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Forces on microparticles in energetic ion beams are investigated experimentally. For this purpose, hollow glass microspheres are injected into a vertically upward directed beam. The particles are illuminated by a diode laser, and their scattered light is recorded with a charge-coupled device camera. From the trajectories, the acceleration and net force on the particles are determined. It is found that the force is significantly higher than the expected ion drag force. This additional part of the total force is explained with fast neutral atoms produced by charge-exchange collisions. Special attention is paid to the momentum contribution from sputtered atoms, which is shown to be negligible in this experiment.