In 2013, Aneta Stodolna and colleagues imaged the hydrogen atom's wave function by measuring an interference pattern on a 2D detector. The electrons are excited to their Rydberg state. In this state, the electron orbital is far from the centre nucleus. The Rydberg electron is in a dc field, which causes it to be above the classical ionization threshold, but below the field-free ionization energy. The electron wave ends up producing an interference pattern because the portion of the wave directed towards the 2D detector interferes with the portion directed away from the detector. This interference pattern shows a number of nodes that is consistent with the nodal structure of the Hydrogen atom orbital