Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics. DOI: 10.1103/PhysRevLett.109.073004 PACS numbers: 32.80.Fb, 32.80.Rm, 34.80.Qb When an atom or molecule is illuminated with a moderately intense femtosecond laser field ($ 10 14 W=cm 2 ), an electron wave packet will tunnel ionize and accelerate in the field before being turned around by the field and returning to the parent ion. The returning electron can either recombine with the parent ion, releasing its kinetic energy as a high-energy photon [1-3], or can elastically scatter from the potential of the ion. The photons and electrons generated by these strong-field processes have the potential to probe the dynamic structure of molecules and materials on the subnanometer length scale and femtosecond-to-attosecond time scale. Several recent papers have suggested that structures seen in angledependent photoelectron spectra may be useful for determining time-resolved molecular structures [4], characterizing attosecond electron wave packets [5], and studying the dynamics of electron wave packet propagation [6]. However, despite extensive analyses [7][8][9][10][11], many features observed in angle-resolved photoelectron spectra still lack a simple physical explanation.The recent development of midinfrared (mid-IR) femtosecond lasers [12] and angle-resolved detection schemes [13] has enabled new advances in visualizing strong-field physics. Electrons that are ionized in a mid-IR laser field reach higher velocities because of the larger ponderomotive energy, given by U P / I 2 , where I is the intensity and is the wavelength. The possibility of harnessing the high-energy electrons that are first ionized and then driven back to a molecule by a strong laser field has inspired several theoretical and experimental efforts to use strongfield ionization to probe molecular structure [4,[14][15][16]. Recently, Huismans and co-workers [17] used 7 m mid-IR lasers, in combination with angle-resolved detection, to observe angular interference structures in the photoelectron spectra. They presented a theoretical model that explains these structures based on the difference in the phase between two different paths that electrons can take to...