Measurements have been made using thermal desorption spectrometry of the entrance probability η of He+ions incident on a W(100) surface at energies 8 eV to 5000 eV. For energies above 250 eV, η for normal incidence, ηn, is within 20 of unity, while for “random” incidence ηn= 0.6 ± 0.1. Below 250 eV, both decrease and become equal forE≤ 25 eV. At 25 eV, η = 0.2 and at 8 eV, η ≈ 0.01.