Product HD kinetic energy distributions are reported for the incident gas phase H atom abstraction of D adsorbed on a monodeuteride-terminated Si(100) surface. The H atoms are generated by laser photolysis of HI and have well-defined kinetic energies in the range of 1-3 eV. For an incident H atom average kinetic energy of [E-H] = 1.1 eV, the HD product kinetic energy distribution has a mean value of [E-HD] = 1.2-1.3 eV and extends up to the nominal available-energy limit, providing dynamical evidence for a direct Eley-Rideal mechanism for this abstraction reaction. For [EH] = 1.5 and 3.2 eV, the HD product kinetic energy distribution broadens relative to that for [E-H] = 1.1 eV while [E-HD] remains unchanged, suggesting that energy loss to the substrate becomes more significant and the reaction becomes less Eley-Rideal-like for these higher energies. The results are compared with recent classical trajectory calculations.