Applying a radio-frequency electric field to an expanding ultracold neutral plasma leads to the observation of as many as six peaks in the emission of electrons from the plasma. These are identified as collective modes of the plasma and are in qualitative agreement with a model of Tonks-Dattner resonances, electron sound waves propagating in a finite-sized, inhomogeneous plasma. Such modes may provide an accurate method to determine the time-dependent electron temperature.

• Received 16 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.105003