SPring-8, the large synchrotron radiation facility

Molecular Science

 Snapshot of core-excited deformed molecules

When a core electron of an atom in a molecule is promoted to an unoccupied molecular orbital, the Auger decay takes place in a few 10^-15 s. Even within this short time scale, the molecule can be deformed. One can snapshot the molecule deformed in a few 10^-15 s, by means of the multiple-ion-coincidence momentum imaging technique. In the experiment described here, a lin-early polarized monochromatic soft X-ray beam available on BL27SU is used to excite an electron in the carbon K shell of a carbon dioxide molecule CO₂ to the lowest unoccupied molecular orbital. The CO₂ molecule has a linear geometry in the ground state. The potential energy surface of the core excited state splits into two due to vibronic interaction. The states of lower and higher potential energies have bent and linear geometries (Fig. 1). The multiple-ion-coincidence momentum imaging technique registers three-dimensional momenta of all ions fragmented from one molecule in coincidence when the molecule breaks up after the multiple Auger decay. The bent and linear geometries of the two split states are successfully probed separately by means of this technique (Fig. 2).

provided by Kiyoshi Ueda, Tohoku University

Fig. 1. Schematic potential energy curves for the core-excited CO2 molecule.
Fig. 2. Momentum distributions (Newton diagrams) for the three fragment ions, formed by three-body breakup of the triply charged molecular ion CO23+ after C 1s excitation to the lowest unoccupied molecular orbital 2πu. Diagram (a) for the breakup of the bent state exhibits a longer tail for each island than the diagram (b) for the breakup of the linear state, illustrating that the C atom goes off significantly transverse to the two O atoms in the bent state.