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Collective modes in free plasmas subjected to a radiation field

In this study we report the effects of an external electromagnetic field on the collective properties of unmagnetized plasmas. The calculations are carried out in the semi-classical approximation, i.e., the electromagnetic field is treated classically and the electrons from a quantum mechanical viewpoint. The results show that the collective modes are damped away more smoothly and in a smaller frequency range than those reported by previous studies. An exponential-like decay of the plasmon frequencies as a function of the external field amplitude is readily observed. We successfully recreate the results of previous studies. We also find that the single photon processes has a pronounced effect on the decrease of the frequency range of modulation.