X-Rays Probe Single Molecules

When chemists want atomic-level structural information about chemical compounds, they often turn to X-ray crystallography. But the technique requires an ensemble of molecules in crystalline form. Researchers have now taken a first step toward using X-rays to obtain precise structures and observe the reaction dynamics of individual gas-phase molecules, reports an international team (Phys. Rev. Lett. 2014, DOI: 10.1103/physrevlett.112.083002).

“This is proof-of-principle work,” says Stephen H. Southworth, leader of the atomic, molecular, and optical physics group in the X-ray Science Division of Argonne National Laboratory. With additional development, he says, the method could be used to probe chemical processes and see structures more directly than is possible with other techniques. Southworth was not involved in the newly reported research.

The technique was developed by a team led by Jochen Küpper, leader of the controlled molecule imaging group at Germany’s Centre for Free-Electron Laser Science, which is affiliated with the DESY synchrotron accelerator center.

Küpper and colleagues took advantage of established methods to align molecular beams using an electric field: When a polarizable molecule interacts with the electric field of a type of laser radiation, the molecules line up to minimize their energy.

The research team then intersected that molecular beam with high-energy, short-duration X-ray pulses produced at SLAC National Accelerator Laboratory’s Linac Coherent Light Source.

In the reported experiments, the researchers looked at X-ray diffraction of 2,5-diiodobenzonitrile. The compound’s iodine atoms strongly scatter X-rays, yielding a two-center interference pattern similar to that from a classic double-slit experiment. The researchers determined that the molecule has an iodine-iodine distance of 800 pm, longer than the expected value of 700 pm.

Küpper’s team is now trying to improve the resolution of the experiment. The X-ray pulses they used to interrogate the molecular beam had a wavelength of 620 pm, similar to the distance between the iodine atoms. To get better resolution—and to narrow in on additional structural detail—scientists need shorter wavelengths. Instrumentation advances since the 2,5-diiodobenzonitrile measurements were completed now allow for wavelengths down to 100 pm, Küpper says. Researchers are also working on creating shorter pulses with faster repetition to minimize radiation damage prior to diffraction.

Shorter pulses will also enable time-resolved experiments, such as following a photochemical reaction in real time, Küpper says. Experiments to create molecular movies showing reaction dynamics would require pulses of just a few femtoseconds, compared with 100-fs pulses the team used to study 2,5-diiodobenzonitrile.