Investigation of Penning Ionization in Helium CCRF Plasma via Optical Emission Spectroscopy

This study investigates energy transfer mechanisms between helium metastable atoms and hydrogen atoms by integrating capacitively coupled radio-frequency (CCRF) plasma with optical emission spectroscopy (OES). Plasma is generated using a 13.56 MHz RF source with helium, argon, and ambient air as working gases, over a pressure range of 0.1 to 5 mbar. Emission spectra are acquired using a high-resolution spectrograph with an EMCCD detector.

In addition to the characteristic lines of the working gases, hydrogen Balmer series and nitrogen molecular bands are detected, primarily due to residual water vapor and air in the chamber. A pronounced enhancement in hydrogen Balmer line intensity is specifically observed in helium discharges—absent in argon and air plasmas.This enhancement is attributed to energy transfer from helium metastables to hydrogen atoms via Penning ionization. As helium metastable energy levels exceed the ionization energy of hydrogen, this interaction leads to ionization and subsequent recombination, producing enhanced Balmer emissions. The intensity of these lines in helium plasma shows a strong correlation with plasma density, supporting this mechanism. No such trend is noted in argon or air discharges.

These results enhance understanding of metastable-driven processes in low-temperature plasmas and suggest a non-invasive spectroscopic method for detecting hydrogen contamination in materials. Detailed correlations with plasma parameters will be presented.