The techniques of computer simulation are used to generate the velocity distribution of ions emerging near close packed planes from a crystal surface. We concern ourselves in particular with protons and α-particles, 5 to 100 keV, near (100) planes in copper. The ions are initially emitted from either a lattice site-the ‘blocking case ‘or any other given point within a unit cell’ intermediate emission’. The resultant profiles are compared with an analytic model due to Andersen(6)based on the continuum approximation, and the validity of this model is thereby assessed. A simpler analytic model in which the interplanar continuum potential is approximated by a pure harmonic is also developed and compared with the previous two. Using the harmonic approximation we obtain a relatively simple relationship between the yield measured exactly parallel to a plane and the position of the original emission site. It is suggested that such a relationship could be of importance for experiments that can measure nuclear lifetimes of the order of 10−18to 10−16seconds.