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An amplitude modulated surface plasma wave, produced via mode conversion of a laser, is shown to resonantly excite a surface plasma wave at the modulation frequency over a rippled metal free space interface. The pump surface plasma wave (SPW) exerts a ponderomotive force on metal electrons, imparting them an oscillatory velocity at the modulation frequency. This velocity beats with the surface modulation to produce a nonlinear current driving the surface plasma wave at the modulation frequency which may be chosen in the terahertz range. The presence of surface modulation is essential for effective coupling of surface plasma wave to terahertz wave.