Electron correlation and multielectron effects are fundamental interactions that govern many physical and chemical processes in atomic, molecular and solid state systems. The process of autoionization, induced by resonant excitation of electrons into discrete states present in the spectral continuum of atomic and molecular targets, is mediated by electron correlation. Here we investigate the attosecond photoemission dynamics in argon in the 20–40 eV spectral range, in the vicinity of the 3s⁻¹np autoionizing resonances. We present measurements of the differential photoionization cross section and extract energy and angle-dependent atomic time delays with an attosecond interferometric method. With the support of a theoretical model, we are able to attribute a large part of the measured time delay anisotropy to the presence of autoionizing resonances, which not only distort the phase of the emitted photoelectron wave packet but also introduce an angular dependence.

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field¹. They are ubiquitous in physics with applications in different areas such as photonics², nano-electronics³, electron microscopy⁴ and quantum information⁵. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light–matter interactions within the rotating-wave approximation⁶. Although intense nanometre-wavelength light sources have been available for more than a decade^7–9, Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser¹⁰ can drive Rabi dynamics between the ground state and an excited state in helium atoms. The measured photoelectron signal reveals an Autler–Townes doublet and an avoided crossing, phenomena that are both fundamental to coherent atom–field interactions¹¹. Using an analytical model derived from perturbation theory on top of the Rabi model, we find that the ultrafast build-up of the doublet structure carries the signature of a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond¹² and few-femtosecond¹³ extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

Significance: In most optics textbooks, one writes the electric field describing an optical wave as a product of temporal and spatial amplitudes. This approximation often breaks down for short optical pulses. An example of such spatiotemporal coupling is chromatic aberrations, where the focal properties of the radiation vary with frequency over the pulse bandwidth. In this work, we point out significant chromatic aberrations of attosecond pulses, which depend on the geometry of the generation process. These aberrations are intrinsic to the generation process and need to be eliminated in applications requiring attosecond pulses to be focused over a small region.