Abstract
An intense laser incident on a plasma relativistically modulates the electron mass at the second harmonic. Near the critical layer, the oscillatory mass couples two oppositely propagating Langmuir waves which grow at a rate Γ ∼ ωpv20/16c2, where ωp is the plasma frequency, v0 is the oscillatory electron velocity and c is the velocity of light in vacuum. The decay is accompanied by the excitation of a low-frequency mode which at large wave number (kc > ωp) enhances the growth rate of the two-plasmon process.