When is a molecule a molecule?

Using ultra-short X-ray flashes, an international team of researchers watched electrons jumping between the fragments of exploding molecules. The study reveals up to what distance a charge transfer between the two molecular fragments can occur, marking the limit of the molecular regime. The technique used can show the dynamics of charge transfer in a wide range of molecular systems, as the scientists around Dr. Benjamin Erk and Dr. Daniel Rolles of DESY and Professor Artem Rudenko of Kansas State University report in the scientific journal Science. Such mechanisms play a role in numerous chemical processes, including photosynthesis.

"The charge transfer takes place at up to approximately ten times the normal bond length," says Erk, a DESY scientist working with the free-electron laser FLASH and at the Center for Free-Electron Laser Science (CFEL), a cooperation between DESY, the University of Hamburg and the Max Planck Society. "A central question posed is: when is a molecule a molecule," Rudenko says, explaining the motivation behind the study. "In this case then, up to what distance do the molecular components share electrons, at what distance does the charge transfer between the two molecular fragments break down? The critical distance we measured marks the transition from the molecular to the atomic regime."

For their study, the scientists shot an infrared laser at molecules of iodomethane (CH3I), made of iodine and a methyl group (CH3), to break the bond of the two partners. "With the help of ultra-short X-ray pulses, electrons were knocked from the inner shells of the iodine atoms, allowing us to then observe how the shared electrons of the disintegrating molecule were distributed between the two fragments," explains Rolles, who heads a junior research group at DESY. The researchers used what is currently the world's most powerful X-ray laser, LCLS, which is located at the SLAC National Accelerator Laboratory in California.

"During each step we delayed the X-ray pulse a bit more after the infrared laser pulse," says Erk. This delay ranged between a few femtoseconds and one picosecond, that is a trillionth of a second. "The later the X-ray pulse arrives, the farther apart the two molecular constituents move from each other." The researchers thus managed to obtain a series of snapshots in which the electron transfer can be observed at an ever-increasing distance between the molecular debris.

"The further apart the fragments move, the more the probability of the charge transfer decreases," explains Erk. "We were able to detect electrons jumping between the two fragments up to a distance of about 20 Ångström." The bond length of iodomethane is only about 2 Ångström, or 0.2 nanometres (millionths of a millimetre).

"Our results are important for a variety of systems," stresses Rudenko. "For instance, in astrophysics X-rays produced by charge transfer processes have been observed. Such mechanisms play an important role in numerous chemical processes, for example, in photosynthesis or in solar cells. And during their research, scientists who study biomolecules using X-rays struggle with radiation damage to their samples. Here, too, the processes we studied play an important role."

These first results also provide a bridge between the study of electron transfer between single atoms and the analysis of the charge flow in larger systems such as those that often occur in biology and chemistry. Further investigations will help to understand the observed process of charge transfer in detail.

Source: Deutsches Elektronen-Synchrotron DESY