Unveiling Under-the-Barrier Electron Dynamics in Strong Field Tunneling

Ever since the advent of quantum mechanics, tunneling has been an intriguing topic and consequently extensively studied and utilized. Investigating both theoretically and experimentally the nonadiabatic tunneling in strong-field ionization across a wide range of laser intensities, we unravel under-the-barrier-recollision dynamics leading to Freeman resonances (FR). The under-the-barrier-recollision model, which goes beyond the traditional direct multiphoton transition description, predicts distinct features of FR phenomena that cannot be explained by the existing direct multiphoton transition scenario. Specifically, it predicts (i) the dominance of high-order FR over above-threshold ionization in the photoelectron energy spectra and (ii) the flat dependence of the FR signal on the laser intensity, both in the nonadiabatic tunneling regime. This Letter experimentally demonstrates these features, corroborating the under-the-barrier-recollision model, and provides intuition into this dynamics, expanding our insights into the control of tunneling dynamics in laser spectroscopy and attosecond physics.