Substantial Doppler broadening of atomic-hydrogen lines in dc and capacitively coupled RF plasmas

The mechanism of extraordinary broadening of the Balmer lines of hydrogen admixed with noble gases in a dc glow discharge and a capacitively coupled rf discharge is studied over a wide range of pressure and gas compositions to test the field acceleration model (Cvetanovic et al 2005 J. Appl. Phys. 97 033302). High-resolution optical emission spectroscopy is performed parallel to the electrode axis (end-on) and perpendicular to the electrode axis (side-on) along with Langmuir probe measurements of plasma density and electron temperature for the parallel plate rf capacitive discharge case. Sharp pin-shaped tungsten dc electrodes are also used to minimize the backscattering of ions that are theorized by a field acceleration model to be heated in the sheath region. An excessively broad and symmetric (Gaussian) Balmer emission line corresponding to 20-60 eV of hydrogen atom energy is observed in Ar/H(2) and He/H(2) plasmas when compared with the majority species atom temperatures. Energy is transferred selectively to hydrogen atoms whereas the atoms of admixed He and Ar gases remain cold (< 0.5 eV). Since there is neither a preferred ion nor atom in the field acceleration model, one should also observe enhanced temperature hydrogen and helium atoms in He/H(2) discharges where the atomic mass is more comparable (4 : 1).