Diagnosing the plasma formed during acoustic cavitation in [BEPip][NTf₂] ionic liquid

Area of Science:

• Physical Chemistry
• Chemical Physics
• Materials Science

Background:

• Sonoluminescence (SL) is a phenomenon involving light emission from collapsing bubbles in liquids under acoustic cavitation.
• Ionic liquids are salts that are liquid at low temperatures, offering unique solvent properties.
• Understanding the plasma conditions within SL bubbles is crucial for various applications.

Purpose of the Study:

• To investigate the time-evolution of sonoluminescence spectra from a dry ionic liquid ([BEPip][NTf2]).
• To determine the rovibronic temperatures of molecular emitters (C2 and CN) during sonication.
• To correlate spectral changes with plasma properties like electron energy and excited state populations.

Main Methods:

• Measurements of sonoluminescence spectra of a dry [BEPip][NTf2] ionic liquid under Argon atmosphere.
• Analysis of molecular emissions (C2, CN, CH) to determine vibrational and rotational temperatures.
• Monitoring spectral evolution over the initial minutes of sonication.

Main Results:

• Observed molecular emissions from C2 and CN with vibrational temperatures around 5800-6000 K and rotational temperatures around 4000 K.
• These temperatures remained remarkably constant during the initial minutes of sonolysis.
• Significant changes in SL spectra occurred concurrently, indicating rapid plasma evolution and decreasing plasma electron energy.

Conclusions:

• The study demonstrates the utility of SL spectroscopy for probing transient plasma conditions in ionic liquids.
• Constant gas-phase temperatures suggest a stable core plasma despite evolving spectral features.
• The observed spectral evolution provides insights into the dynamics of the sonochemical plasma in the early stages of sonolysis.