Explanation for dip in literature
1. Rescattering:
J. Chen et al, PRA 63  11404(R) (2000)
2. Coulomb potential:
K. Dimitriou et al,  PRA 70  061401(R) (2004)
3. Position of ATI peaks:  (in tunneling regime)
F. H. M. Faisal et al,   J. Phys. B  38  L223  (2005)
4. Freeman Resonance:
A. Rudenko et al,  J. Phys. B 37  L407  (2004)