Alpha to gamma mode transition in hydrogen capacitive radio-frequency discharge

Electron energy distribution functions (EEDFs) were measured with increasing discharge voltages in hydrogen capacitively coupled plasmas by means of radio-frequency compensated Langmuir probe. The results are compared with EEDF in argon plasmas. It was found that, in the hydrogen capacitive discharge, abnormally low-energy electrons became highly populated and the EEDF evolved to a non-Maxwellian distribution as the discharge voltage was increased. This voltage dependence of the EEDF in the hydrogen is contrary to argon capacitively coupled plasma, where at high discharge voltage, low-energy electrons are significantly thermalized due to γ heating and the EEDF evolves to the Maxwellian distribution. The highly populated low-energy electrons at high gas pressure, which was not observed in capacitively coupled argon plasma, show that the γ heating mechanism is somehow inefficient in terms of the molecular gas in capacitive discharges. It appears that this inefficient γ heating seems to be attributed to an efficient vibrational excitation in hydrogen capacitive plasma.