Full particle particle-in-cell simulations are performed to study the collisionless electron-ion coupling during ion beam emission and neutralization. Simulations show that ion beam neutralization and propagation are two closely coupled processes. Electron-ion coupling is achieved through interactions between the trapped electrons and a potential well established by the propagation of the ion beam front along the beam direction and not through plasma instabilities as previous studies suggested. In the transverse direction, the beam emission generates an expansion fan similar to that of the expansion of a mesothermal plasma into vacuum. The expansion process determines the beam potential with respect to the ambient. This suggests that a plasma beam in a vacuum chamber and a plasma beam in space may have similar charge density profiles but different beam potentials with respect to the ambient due to the limit imposed by the boundary on plasma expansion.