We know very well how the involved photons transfer their energy to the ion and the photoelectron during multi-photon ionization of an atom. However, our understanding of how the photon linear momentum is transferred is still incomplete. Physicists of ETH Zürich have now measured the transfer of linear momentum along the laser pulse propagation direction.
The interaction of light and matter is one of the most important interactions in the atomic world that forms the basis of many technologies. One of the most famous examples in this field is the photoelectric effect. In this phenomenon, the electron emits when the material is exposed to appropriate light. In such interactions, not only the transfer of energy but also the transfer of linear momentum from photons to electrons is important.
Despite the fundamental importance of momentum transfer, its details are not yet fully understood. One reason is the high speed of this phenomenon. In fact, most of the information available is related to the time-average behavior of linear momentum transfer during photoionisation. Now the researchers have used an approach called attoclock to address this problem
Using the Atoclock technique, they showed that the transfer of linear momentum to the photoelectron depends on the ionization time within the laser cycle. Interestingly, the researchers were able to explain the measured linear momentum transfer within a classical model for a free electron in a laser field. This is while, many light-matter interaction schemes, such as the Compton effect, can only be expressed within a quantum model. However, due to the parent-ion interaction, some corrections are needed. By all accounts, this research opens up an exciting path to discovering the fundamental nature of light-matter interaction.