The electric dipole approximation is widely used in atomic, molecular and optical physics and is typically related to a regime for which the wavelength is much larger than the atomic structure. However, studies have shown that in strong laser fields another regime exists where the dipole approximation breaks down. During the ionization process in intense laser fields and at long wavelengths the photoelectrons can reach higher velocities such that the magnetic field component of the laser field becomes significant. The ionization dynamics and the final momentum of the electron is therefore modified by the entire Lorentz force. In contrast the magnetic field interaction is neglected in the dipole approximation. Rapid developments in laser technology and advancements in the accuracy of the measurements techniques have enabled the observation of the influence of such non-dipole effects on the final angular photoelectron momentum distributions. More recently the number of studies on ionization beyond the dipole approximation has increased significantly, providing more important insight into fundamental properties of ionization processes. For example we have shown that the final three dimensional photoelectron momentum spectra is significantly affected by the non-dipole drift with the parent–ion interaction, the linear multiphoton momentum transfer on a sub-cycle time scale and the sharing of the transferred linear photon momenta between the electron and the ion. In this article we present an overview of the underlying mechanisms and we review the experimental techniques and the achievements in this field. We focus on ionization in strong laser fields in the regime where the dipole approximation is not valid but a fully relativistic description is not required.

电偶极子近似广泛用于原子、分子和光学物理，并且通常与波长远大于原子结构的状态有关。然而，研究表明，在强激光场中，存在另一种偶极子近似失效的状态。在强激光场和长波长的电离过程中，光电子可以达到更高的速度，使得激光场的磁场分量变得重要。因此，整个洛伦兹力会改变电子的电离动力学和最终动量。相反，在偶极子近似中忽略磁场相互作用。激光技术的快速发展和测量技术精度的进步使得能够观察到这种非偶极子效应对最终角光电子动量分布的影响。最近，关于超出偶极子近似的电离的研究数量显着增加，提供了对电离过程基本特性的更重要的见解。例如，我们已经表明，最终的三维光电子动量谱受到母离子相互作用的非偶极漂移、子周期时间尺度上的线性多光子动量传递以及传递的线性光子动量共享的显着影响。在电子和离子之间。在本文中，我们概述了基本机制，并回顾了该领域的实验技术和成就。我们专注于强激光场中偶极子近似无效但不需要完全相对论描述的情况下的电离。