Hot hydrogen atoms in a water-vapor microwave plasma source

A study of the hydrogen Balmer line shape in a water-vapor, microwave slot-antenna excited plasma source operated at 2.45 GHz is reported. The emission profiles of the Hα and Hβ lines are well fitted by Gaussian profiles. Excited hydrogen atoms are detected in the remote plasma zone of the source up to 30 cm distance from the exciting antennas. The measured Doppler temperature corresponding to the Hβ line broadening is about three times higher than the rotational temperature of the hydrogen molecular Fulcher-α band. It has been found clear evidence for the existence of a local source of excited “hot” hydrogen atoms in the “microwave field free” remote plasma zone. The measured Doppler broadening of the O(777.4 nm) triplet line indicates that “hot” oxygen atoms, with an energy around 0.3 eV, are also created in this source. Exothermic electron–ion and ion–ion recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are possible local sources of “hot” atoms in the far remote plasma zone.