Watching as helium goes topsy-turvy: Theorists have long pondered the underpinnings of the Fano resonance, a spectral feature that resembles adjacent rightside-up and upside-down peaks. An especially well-studied instance of this feature appears in the electronic spectrum of helium as a transient state undergoes delayed ionization. Two studies have now traced the dynamics of this state in real time. Gruson et al. used photoelectron spectroscopy to extract the amplitude and phase of the electron wave packet after inducing its interference with reference wave packets tuned into resonance at variable delays. Kaldun et al. used extreme ultraviolet absorption spectroscopy to probe the transient state while variably forcing ionization with a strong near-infrared field. Science , this issue pp. 734 and 738

Clocking departures from chiral origins: Just as the atoms in a molecule can be arranged in a left- or right-handed manner, the field in a beam of light can circulate like a left- or right-handed corkscrew. Matches or mismatches in this mutual handedness give rise to an asymmetric distribution of trajectories as electrons are ejected during photoionization. Beaulieuet al.used an interferometric approach to uncover the temporal dynamics associated with this asymmetry. They probed the mirror-image isomers of camphor with circularly polarized light, which revealed the angle-dependent delays between trajectories that spanned up to 24 attoseconds.Science, this issue p.1288

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon’s angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a $0.9\pi$ shift for the electrons produced in the ionization channels leading to the $X{{}^2\Sigma }_g^{+}$, $v^{\prime }=1$, and $v^{\prime }=2$ states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise.