We develop a theoretical approach to characterize the dynamics of a plasma
surface irradiated by a high-intensity electromagnetic wave. The method is
based on the analysis of the harmonic content in the reflection of a probe
field, which is aimed at the plasma simultaneously with the strong field. I
n particular, a explicit formula for the angles of reflection of these harm
onics is derived, showing a particular distribution directly related to the
frequency of oscillation of the vacuum-plasma interface. Also, a system of
equations for the reflected field amplitudes is formulated, and solved num
erically in some particular cases. The experimental implementation of this
scheme would provide a direct test of the so-called moving mirror models, w
hich presently provide a basis for the understanding of the intense field-p
lasma interaction.