Semiclassical and quantum form factors in Rydberg-neutral collisions

Semi-analytical methods are presented for computing quantum form factors in the general non-hydrogenic case. For transitions between hydrogenic states, semiclassical and quantum form factors are compared in a large domain of momentum transfer. The free electron model is applied to the l-changing Rydberg-atom plus rare-gas collisions between nearly degenerate states on the one hand, and to inelastic n-changing Rydberg-atom plus rare-gas collisions on the other hand. It is shown that the free electron model can be justified in the first case for high values of n(>or=15) and that it breaks down in the second case when large momentum transfers are involved.