The PLEIADES group of experimentalists at the French synchrotron radiation facility SOLEIL, in collaboration with theoreticians from Japan and Sweden, has found a way to “pull-out” molecular states from the regions of overlap and reconstruct their potentials, otherwise inaccessible by other methods. In their complex technique, the molecular states of interest are reached not directly, as in traditional photo-electron spectroscopic methods, but indirectly via an intermediate state. This neutral, highly excited state is produced by absorption of a soft x-ray photon of a well-defined energy. Several femtoseconds after this absorption (1 fs = 10-15 s), the highly excited species thus produced relax by emitting a valence electron. However, in the intermediate state the molecule vibrates and has sufficient time to change its bond length. The probed part of the final state potential is different, therefore, from that probed by direct photo-electron spectroscopic methods, which can access only the so-called vertical transitions. In such transitions, the geometry is the same as in the ground state and no change of bond length takes place. By tuning the photon energy of the excitation radiation however, different vibrational levels of the intermediate state can be reached which allows the width of the probed region in the final states to be controlled.
Figure 1: Schematic diagram of the Resonant Auger decay process used as a method to obtain the results. In the first step (indicated by the grey arrow) the molecule is excited by an X-ray photon and an inner-shell electron (the closest to the atomic nucleus) is promoted to the molecule’s periphery (unoccupied valence electron shell). The interatomic distance in the excited state begins to change and at the same time the electronic relaxation takes place. The vacancy in the inner-shell orbital is filled by an electron (indicated by the pink arrow), and another electron is ejected. This ejected electron is then detected and its energy provides information about the intrinsic properties of the molecule.
This technique is very demanding and requires high-brightness radiation sources owing to the dramatic decrease of the signal when tuning the excitation energy to higher vibrational levels of the intermediate state. The state-of-the-art, soft x-ray PLEIADES facility at the SOLEIL synchrotron in France is for the moment, the only place worldwide where such a study can be performed. Similar studies will soon however be possible at the PETRA III synchrotron at DESY in Germany, and at MAX IV at Max-Lab in Sweden where beamlines similar to PLEIADES are planned.
Figure 2: Schematic diagram of the potential energy curves for a diatomic molecule – the dependence of the electron energy on the interatomic distance.
Reference :
Catalin Miron, Christophe Nicolas, Oksana Travnikova, Paul Morin, Yuping Sun, Faris Gel’mukhanov, Nobuhiro Kosugi, and Victor Kimberg, « Imaging molecular potentials using ultrahigh resolution resonant photoemission », Nature Physics (2011) DOI 10.1038/NPHYS2159.
