Failure of classical scaling in low-velocity charge transfer from Rydberg atoms

Recently measured cross sections for electron loss and charge transfer from laser-excited Na Rydberg targets are compared with experimental and theoretical cross sections for capture to all states in p+H(1s) collisions at intermediate velocities. For projectile velocities greater than 1.2 times the Bohr velocity of the target initial state the classical $n^4$ scaling rule correctly relates ground-state and Rydberg charge-transfer cross sections. However, for projectile velocities between 0.27 and 1.2 times the Bohr velocity a significant departure from $n^4$ scaling occurs, and after scaling, the capture cross section from Rydberg targets is up to 4 times greater than that from H(1s). The effect is the same for l=0 and for l-mixed Na targets and is evident also in independently obtained data on capture from hydrogen Rydberg atoms. A significant difference between the dynamic response of Rydberg and H(1s) states is discussed, and theoretical investigation is called for.