  Ionization of an atom or molecule presents surprising richness beyond our
current understanding: strong-field ionization with low-frequency fields
recently revealed unexpected kinetic energy structures (1, 2). A solid grasp on
electron dynamics is however pre-requisite for attosecond-resolution
recollision imaging (3), orbital tomography (4), for coherent sources of keV
light (5), or to produce zeptosecond-duration x-rays (6). We present a
methodology that enables scrutinizing strong-field dynamics at an unprecedented
level. Our method provides high-precision measurements only 1 meV above the
threshold despite 5 orders higher ponderomotive energies. Such feat was
realized with a specifically developed ultrafast mid-IR light source in
combination with a reaction microscope. We observe electron dynamics in the
tunneling regime ({\gamma} = 0.3) and show first 3D momentum distributions
demonstrating surprising new observations of near-zero momentum electrons and
low momentum structures, below the eV, despite quiver energies of 95 eV.

Atomic Physics