High-resolution attosecond imaging of an atomic electron wave function in momentum space

An electron wave function is characterized by the phase and amplitude distributions over position or momentum space. Recent attosecond technologies allow us to obtain the phase information of photoelectrons using interference between several optical transition pathways. We demonstrate that by employing a two-path photoionization interference process, the complex wave function of the photoelectron is fully mapped in two-dimensional momentum space. We ionize neon gas by an extreme ultraviolet (XUV) attosecond pulse train consisting of both odd and even harmonics in the presence of an infrared (IR) laser field. By controlling the generation process of the attosecond pulse train, we isolate two ionization pathways for interfering with the photoelectrons: one is the two-photon ionization process due to the odd harmonic excitation with one IR photon absorption, and the other is the one-photon ionization by an even harmonic. We record the photoelectron momentum distributions via velocity map imaging as a function of the XUV and IR delay. Using three different experimental conditions, we show that the detailed structure of the amplitude and phase distributions of photoelectrons can be resolved in the two-dimensional momentum space within the bandwidth determined by the attosecond XUV pulse. We separate the measured photoelectron wave function into those produced by each ionization pathway. Our method will be applicable to more complex molecules.