  We consider the parametric excitation of a Langmuir wave and an
electromagnetic wave by gravitational radiation, in a thin plasma on a
Minkowski background. We calculate the coupling coefficients starting from a
kinetic description, and the growth rate of the instability is found. The
Manley-Rowe relations are fulfilled only in the limit of a cold plasma. As a
consequence, it is generally difficult to view the process quantum
mechanically, i.e. as the decay of a graviton into a photon and a plasmon.
Finally we discuss the relevance of our investigation to realistic physical
situations.
