7 and are mainly related to the relativistic regime for high intensities, to radiation pressure, and magnetic field displacement. Possible sources of dipole violations at longer wavelengths are summarized in ref. Recent investigations with optical lasers have demonstrated a breakdown of the dipole approximation under strong-field conditions in the long-wavelength regime 6, 7, 8. With the rapid evolution of ultrabright X-ray light sources such as free-electron lasers (FELs), the relevance of nondipole effects for photoionization of ionic targets in nonlinear and time-resolved studies gains further importance. Generally, the importance of this nondipole effect and its strong influence on various phenomena were demonstrated in numerous studies ranging from fundamental research in atoms and molecules 1, 2, over applications in condensed matter 3 and realization of high-harmonic generation lasers 4 to investigations of astrophysical interest 5. The momentum transfer can lead to a symmetry breakdown in terms of a forward-backward asymmetry in the photoelectron angular distribution (PAD). However, at very high intensities or at shorter wavelengths the photon’s linear momentum has been shown to become essential for an accurate description of the electron emission. This is the so-called dipole approximation, which constitutes one of the common approaches to describe light-matter interaction in the wavelength regime up to the extreme-ultraviolet (XUV). In photoionization, it is typically assumed that the linear momentum of photons for wavelengths much larger than the size of the absorbing target can be neglected. Nature Communications volume 9, Article number: 4659 ( 2018) Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon
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