On the enhanced Balmer emission of hydrogen in a helium capacitively coupled radio frequency (CCRF) plasma

Abstract

The present study investigates the observation of the hydrogen Balmer series emission in a capacitively coupled radio frequency (CCRF) helium plasma using optical emission spectroscopy (OES). Experimental evidence suggests that helium metastables transfer energy to trace amounts of water vapor present in the vacuum chamber. The plausible mechanism is the energy exchange between metastable helium and water molecules via Penning ionization. This occurs because the energy of helium metastables exceeds the dissociation energy of a water molecule and the ionization energy of hydrogen. The hydrogen ions that are formed subsequently combine with electrons in the plasma, resulting in the emission of the Balmer series during the subsequent relaxation process. The intensity observed in the Balmer lines indicates unconventional ratios between them, demonstrating a strong dependence on neutral pressure and RF power. This phenomenon is indicative of the role of the energetic tail population. The observation reveals a direct correlation between the intensity of Balmer line emission and the electron density. In order to comprehend the dependence of helium metastables on the Balmer series intensity, the collisional radiative (CR) model is employed to estimate the metastable population enhancement under the present experimental conditions. The observed enhancement can be attributed to the increase in metastable densities and the radiative recombination processes. The results of this study hold significant potential for applications, in addition to providing a fundamental understanding of energy transfer between the metastables of helium and water vapor.