Radial-momentum-resolved Measurement of the Tunneling Ionization Time in Attoclock Experiments

We studied the tunneling ionization time in the elliptically polarized laser field with the recently proposed phase-of-the-phase spectroscopy, wherein a weak linearly polarized second-harmonic field was introduced to the attoclock frame. By monitoring the oscillation of the photoelectron yield with the relative phase between the two fields, the ionization time of the photoelectrons can be determined without any effort on handling the Coulomb interaction between the escaping electron and the parent ion. This advantage enables us to resolve the ionization time in the full photoelectron momentum distribution. By solving the time-dependent Schrodinger equation, we showed that the ionization time depends on the radial momentum of photoelectron. The photoelectron with larger radial momentum ionizes earlier at the emission angle before the most probable emission angle, while this radial-momentum dependence is reversed at the emission angle after the most probable emission angle. These results are confirmed by our experiments.