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

Abstract

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 N2 plasma, the rotational temperatures of the second positive system (2PS) and first negative system (1NS) are measured, whereas for O2 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 N2 plasma, while both have almost the same temperature for the O2 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.

Nonequilibrium characteristics in the rotational temperature of CO excited states in microwave discharge CO2 plasma

“…Electron temperature and density measured by the Langmuir double probe First, to understand the basic characteristics of the CO 2 plasma, the Langmuir double probe measurement was conducted at the longitudinal position z = 50 and 100 mm on the discharge tube axis. 34,35) The electron temperature T ele = 2.2 eV at z = 50 mm and T ele = 1.8 eV at z = 100 mm, while the electron density N ele = 1.5 × 10 12 cm −3 at z = 50 mm and N ele = 8.0 × 10 11 cm −3 at z = 100 mm. These observed values are considerably common to the microwave discharge plasmas of any gaseous species in the present device.…”

“…These observed values are considerably common to the microwave discharge plasmas of any gaseous species in the present device. 34,35,39,40,43) The characteristics of the nonequilibrium were considered to have been caused by the low ionization degree and the resultant electron collisions with atoms and molecules, which were more essential than the Coulomb collisions between the charged species.…”

“…It is identical to the one applied in the previous experiments. [34][35][36][37][38][39][40][41][42][43][44][45][46] The magnetron generates a microwave with a frequency of 2.45 GHz, as a general-purpose variable output microwave oscillator with its output of 0.2-4.9 kW equipped with a built-in isolator. It was connected to the discharge tube via a set of waveguides with a power monitor and a 3-stub tuner, which was finally terminated with a movable short-circuited plunger.…”

“…Finally, even if the rotating structure is not separated, each transition emission line is superimposed, thereby assuming a Gaussian broadening that comprises instrumental functions. [34][35][36][37]49) Thus, the superimposed overall band spectrum is obtained as a function of T v and T r . This theoretical calculation was performed for the CO 3PS and CO Ångstrom bands.…”

“…Then, theoretical fitting is performed on the obtained emission spectra to obtain the values of temperatures T v and T r of each band. To determine T v and T r using a spectral fitting procedure with the band spectra, the same procedure can be applied for the CO radicals as the OH A-X transition 34) or O 2 atmospheric absorption band in the O 2 plasma, 35) the N 2 1PS or 2PS bands in the N 2 plasma, 36) the NO γ band of the N 2 -O 2 -mixture plasma, 37) and so on. From the obtained parameters, a deeper discussion is provided on the CO excited states by applying the Boltzmann plots of rotational levels or by employing molecular collisional processes in the plasma.…”