Spectroscopic determination of cold electrons in electron cyclotron resonance discharges with highly charged ions

Abstract

A spectroscopic method based on the analysis of molecular and atomic spectra of nitrogen was used for the determination of electron distribution functions (EDFs) in electron cyclotron resonance (ECR) discharges run in nitrogen. EDFs were determined for the plasma region seen by the VUV-monochromator. This region includes the hot core plasma surrounded by a less dense and not so hot halo plasma. The EDF between 1.5 and 4.5 eV was determined from the vibrational distribution of excited molecular states of nitrogen, and that in the energy range above 8.5 eV was evaluated from the intensities of different emission lines and bands of NI and N2. The shape of the EDF in the energy range between 4.5 and 8.5 eV was interpolated by using the electron density measured with Langmuir probes and normalizing the EDF to the same density. Due to the low gas pressure of 2×10−5 Torr vibrational relaxation processes on the discharge chamber wall had to be taken into account. In the wavelengths range 30–400 nm the emission spectra of the ECR discharge were measured for the input microwave power range Pin=100–1500 W. This wavelength range includes the emission bands of the molecular transitions N2(C3Πu→B3Πg), N2(a1Πg→X1Σg−), and N2+(B2Σu−→X2Σg+) and emission lines of the atoms and ions e.g., transitions N(4P→4S, 120.0 nm), N(2P→2P, 124.3 nm), and N+(3P→3P, 91.6 nm), N+(3D→2P, 108.4 nm). For these transitions the experimental emission cross sections for the excitation from N2(X) and the some intermediate stable states [e.g., from the atomic ground state N(4S)] are known from the literature. The other cross sections used in our EDF calculations especially N and N+ excitation cross sections were calculated in this work.