Ion–ion oscillation and periodic emission pattern in electronegative capacitively coupled radio frequency plasmas

Experimental observation and numerical analysis of low-frequency (LF) waveform-induced ion–plasma oscillations in low-pressure capacitively coupled plasmas operated in argon/oxygen mixtures are reported. We investigate the spatiotemporal excitation dynamics of dual-frequency (high-frequency, HF = 13.56 MHz and low-frequency, LF = 135–400 kHz) voltage-driven discharges using phase-resolved optical emission spectroscopy. Under oxygen-rich and low-pressure conditions (<10 Pa), distinct megahertz-range oscillations emerge in the excitation rates, indicating the onset of ion–ion plasma wave phenomena. These oscillations are predominantly triggered by the sharp transitions of external LF waveforms and persist throughout the LF cycle. One-dimensional particle-in-cell/Monte Carlo simulations reproduce the observed oscillation patterns and confirm their origin as electrostatic waves driven by LF-induced charge separation between positive and negative ions. The oscillation frequency is found to depend on ion density and gas composition, providing sheath modulation at a time scale intermediate between the HF and LF components. These waves offer new insight into plasma heating mechanisms in dual-frequency discharges and serve as a sensitive diagnostic of plasma conditions.