08.07.2009

Between Atom und Molecule

HZB researchers observe molecule dissociation in real-time

When two atoms join, their mass-rich nuclei are held together by sharing electrons, the chemical bond, which behave like a ductile glue. As you tear both atoms apart the glue stretches until the bond finally brakes and the molecule resolves into two atoms. Such processes are extremely fast and happen on a time-scale of less than a trillionth of a second Philippe Wernet and colleagues at HZB in collaboration with a scientist of Stockholm University studied for the first time how the electronic structure evolves or what happens to the "glue" as a molecule dissociates. (Ph. Wernet, M. Odelius, K. Godehusen, J. Gaudin, O. Schwarzkopf, and W. Eberhardt, Phys. Rev. Lett. 103, 013001 (2009))In their studies, Wernet et al. used Bromine molecules, Br2 and excited them with extremely short Laser pulses at 395 nm. This excitation turns the forces on and the nuclei start flying apart from each other. Using pulses of a second frequency - the 15th harmonic at 23.5. eV - they probed the electronic structure of the valence electrons at various delay times achieving a time resolution suitable to observe the rupture of the bond in real-time.The result is surprising: Already after 85 +/-15 fs the properties of the valence electrons resemble to those of isolated atoms, in other words: the bond has broken. During this time, the atoms moved apart only a little more than 1.5-times their distance in the molecule, less than the often assumed increase for broken bonds of 2.Together with other investigations, this study sets a benchmark for future research with high-intensity coherent soft x-ray light sources like Free Electron Lasers, testing how atoms and molecules form and break bonds.

For more Information see original paper:
Real-Time Evolution of the Electronic Structure in a Dissociating Molecule

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