The inertia of a collapsing bubble generates high pressures and temperatures capable of ionizing a small fraction of the noble gas within the volume of the bubble. This small fraction of ionized gas is transparent and allows for volume emission to be detected. Free electrons from the ionized noble gas begin to interact with other neutral atoms causing thermal bremsstrahlung radiation. Surface emission emits a more intense flash of light with a longer duration and is dependent on wavelength. Experimental data suggest that only volume emission occurs in the case of sonoluminescence. As the sound wave reaches a low energy trough the bubble expands and electrons are able to recombine with free ions and halt light emission.  Light pulse time is dependent on the ionization energy of the noble gas with argon having a light pulse of 160 picoseconds.