Comment on "Observation of Collective Modes of Ultracold Plasmas"

We discuss a theoretical interpretation of the experiment by Fletcher, Zhang, and Rolston [PRL, 96, 105003 (2006)], who revealed a number of subharmonics of the electron emission under external irradiation of the freely-expanding clouds of ultracold plasmas. The original interpretation of these subharmonics as standing waves of plasma oscillations (the so-called Tonks-Dattner resonances) encounters a number of problems, such as the lack of adequate boundary conditions and incorrect dependence on the plasma temperature and the degree of neutrality. It is the aim of the present comment to mention that a more promising interpretation may be based on the peaks in the efficiency of ionization of the secondary Rydberg atoms, formed during the plasma cloud expansion and cooling. The presence of the last-mentioned peaks is well supported by the independent experiment by Maeda and Gallagher [PRL, 93, 193002 (2004)].