21 May 2020
Photo: DESY, Andrea Trabattoni
Filming a quantum molecular movie. A carbonyl sulphide molecule is ionised by ultraintense light and the photoelectrons are projected onto a detector, carrying the signature of electrons’ ballet.
Imaging the structure of a molecule and capturing its dynamics during a chemical reaction with atomic resolution and in real time is one of the holy grails for unravelling chemistry. An interdisciplinary team of scientists from the Center for Free-Electron Laser Science (CFEL) now took a crucial step forward in this direction. The researchers could capture the details of the strong-field photoionization process, where an electron is ejected from a molecule by ultraintense light, and the subsequent intricate electronic dynamics of the molecule in real time. The team including scientists from DESY, Universität Hamburg, and the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) reports its results in the journal Nature Communications.
“Understanding exactly how chemical reactions proceed can not only advance our fundamental insight into the dynamics of matter, it may also help us to design better or completely new chemical reactions for specific applications,” explains DESY scientist Jochen Küpper, head of the Controlled Molecule Imaging (CMI) group at CFEL, where the experiment was performed. Küpper is a professor at Universität Hamburg and a member of the Cluster of Excellence "CUI: Advanced Imaging of Matter". “However, filming chemical reactions requires a series of ultrafast snapshots with atomic resolution, something that has been coined ‘molecular movie’ but could not yet be realised to the necessary extend of the actual quantum nature of the process.”
Scattered electrons create a diffraction pattern
The team now got a big step closer to this goal. The researchers investigated the interaction between carbonyl sulphide molecules, which were fixed in space using laser alignment, and a very intense laser field – applying the method of laser-induced electron diffraction. Here, the strong laser field extracts an electron from the molecule, accelerates it, and then makes this electron recollide with the molecule itself at high energy. The scattered electrons create a diffraction pattern that allows for the extraction of the structure of the molecule with very high spatial and temporal resolution.
A combined experimental, theoretical, and computational approach enabled the team to follow the electrons as they danced around the molecules in the strong laser field. “Surprisingly, it is the molecule itself that sets the rhythm and the pace of the electrons’ ballet,” says first author and CUI researcher Andrea Trabattoni from DESY.
The molecule sets a clock for the motion of the electron
So far, the dynamics of the electron has been considered to be governed mostly by the external laser field. However, the new result indicates that this common mindset is apparently wrong: the molecule has a crucial role in the motion of the electron and sets a clock for its ejection and acceleration. “The molecule not only defines the initial direction of the ejected electron, but it also pulls the electron towards itself multiple times, and sets the recollision time,” explains Trabattoni. “In light of this new understanding, the interpretation of previous studies employing laser-induced electron diffraction should be reconsidered.”
“In our laboratory, we developed cutting-edge technology to control the direction of molecules in space and their interaction with laser fields,” says Küpper. “This capability was key to disclose the interplay between the ultraintense laser field and the molecule during the electron dynamics.” In addition, the molecular strong-field interaction was modelled by two innovative theoretical approaches: On the one hand a fully quantum-mechanical treatment was implemented by Umberto De Giovannini from the group of Angel Rubio at MPSD, and on the other hand a semiclassical quantum-trajectory calculation was developed by Joss Wiese, a doctoral student in the chemistry department of Universität Hamburg and in Küpper's CMI group.
“Our result redefines the seminal statements of strong-field molecular physics and opens new ways to image molecules with unprecedented spatiotemporal resolution,” concludes Trabattoni. Text: DESY, ed.
Andrea Trabattoni, Joss Wiese, Umberto De Giovannini, Jean-François Olivieri, Terry Mullins, Jolijn Onvlee, Sang-Kil Son, Biagio Frusteri, Angel Rubio, Sebastian Trippel, and Jochen Küpper
"Setting the photoelectron clock through molecular alignment"
Nature Communications (2020)