When A Molecule Is A Molecule?

Researchers observe fast electron jumps in exploding molecules

With ultra-short X-ray flashes by an international research team has observed electron transitions in exploding molecules. The study shows maximum distance to which a charge transfer takes place between the two fragments of a molecule. With the technique used allows the dynamics of charge transfer study in a wide range of molecular systems, the scientists write to Dr. Benjamin Erk and Dr. Daniel Rolles of DESY and Prof. Artem Rudenko from Kansas State University in the U.S. journal "Science ". Such processes play in numerous chemical processes play a role, such as in photosynthesis.

"Charge transfer takes place until around for ten times the normal bond length instead," says Erk, who conducts research at DESY at the free-electron laser FLASH and at the Center for Free-Electron Laser Science (CFEL), a collaboration of DESY, Hamburg University and the Max . Planck Society "A central question is: When is a molecule is a molecule," Rudenko explained the motivation behind the investigation. "In this case, maximum distance to which the molecular constituents share the electrons, from what distance breaks the charge transfer between the two molecular fragments together. We measured critical distance marks the transition from molecular to atomic regime. "

For their study, the researchers bombarded molecules of iodine and a methyl group (CH3), so-called iodomethane (CH3I), with an infrared laser and so broke the bond between the two partners. "With the help of ultrashort X-ray flashes, which also beat electrons from the inner shells of the iodine atoms, we could then watched as the shared electrons of shattering molecule between the two fragments is distributed," says Rolle, who heads a research group at DESY. Purpose used The researchers currently the world's most powerful X-ray laser LCLS at the U.S. SLAC National Accelerator Laboratory in California.

"We have the X-ray flash behind a little later sent the infrared laser pulse at every step," says Erk. This delay was between a few femtoseconds and a picosecond, ie up to a trillionth of a second. "The later the X-ray flash is, the farther the two molecular constituents already apart. "In this way, the scientists won a series of recordings on which the migration of the electrons can be observed at an ever increasing distance between the molecule debris.

"The further removed the fragments has, the more the charge transfer from," says Erk. "We have up to a distance of about 20 angstrom electron migration between the two fragments of evidence." The bond length of iodomethane is, however, only slightly more than 2 angstroms, or 0.2 nanometers (millionths of a millimeter).

"Our results are for a number of systems of meaning," says Rudenko. "So you have about observed in astrophysics x-ray radiation generated by such charge transfer processes. Such processes play in numerous chemical processes play a role, such as in photosynthesis or in solar cells. And in research for scientists who study with X-rays biomolecules to fight with radiation damage to their samples. too, the we have studied processes are important. "

These initial results suggest also a bridge between the study of charge transfer between single atoms and the analysis of the electric charge flow in larger systems such as occur frequently in biology and chemistry. Further measurements should help to understand the process of charge transfer in detail.