We report the desorption of a chemisorbed polyatomic adsorbate from a transition metal surface by a beam of neutral, energetic Ar atoms. From direct measurement of the threshold energy for collision-induced desorption we calculate the surface-adsorbate bond energy. The absolute cross sections for NH3 desorption at one-quarter of a monolayer coverage were measured for Ar beam energies up to similar to 3 eV at incident angles of 0 degrees, 30 degrees, 45 degrees, and 60 degrees. For the NH3/Pt{111} adsorbate-surface system, the threshold desorption energy is found to be 1.95+/-0.17 eV. Using a classical energy transfer mechanism this threshold energy corresponds to a bond energy of 1.1+/-0.1 eV using an effective mass of 1 Pt atom. The threshold desorption energy scales with the total energy of the noble gas atoms for each angle of incidence. This result is consistent with a strong lateral corrugation in the NH3/Pt{111} potential energy surface and a similar ejection mechanism at each angle.