Detection of Intermediate Energy Structures from Re-Scattered Electrons Following Strong-Field Ionization of Alkali Atoms*

We explain experimental measurements of "bump-like" enhancements in electron emission, observed in the energy distribution for electrons emitted from alkali atoms driven by infrared lasers at non-perturbative intensities. These emissions occur at energies approaching twice the ponderomotive energy ($2U_\text{p}$). Our findings rule out the half-cycle interference mechanism, previously known to produce similar emissions from "direct" electrons produced from negative ions. Instead, we observe a different mechanism where electrons "re-scatter" off the Coulomb potential. Our results are interpreted in terms of Keldysh's adiabaticity parameter ($\gamma_\text{K}$) using a Coulomb-corrected version of the strong-field approximation (SFA) and "complex" electronic trajectories.