A rotational temperature comparative study of OH radical vs. N<sub>2</sub><sup>+</sup> was carried out on a low-power helium microwave-induced plasma. Under the prevailing conditions, N<sub>2</sub><sup>+</sup> was found to provide twice as many usable lines for temperature measurement than did hydroxyl radical. For the particular torch design used, both species exhibited slightly increasing rotational temperatures at lower flow rates. At fixed conditions, OH consistently indicated higher rotational temperatures than those of the molecular nitrogen ion. Positional studies revealed a slightly increasing temperature near the center of the plasma. This work suggests that N<sub>2</sub><sup>+</sup> may provide a number of advantages over OH radical as a thermometric probe species in the determination of plasma rotational temperature.
John M. Workman, P. A. Fleitz, Harry B. Fannin, Joseph A. Caruso, and C. J. Seliskar, "A Comparative Study of Rotational Temperatures in a Microwave Plasma: OH Radical Versus N2+," Appl. Spectrosc. 42, 96-100 (1988)