Open Access

Sven Eric Grundmann

• The present work deals with photoionization in the realm of the absorption of one single photon. The formal treatment of one-photon ionization usually employs a semi-classical approach, where the electron’s initial and final states are described as quantum-mechanical wave functions but the photon is treated as a classical electromagnetic wave. In the calculation of photoionization cross sections with this semi-classical method, there is an often used approximation which is called the electric dipole approximation. Mathematically, the application of the dipole approximation corresponds to truncating the series expansion of an exponential after the leading term. Physically, this means neglecting the linear photon momentum and the spatial dependence of the light field. The dipole approximation is valid if the wavelength of the light is much larger than the spatial extent of the target and if the photon momentum is small compared to the momenta of the reaction products, which is generally the case for photon energies short above the electron binding energy. For the present work, we experimentally investigated nondipolar photoionization, i.e., one-photon ionization at high photon energies where the dipole approximation breaks down. In our experiments, we irradiated single atoms and molecules with such high-energetic photons and measured the three-dimensional momentum distributions of the reaction fragments to uncover the effects of the linear photon momentum and the spatially-dependent light field on photoionization. Our observations allow the first profound insight into photoionization that reveals all photon properties, i.e., photon energy, spin, linear momentum, and the speed of light. Hopefully, our efforts make a constructive contribution to the understanding and the further exploration of light-matter interaction.