The first complete experimental study of photons scattering electrons from free atoms opens a new platform for testing quantum theories.
When photons of light have sufficient energy, they can knock electrons away from their atoms like billiard balls. This process, called Compton scattering, was an important discovery in early quantum physics because it showed that light cannot solely be thought of as waves, but also as particles with momentum.
Almost a century later, the theories explaining Compton scattering have grown exponentially. However, due to the challenge of directing powerful beams onto free atoms and molecules, experiments have been mostly restricted to solids. Now, an international team, including Max Kircher and Reinhard Dörner at Goethe Universität, Frankfurt, Germany, and Salim Houamer at University Sétif-1 in Algeria, has used a highly efficient method, called cold-target recoil ion momentum spectroscopy (COLTRIMS), to study Compton scattering from free atoms. The researchers directed a powerful photon beam from the Petra III synchrotron in Hamburg through a supersonic helium gas jet. The COLTRIMS technique enabled them to measure the momentum vectors of not just scattered electrons, but also the recoiling helium ions from individual scattering events. The team focused on investigating one interesting quantum phenomenon. In the billiard-type picture of Compton scattering, when the kick of the photons is too small to lift the electron over the ionization threshold, it should be impossible for the electrons to escape. Yet sometimes they can. This is explainable with quantum wavefunctions. “We finally managed to access an experimentally challenging regime of Compton scattering,” says Kircher, “and we hope this will pave the way towards further sensitive testing of quantum theories.”