Page "Sonoluminescence" Paragraph 22

During bubble collapse, the inertia of the surrounding water causes high pressure and high temperature, reaching around 10, 000 kelvins in the interior of the bubble, causing the ionization of a small fraction of the noble gas present.

The amount ionized is small enough for the bubble to remain transparent, allowing volume emission ; surface emission would produce more intense light of longer duration, dependent on wavelength, contradicting experimental results.

Electrons from ionized atoms interact mainly with neutral atoms, causing thermal bremsstrahlung radiation.

As the wave hits a low energy trough, the pressure drops, allowing electrons to recombine with atoms and light emission to cease due to this lack of free electrons.

This makes for a 160-picosecond light pulse for argon ( even a small drop in temperature causes a large drop in ionization, due to the large ionization energy relative to photon energy ).

This description is simplified from the literature above, which details various steps of differing duration from 15 microseconds ( expansion ) to 100 picoseconds ( emission ).