Mode conversion by double Cerenkov processes in a plasma

It is shown that the conversion of an electromagnetic wave (i.e., a transverse photon) to an electrostatic wave (i.e., a longitudinal plasmon) and vice versa can occur in a uniform, homogeneous and isotropic plasma by Cerenkov absorption of one kind and the subsequent emission of the other kind. Furthermore, it is shown that there also exists an additional 'stimulated mode conversion process'. These mode conversions by double Cerenkov processes are of second order in the Heitler-Ma formulation of the quantum time-dependent perturbation theory. Explicit expressions for the cross sections appropriate to these mode conversion processes are also presented.