Difference in Rotational Temperatures between Neutral Molecules and Molecular Ions of Low-Pressure Discharge N2-O2 Plasmas

For a microwave discharge nitrogen plasma with its discharge pressure about 1 Torr, our OES measurement showed that the rotational temperature of N$_2^+$ B state by the first negative system (1NS) is about 1.5 times higher than that of N$_2$ C state by the second positive system (2PS). Meanwhile, it is found that the rotational temperature of O$_2^+$ b state by 1NS is almost the same as that of O$_2$ b state by the atmospheric absorption band, which is quite different from N$_2$ plasma. We consider that the rotational temperature of the ground state O$_2^+$ X ion should be higher than that of O$_2^+$ b state due to difference in the internuclear distance, where that of the O$_2^+$ b state is much larger than that of the ground state O$_2^+$ X. The angular momentum of both X and b states are almost conserved before and after the electron impact excitation due to small mass of an electron. Therefore, the rotational temperature of the X state of O$_2^+$ ion should be estimated to be about 1.3 times of that of O$_2^+$ b state. This value gives a similar result with that of nitrogen plasma, where the internuclear distances of B and X states of N$_2^+$ are almost the same. It is considered that the ground-state molecular ion has higher rotational temperature than neutral molecule.