Hydrogen Balmer-α line broadening in a microwave plasma source

Abstract

Emission spectroscopy was used to investigate the Balmer-α line broadening in a slot-antenna excited microwave plasma source operating in helium–hydrogen and argon–hydrogen mixtures, and in pure hydrogen at low pressure (p = 0.3 mbar). No evidence was found for excessive broadening of the Hα line under the present operating conditions, even though this line was found to be broader than the helium singlet line at 667.8 nm. The Doppler temperatures corresponding to this helium line (∼650 K) are close to the rotational temperatures (∼500 K) determined from the Q-branch of the Fulcher-α band under the same conditions. The Doppler temperatures corresponding to the Balmer-α broadening are about 3000 K. The Hα line shape does not change as the amount of hydrogen (up to 50%) in the He–H2 mixture is changed. The results indicate selective heating of H atoms in low-pressure He–H2, Ar–H2 and H2 microwave plasmas.