Attosecond-resolved non-dipole photoionization dynamics

Light–matter interactions are usually described within the electric-dipole approximation, where the magnetic-field component and the spatial variation of the light electric field over the relevant electronic length scales are both ignored. Non-dipole effects in photoionization were revealed to be tiny from the infrared to the soft X-ray domains, and all non-dipole observations reported so far were limited to single-pulse, single-colour measurements. Here we advance attosecond time-resolved spectroscopy into the non-dipole interaction regime. Using a self-referenced attosecond photoelectron interferometry on helium atoms, we resolve the electron subcycle motion along the light propagation direction in the 15 pm range driven by the magnetic component of a near-infrared laser field. Furthermore, we measure a time delay of 15 ± 10 as between the electric-dipole and electric-quadrupole transitions by resolving the asymmetry of the photoelectron forward–backward yields with attosecond resolution. These fundamental findings are supported by ab initio calculations based on the non-dipole time-dependent Schrödinger equation.