogen dissociation degree in the diffusion region of a helicon plasma source obtained by atomic lines to molecular band intensities ratio

Estimates of the dissociation degree in the diffusion region of a nitrogen helicon plasma source based on optical emission spectroscopy and Langmuir probe measurements are presented. The estimation procedure relies on measurements of the ratios of the intensities of the atomic triplet 3pS04→3sP4 (742.36, 744.23, and 746.83nm) to the intensity of the 4-2 band of the first positive system (AΣu+3→BΠg3) at 750.39nm and the measured relative vibrational distribution of the BΠg3 state. The electron energy distribution function, obtained from the second derivative of the Langmuir probe characteristic, and published excitation cross sections are used to calculate the electron-impact excitation rate coefficients—which are then compared to the atomic line and molecular band intensities to calculate the dissociation degree. For two distinct operating regimes, capacitively and inductively coupled, dissociation fractions of 5% and 13% are obtained in the expansion region of a 10mTorr, 500W, 10.74MHz helicon generated ...