A sub-femtosecond study of linear photon momentum transfer during an ionization process, done by a team at ETH Zürich, could provide insight into the “birth” of photoelectrons. Physicists understand how photons transfer their energy to an ion and a photoelectron during multiphoton ionization of an atom. However, the precise details of how light passes its impulse onto matter are still not fully understood. One reason, said the ETH Zürich team, is that changes to the transferred impulse occur during an optical cycle on extremely fast, sub-femtosecond timescales. Existing studies provide information on time-averaged behavior, but not on the time-dependent aspects of the linear-momentum transfer during photoionization. Scientists at ETH Zürich devised a time-resolved measurement of linear momentum transfer along the laser pulse propagation direction and showed that the linear momentum transfer to the photoelectron depended on the ionization time within the laser cycle. The experiments focused on high laser intensities, where multiple photons were involved in the ionization process, and explored how much momentum was transferred in the direction of laser propagation. Reconstructed 3D photoelectron momentum distribution, together with a sketch of the polarization ellipse and the beam direction. (Adapted from Willenberg et al., Nature Communications 10, 5548, 2019.) Courtesy of ETH Zürich, D-PHYS, Keller group. To achieve sufficient time resolution, the scientists employed the attoclock technique, an approach developed by the group of Ursula Keller at the Institute for Quantum Electronics. With this method, attosecond time resolution is achieved without the need to produce attosecond laser pulses. Instead, information about the rotating laser-field vector in close-to-circular polarized light is used to measure time relative to the ionization event with attosecond precision. The attoclock approach is similar to the hand of a clock, only the attoclock hand is rotating through a full circle within one optical cycle of 11.3-femtosecond duration. The ETH physicists were able to determine how much linear momentum the electrons gained depending on when the photoelectrons were “birthed.” The scientists found that the amount of momentum transferred in the propagation direction of the laser depended on when in the laser’s oscillation cycle the electron was released from matter (in this experiment matter was composed of xenon atoms). The scientists concluded that, at least for the scenario they explored, the time-averaged radiation pressure picture was not applicable. The team could reproduce the observed behavior almost fully within a classical model (whereas many scenarios of light-matter interaction, such as Compton scattering, can be explained only within a quantum mechanical model). However, the classical model had to be extended to account for the interaction between the outgoing photoelectron and the residual xenon ion. In their experiments, the scientists showed that this interaction induced an additional attosecond delay in the timing of the linear momentum transfer, compared to the theoretical prediction for a free electron birthed during the pulse. Whether such delays are a general property of photoionization, or whether they apply only to the type of scenarios investigated in the present ETH Zürich study, remains open. However, with this first study of linear momentum transfer during ionization on the natural timescale of the process, the Keller group has opened up a possible new route to explore the fundamental nature of light-matter interactions — thus making good on a central promise of attosecond science. The research was published in Nature Communications (www.doi.org/10.1038/s41467-019-13409-6).