J.-B. Kim, G. Lan, Q. Li, T.B. Lucatorto, T.J. McIlrath, and T.R. O'Brian
When an atom is subject to an intense, time-varying electric field (such as the electromagnetic field produced by intense laser radiation), the atomic energy levels are shifted by the external field, an effect called the light shift or ac Stark shift. A thorough understanding of these shifts is necessary to interpret results from experiments that use intense lasers to probe atomic structure. For a highly monochromatic laser not tuned to any atomic transition and for a Rydberg level with binding energy much less than the laser photon energy, theory predicts that the energy shift in the level will approach the value given by the so-called ponderomotive potential. In a Rydberg level, a single electron is in a large orbit loosely bound to the nucleus (which is embedded in the core of the remaining electrons), and this electron exhibits near-classical behavior in certain circumstances. The ponderomotive potential is simply the classical average kinetic energy a free electron gains when driven into oscillation by an external electromagnetic field.
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