Compton Streuung - Highly differential studies

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Compton scattering is one of the fundamental processes by which light interacts with matter, and it is the textbook example for demonstration of the particle nature of light. The process is a billiard type binary collision between a (high energy) photon and an electron, in which the photon is deflected and transfers part of its energy and momentum to the electron. Despite of its elementary and fundamental nature Compton scattering still poses major open questions today as emphasized by the programmatic title "Need for further inelastic scattering measurements at X-ray energies" of a review on the subject. There are three regimes, which are of particular interest: Firstly, the domain of low photon energies and/or small momentum transfers, where the binding energy of the electron is of the order of the energy transfer implied by the billiard-like collision scenario. Secondly, the regime of all processes in which electrons cannot be treated as independent particles, but the correlations are essential and, thirdly, the realm of molecular targets. It is these three regimes, which we will address, in the current project. To this end, we will perform the first ever coincidence experiments on Compton scattering on individual atoms and molecules in the gas phase. The experiments will be pursued at European synchrotron radiation facilities (PETRA3 in Hamburg, ESRF in Grenoble) where so called „pink“ beams (i.e. photon beams bypassing the monochromator) of sufficient photon flux are available for such measurements at extremely small low cross sections. We will use a COLTRIMS reaction microscope, co-developed by the group of the applicant, to measure the momentum vectors of electrons and ions created by Compton scattering with high resolution and 4π collection solid angle. From these two momentum vectors, the momentum and energy transfer of the scattered photon can be deduced from momentum conservation. Thus, from our experiment, we will obtain for all photon deflection angles and all energy transfers the respective angular emission distribution and energy spectrum of the ejected electron. Our experiments will be, 95 years after Arthur H. Compton’s fundamental discovery, be the first study in which for a free atom or molecule (gas phase) the momenta and directions of the scattered photon and the kicked electron are determined in coincidence. This will provide the most detailed and most complete view at Compton scattering ever achieved. Thus, besides their textbook character, our coincidence gas phase experiments will allow to address the open questions of Compton scattering in the afore mentioned three regimes. We will perform studies at low energies, where Compton scattering at a bound electron is energetically suppressed, investigate cases of Compton scattering, where electron-electron correlation is essential and explore Compton scattering in a molecule breaking the rotational symmetry of the process.