A Momentous View on the Birth of Photoelectrons

Reconstructed 3D photoelectron momentum distribution, together with a sketch of the polarisation ellipse and the beam direction. (Source: Keller group, D-PHYS, ETHZ)

The interaction between light and matter is the basis of both many funda­mental phenomena and various practical technologies. Most famously, in the photo­electric effect, electrons are emitted from a material that is exposed to light of suitable energy. For long, the origin of the phenomenon remained a riddle, and only with the advent of quantum theory and thanks to  Albert Einstein was the effect fully understood. Einstein received the 1921 Nobel Prize in Physics for his discovery of the underlying laws, and since then the effect has been harnessed in appli­cations ranging from spectro­scopy to night-vision devices. In some important cases, the key principle is the transfer not of energy but of linear momentum from photons to electrons. This is the case, for instance, when laser light is used to cool micro­scopic and macro­scopic objects, or to understand the pheno­menon of radiation pressure.

Despite the funda­mental importance of momentum transfer, the precise details of how light passes its impulse on to matter are still not fully understood. One reason is that the transferred impulse changes during an optical cycle on extremely fast, sub-femto­second timescales. So far, studies revealed mainly information on time-averaged behaviour, missing time-dependent aspects of the linear-momentum transfer during photo­ionisation. This gap has now been filled by the group of Ursula Keller at the Institute for Quantum Electronics at ETH Zurich.

They looked at the case of high laser inten­sities, where multiple photons are involved in the ioni­sation process, and investigated how much momentum is trans­ferred in the direction of laser propa­gation. To achieve sufficient time resolution, they employed their attoclock technique, which has been developed and refined in the Keller lab over the past decade. In this method, attosecond time resolution is achieved without having to produce atto­second laser pulses. Instead, information about the rotating laser-field vector in close to circular polarised light is used to measure time relative to the ionisation event with atto­second precision. Very similar to the hand of a clock – just now this clock hand is rotating through a full circle within one optical cycle of 11.3-fs duration.

With this versatile tool at hand, the physicists were able to determine how much linear momentum electrons gained depending on when the photo­electrons were born. They found that the amount of momentum trans­ferred in the propa­gation direction of the laser does indeed depend on when during the oscil­lation cycle of the laser the electron is ‘freed’ from the matter, in their case xenon atoms. This means that at least for the scenario they explored, the time-averaged radia­tion pressure picture is not appli­cable. Intri­guingly, they can reproduce the observed behaviour almost fully within a classical model, whereas many scenarios of light-matter inter­action, such as Compton scattering, can only be explained within a quantum mechanical model.

The classical model had to be extended though, to take into account the inter­action between the outgoing photo­electron and the residual xenon ion. This interaction, they show in their experiments, induces an addi­tional attosecond delay in the timing of the linear momentum transfer compared to the theo­retical prediction for a free electron born during the pulse. Whether such delays are a general property of photo­ionisation or if they apply only for the sort of scenarios inves­tigated in the present study remains open for now. What is clear, however, is that with this first study of linear momentum transfer during ionisation on the natural timescale of the process, the Keller group opened up a new exciting route to explore the very funda­mental nature of light-matter inter­actions — thus making good on a central promise of atto­second science. (Source: ETHZ)

Reference: B. Willenberg et al.: Sub-cycle time resolution of multi-photon momentum transfer in strong-field ionization, Nat. Commun. 10, 5548 (2019); DOI: 10.1038/s41467-019-13409-6

Link: Ultrafast Laser Physics (U. Keller), Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland

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