Filming real time ultrafast molecular bending motion with laser-induced electron diffraction

Using ultrafast infrared lasers, researchers at K-State and three international teams reported the success of filming in real time the bending of a linear triatomic molecule from 180° to 140° in one-hundred-thousandth (10-5) of a nanosecond (10-9 s).


Chemical reactions and biological processes operate at the spatial and temporal scales of the constituent atoms in the molecules. The basic principle of measurements lies in the fact that the measuring scale on the instrument has to be similar to or even finer than the scale of the object of measurement. For atoms in a molecule, the length and time scales are Angstrom and femtosecond. These call for experimental tools that are still not available today. While X-ray and electron diffractions can probe positions of atoms precisely, their temporal resolution is limited to a few-hundreds of femtoseconds. Today's ultrafast infrared lasers of a few to tens of femtoseconds are routinely available, yet their wavelengths are hundreds of times longer than the typical bond lengths in molecules.

Laser induced electron diffraction (LIED) is a technique which can locate positions of individual atoms inside a single molecule by using high intensity mid-infrared lasers. The theoretical foundations of LIED were first established at K-State in 2010. When an intense laser impinges on a molecule, the strong field can strip an electron out of the molecule. The photoionized electron is initially accelerated by the oscillating laser field away from the molecule to be finally driven back and to recollide with the molecular ion. The re-collision of the electron wave with individual atoms inside the molecule produces interference patterns that encode the positions of the atoms. In LIED, the release of the electron triggers prompt rearrangement of atoms inside the molecule. Such changes are imprinted into the diffraction image at the time of the re-collision. Since this whole process occurs within one cycle of the laser pulse, LIED would reveal new positions of the atoms after about ten femtoseconds, which is the optical period of the 3.2-micron infrared laser used in the experiment.

In a study recently published in Nature Communications, K-State postdocs Su-Ju Wang and Jiří Daněk, both led by University Distinguished Professor Chii-Dong Lin at J. R. MacDonald Laboratory of Physics Department, in collaboration with researchers from ICFO at Barcelona, Max-Planck-Institut für Kernphysik, Heidelberg, and Friedrich-Schiller-Universität Jena have reported the recent success of LIED in the gas-phase Carbonyl Sulfide molecule (OCS). This international team has collaborated over the past ten years and has proven to be mutually beneficial. The laser experiment was carried out at Barcelona with the expertise in high-resolution detectors from Heidelberg. The group from Jena provided theoretical quantum chemistry simulations on laser-molecule interaction and the K-State team was invested in the retrieval of molecular structures from experimental data. This collaboration exemplifies that it takes a group of people with different expertise in today’s scientific endeavors.

Many LIED experiments have been successfully carried out in the last few years by our team, for example, on water, carbonyl sulfide, or carbon disulfide. However, the extraction of structural information of molecules has always been very tedious. In our recent Nature Communications paper, a novel retrieval method was introduced and it has been successfully applied to the OCS molecule. This new method analyzes diffraction images directly from the experimental data taken at the laboratory. It also uses the two-dimensional diffraction image (instead of one-dimensional data used previously) and focuses on the so-called zero-crossing points (ZCP) of the diffraction image at each angle. Combining this new retrieval method with previously existing methods offers an opportunity to assure the uniqueness of the extracted molecular structures with high confidence. This advantage actually helped us to identify that the bond angle in OCS indeed has changed from 180° to 140° and that the OCS is asymmetrically stretched.

The new ZCP analysis provides an alternative approach for retrieving the molecular structure in LIED experiments. The simplicity of this new method together with the synergy benefit of combining with prior approaches would contribute to a powerful tool for LIED experiments, for more complex molecules and for transient molecules where the atoms undergo large changes in their arrangement. Looking forward, with the rapid progress of the high-repetition rate mid-infrared laser technology and the improved retrieval methods, LIED is expected to complement ultrafast electron diffraction (UED) with electron beams around 3.2 MeV. With UED, the electron beam duration, which is directly connected to the time-resolution of the method, is still on the order of a few hundreds of femtoseconds. They would be more suitable for probing molecules with heavy atoms. Instead, LIED would be more favorable for small and light molecules.

Reference: Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction,  A. Sanchez, K. Amini, S.-J. Wang, T. Steinle, B. Belsa, J. Danek, A.T. Le, X. Liu, R. Moshammer, T. Pfeifer, M. Richter, J. Ullrich, S. Gräfe, C.D. Lin, J. Biegert, Nature Communications,