A study describes a method of determining temperatures based on the formation of plasmas. Using argon bubbles in sulfuric acid, the data shows the presence of ionized molecular oxygen O 2, sulfur monoxide, and atomic argon populating high-energy excited states, which confirms a hypothesis that the bubbles have a hot plasma core. The ionization and excitation energy of dioxygenyl cations, which they observed, is 18 electronvolts (2.9 × 10 J). From this observation, they conclude the core temperatures reach at least 20,000 K (19,700 °C; 35,500 °F) —hotter than the surface of the Sun.