Plasma formation and temperature measurement during single-bubble cavitation.

Area of Science:

• Physical Chemistry
• Plasma Physics
• Spectroscopy

Background:

• Single-bubble sonoluminescence (SBSL) involves extreme conditions within collapsing bubbles.
• Previous studies predicted a plasma core but lacked direct experimental evidence.
• Standard SBSL spectra offer limited insight into intra-bubble conditions.

Purpose of the Study:

• To investigate the physical conditions and chemical processes occurring during SBSL.
• To provide experimental evidence for the existence of a plasma core in SBSL.
• To analyze SBSL spectra for characteristic emission lines and molecular bands.

Main Methods:

• Generated SBSL in concentrated aqueous sulfuric acid solutions.
• Analyzed the emission spectra using high-resolution spectroscopy.
• Identified atomic (Ar), molecular (SO), and ionic (O2+) emission features.

Main Results:

• Observed distinct atomic (Ar) and molecular (SO) emission lines, enabling temperature determination (4,000–15,000 K).
• Detected extensive molecular (SO) and ionic (O2+) progressions in the SBSL spectra.
• Identified highly energetic excited states (>13 eV for Ar) inconsistent with thermal population.

Conclusions:

• The observed spectral features, particularly high-energy states, strongly indicate the presence of a plasma core.
• Emitting species originate from collisions with high-energy electrons, ions, or particles within the plasma.
• This study provides the first robust experimental evidence for a plasma core in SBSL.