Experimental study on difference in molecular rotation temperatures between neutral molecules and molecular ions in nitrogen plasma and oxygen plasma by optical emission spectroscopy measurement

The differences in the rotational temperatures between neutral molecules and molecular ions are examined for low-pressure microwave discharge nitrogen plasma and oxygen plasma. For N-2 plasma, the rotational temperatures of the second positive system (2PS) and first negative system (1NS) are measured, whereas for O-2 plasma, those of the atmospheric absorption band ( A-band) and 1NS are measured for neutrals and ions, respectively. The experimentally measured spectra are compared with the theoretically calculated ones to determine the vibrational and rotational temperatures as best fitting parameters. It is found that the ions have a higher rotational temperature than neutrals for the N-2 plasma, while both have almost the same temperature for the O-2 plasma, for which a discussion is made on the basis of atomic and molecular processes. The difference in the rotational temperature between the neutrals and the ions is also discussed from the viewpoint of the atomic and molecular processes in the plasmas. (C) 2017 The Japan Society of Applied Physics