How do electrons that form a bond rearrange as the bond breaks?

When two atoms join, their mass-rich nuclei are held together by sharing electrons, the chemical bond, which behave like 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.

Schematic depiction of concurrent nuclear
wavepacket and valence electronic structure
evolution upon molecular dissociation.

Researchers at HZB in collaboration with scientists from Stockholm University studied for the first time how the electronic structure evolves at the atomic level or and 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) mit "http://prl.aps.org/abstract/PRL/v103/i1/e013001" )

In their studies Philippe Wernet from the institute of Methods and Instruments of Synchrotron Radiation Research at HZB and colleagues used Bromine molecules, Br2, and excited them with extremely short optical laser pulses with a wavelength of 400 nm. This excitation with the second harmonic of an 800 nm femtosecond Titanium:Sapphire laser turns the forces on and the nuclei start flying apart from each other.

They used ultrashort pulses with a wavelength in the soft x-ray region, namely the 15th harmonic with a wavelength of 53 nm or corresponding photon energy of 23.5. eV, to probe the valence electronic structure of the molecules as it evolves during dissociation with photoelectron spectroscopy. With their ultrashort soft x-ray pulses they were able to map the electronic structure evolution in real-time during dissociation for the first time.

The result is surprising: Already after 50±15 femtoseconds the properties of the valence electrons that are characteristic of the intact molecules are lost. After 85±15 femtoseconds the electronic structure of the free atoms has evolved and the bond is broken. Strikingly, during this time, the atoms have moved apart from each other only a little more than 1.5-times their distance in the molecule and much less than the often assumed 2-fold increase of nuclear distance for broken bonds.

This research with ultrashort soft x-ray pulses was enabled by recent developments of x-ray optics and spectroscopy at the institute of Methods and Instruments of Synchrotron Radiation Research at HZB and, in particular, by combining ultrafast laser and x-ray technologies. The results and methodology provide a benchmark for future experiments using time-resolved x-ray methods to map the evolution of the electronic structure in real time and at the atomic level during ultrafast processes such as chemical reactions and phase transitions.  

Dr. Ph. Wernet

More information on this research can be found in a recent publication in the Physical Review Letters:

Real-time evolution of the Valence Electronic Structure in a Dissociating Molecule, Ph. Wernet, M. Odelius, K. Godehusen, J. Gaudin, O. Schwarzkopf, and W. Eberhardt, Phys. Rev. Lett. 103, 013001 (2009) mit "http://prl.aps.org/abstract/PRL/v103/i1/e013001" .

This research was selected for a Synopsis in Physics "http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.103.013001" and it is an “Editor’s Suggestion” in the Physical Review Letters and an “Editors’ Choice” of highlights in the recent literature in Science "http://www.sciencemag.org/content/vol325/issue5940/twil.dtl#325/5940/519-c"

The movies depict how two of the valence orbitals of Br2 evolve during dissociation from molecular to atomic orbitals according to a calculation and in agreement with the measured results.