X-ray diffractive imaging of controlled gas-phase molecules reveals the molecular structure of 2,5-diiodothiophene

Time-resolved diffractive imaging of gas-phase molecules is a promising tool to unravel the intrinsic molecular structure with atomic resolution at the femtosecond timescale, particularly for molecules that can’t be crystalized. The challenge is to capture the random orientation of molecules in the gas phase and to record images in the molecular frame.

Kierspel et al. have taken the next step in realizing the benefits of this ultrafast imaging method by using it to record the structure of 2,5-diiodothiophene, a stable molecule that shows potential in biomolecular engineering research among other applications.

The molecules, seeded in helium and supersonically expanded into a vacuum, were aligned three dimensionally by chirped near-infrared laser pulses, which were synchronized with an X-ray free-electron laser (XFEL) at a 120 hertz repetition rate.

The researchers integrated 2.2 million individual diffraction patterns of the molecular ensemble, which was probed at a photon energy of 9.5 kiloelectron volts, enabling the measurement of intramolecular atomic distances. The patterns were recorded with a Cornell-SLAC pixel array detector camera with hard X-rays.

The extracted distance between the two iodine atoms concurred with the calculated molecular structure within a few percentage points. The results were limited to the structure’s diffraction pattern only in static equilibrium.

“The signal-to-noise ratio was limited by background contributions from the beamline and the last X-ray aperture before the interaction region,” author Thomas Kierspel said. “Still, our results provide a good benchmark.”

“Aligned molecules and femtosecond pulses of hard X-rays provide crucial steps toward the coherent diffractive imaging of ultrafast molecular dynamics at the atomic scale, the long-sought after ‘molecular movie,’” author Jochen Küpper said.

The researchers expect to build on their findings at more advanced XFEL facilities.

Source: “X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame” by Thomas Kierspel, Andrew Morgan, Joss Wiese, Terry Mullins, Andy Aquila, Anton Barty, Richard Bean, Rebecca Boll, Sébastien Boutet, Philip Bucksbaum, Henry N. Chapman, Lauge Christensen, Alan Fry, Mark Hunter, Jason E. Koglin, Mengning Liang, Valerio Mariani, Adi Natan, Joseph Robinson, Daniel Rolles, Artem Rudenko, Kirsten Schnorr, Henrik Stapelfeldt, Stephan Stern, Jan Thøgersen, Chun Hong Yoon, Fenglin Wang, and Jochen Küpper, The Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5133963 .