Non-dipole effects and the role of the photon momentum in strong-field ionization

• Applicant: Professor Dr. Reinhard Dörner
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431657260

This experimental project aims to elucidate the role of the photons’ linear momentum for the ionization of an atom or molecule if multiple photons are absorbed from a strong femtosecond laser pulse. The central question is how this photon momentum is shared between the fragment particles, i.e. the electron(s) and ion(s).In the recent 2 years about once a month, a new theory paper on the above topics has been published. However, there are only few experiments reported. The pioneering one by the Ottawa group (circularly polarized light) and experiments by the ETH group of Ursula Keller (linearly and elliptically polarized light). The reason for this lack of experiments is, that the photon momentum is very small compared to the width of the electron and ion momentum distribution. This calls for an excellent control of all systematic errors in any experiment trying to investigate the role of the photons’ linear momentum. As we demonstrate with preparatory work, the unprecedented precision needed for the success of this proposal is achievable using a novel type of a COLTRIMS reaction microscope. The new setup works without magnetic fields and is tailored to eliminate the main sources of systematic errors by a differential measurement. The key feature is the use of two counter propagating femtosecond laser pulses. The geometry allows one to use three different modes: light coming from one or the opposite direction or a standing wave that can be generated, if light is sent in from both directions simultaneously. The standing wave can be used to calibrate the zero point in momentum space. Toggling on a minute time scale between these three modes eliminates most of the possible sources of systematical errors. In more detail the goals are: we will explore how the light polarization, the binding energy and the continuum energy of the electrons modify the sharing of the photon momentum. We will investigate how the photon momentum sharing changes when two electrons are emitted. We will furthermore search for a dependence of these so-called ”nondipole effects’’ on the orientation of a diatomic molecule. The latter would demonstrate the influence of the retardation of the electric field (rather than the magnetic field) in strong-field ionization for the first time. At the end of the project, we will have answered how the photons’ momentum (the light pressure) modifies the electron and ion momenta in strong-field ionization in the most relevant and most studied regime (non relativistic, 1013-1015W/cm2). We will have set the future experimental benchmark for studies of the role of the magnetic component of the light field and the role of the retardation of the electric field in strong-field ionization.

• DFG Programme: Research Grants