Molecular Photoionization beyond the Dipole Approximation

• 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 447234583

The photoelectric effect describes the emission of an electron by absorption of a photon. At a single molecule the photon spin is transferred to the ion and the electron. With increasing photon energy additional orbital angular momentum can be transferred, which results from the photon’s linear momentum. Then the much used dipole approximation for the description of light matter interaction loses its validity. What are the consequences of the additional angular momentum in molecular photoionization? This is the central question of the present experimental project. How does the angular distribution of the photoelectrons change? Does the fragmentation direction of the molecule change and is there also an influence on secondary electron emission by Auger decay?Despite the fundamental importance and the broad applications of the photoelectric effect there have been only a very few (conflicting!) experiments reported addressing non dipole effects in molecular photoionization and we are aware of only one single experiment on non-dipole effects for a spatially oriented molecule.We will address the following open questions: How do non dipole effects depend on molecular orientation? Do they depend on the parity of the bound electronic state? How are the influenced by electron-electron correlation? Does the photoionization cross section for a fixed in space molecule change of the photon direction is inverted; i.e. for example does it matter for the ionization cross section of CO if the photon hits the O or the C side first? Is there an influence of non-dipole effects also on the Auger electron emission which often follows the ionization step at an inner shell? For some of these questions theoretical predictions exist, however, none of them has been experimentally tested.We will answer these questions focusing on small molecules (CO,H2,N2) exploiting the most powerful experimental technique available for studies of photoionization today. Using a COLTRIMS reaction microscope, a technique co-developed by the applicant, we will measure the angle and energy of the photoelectron jointly with the fragments direction and energy of the molecule and the Auger electron. This complete picture of the ionization process including the subsequent relaxation processes in the molecule will allow us to determine the spatial orientation of molecular axis, the parity of the photoelectron and correlation driven excitation of other electrons. The unprecedented detail and completeness of this information on non-dipole effects in molecular photoionization and its consequences for the molecule will give access the relevant physical mechanisms.

• DFG Programme: Research Grants