Langmuir probe diagnostics of electron energy distributions with optical emission spectroscopy in capacitively coupled rf discharge in nitrogen

Measurements with a rf compensated Langmuir probe and optical emission spectroscopy are carried out in capacitively coupled rf (13.56 MHz) pure nitrogen N-2 discharges at fixed rf voltage over a wide range of pressure, 30 to 400 mTorr. The electron energy probability function (EEPF) measured below 100 mTorr resembles a bi-Maxwellian-type distribution. At pressure range of 100-200 mTorr, the EEPF has non-Maxwellian distribution with a "dip" near 4.5 eV. At the highest pressure of 400 mTorr, the EEPF evolves into a Druyvestein-like distribution and the "dip" disappears. The electron density significantly decreases with increase in the N-2 pressure. On the other hand, the electron temperatures gradually decrease with an increase in N-2 pressure, reaching minimum at 150 mTorr, beyond which it abruptly increases. Such evolution of the EEPFs shape with gas pressure has been discussed in terms of non-local electron kinetics and heating mode transition. The emission intensities of nitrogen (0-0) band of second positive system at 337.1 nm and (0-0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states N-2(C-3 Pi(u)) and N-2(+)(B-2 Sigma(+)(u)) with nitrogen pressure. It is observed that the pressure influences the radiative states differently owing to their different populating mechanisms. The vibrational temperature T-vib and rotational temperature T-rot are measured for the sequence (Delta nu = -2) of N-2 second positive system (C-3 Pi -> B-3 Pi(g)) using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that both T-vib and T-rot have similar dependences with N-2 pressure; peaked at 100 mTorr beyond which it monotonically decreases with increase in the N-2 pressure. The correlation between the observed maximum value of T-vib around 100 mTorr and the detected "dip" in the EEPF in the same pressure range has been discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3664858]