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We discuss the second-harmonic generation of light at metal surfaces within the hydrodynamic theory of the electron gas; expressions for the phenomenological parameters a and b of Rudnick and Stern are presented, and the possibility of a resonance in a at optical or near-uv frequencies is discussed. A recent plasmon-enhanced experiment of Simon et al. is analyzed, and the use of such experiments to determine a and b is considered; new experiments are proposed to aid in such a determination.
Sipe et al. (Thu,) studied this question.