Low-energy peak structure in strong-field ionization by midinfrared laser pulses: Two-dimensional focusing by the atomic potential

We analyze the formation of the low-energy structure (LES) in above-threshold ionization spectra in strong-field ionization by midinfrared laser pulses by using both quasiclassical and quantum approaches. We show this structure to be largely classical in origin, resulting from a two-dimensional focusing in the energy--angular-momentum plane of the strong-field dynamics in the presence of the atomic potential. The latter is shown to cause the LES even in the absence of a long-range Coulomb tail. The peak at low energy is strongly correlated with high angular momenta of the photoelectrons. Quantum simulations confirm this scenario. We find the LES to be remarkably sensitive to the carrier-envelope phase of the laser field.