Deciphering molecular chirality with a new ultrafast imaging technique

An international team of researchers has identified a phenomenon that could allow them to image molecular chirality with extremely high temporal resolution.

The dynamics of molecules is a phenomenon that occurs on extremely short time scales. To be able to see and understand, for example, how electrons and nuclei rearrange during a chemical reaction, the imaging of these reactions can only be achieved through ultrafast snapshots. Short laser pulses provide the required time resolution to obtain such snapshots, but the interpretation and reconstruction of the molecular dynamics from the obtained images and measurements ends up being rather challenging, especially if one is interested in imaging more complex and non-symmetric molecules with more than a couple of nuclei.

Now, a particularly interesting and important property that emerges with molecules holding more than three nuclei is the known as chirality. Chirality (or handedness) is fundamental in chemistry because it plays an important role in the recognition phenomenon between biologically active molecules, and it can modify the outcome of a chemical reaction depending on the handedness of molecular structure.

The phenomenon of chirality relates to the study of the three-dimensional structure of molecules and it is linked to the fact that a molecule cannot be superposed on its mirror image by any combination of rotations and translations (the right and left hand effect). In addition, molecules that show chirality have the special property that when being shined by light, they can rotate the plane of polarized light, where this degree of rotation is known as a specific rotation or optical rotation. However, this well-known optical rotation effect is intrinsically weak, making its extension to the ultra-fast regime very challenging. These challenges hinder the ultrafast imaging of molecular chirality, and with it our ability to image and interpret the electron dynamics in complex and relevant biological molecules.

Developing methods to image molecular chirality with high temporal resolution is a key step towards understanding the molecular dynamics of the not-so-simple systems. This is what was achieved in a recent study published in Physical Review Letters, where ICFO researchers Xavier Barcons (now at the German Aerospace Center), Andres Ordoñez, and Andrew Maxwell (now at Aarhus University), led by ICREA Prof at ICFO and Dynamite project coordinator Maciej Lewenstein have identified a phenomenon that could allow them to image molecular chirality with extremely high temporal resolution (on the order of 10-16 s), which depends on a fundamental property of electrons, unexploited in this way until now.

In their theoretical approach, they considered the photoionization of a chiral molecule using a very short and rather intense infrared pulse. As Andres Ordoñez explains, “the energy of the photons of this pulse are not enough to ionize the molecule, but the intensity of the pulse is so strong that the molecule can absorb several photons at once. In fact, the ionization caused by such a pulse is better understood as the electric field of the pulse being so high that it basically rips away the electron from the molecule. Before being ripped away, the electron cloud has a shape determined by the chiral nuclear arrangement of the molecule”.

Now, despite the severe deformation that the electron cloud suffers as it is ripped away from the molecule, its structure conserves an imprint of the initial state and, consequently, of the chirality of the molecule. This structure can be quantified in terms of the orbital angular momentum (OAM) of the electron cloud and the direction in which it flies away from the molecule. What the team found is that upon ionization of the molecule with linearly polarized light, the molecular chirality is imprinted in the OAM-helicity of the photoelectron. They observed that emitted electrons carry information about the chirality of the molecule they are emitted from, through the OAM retrieved information, a fundamental phenomenon that they were completely unaware of until now.

The results of this work prove it to be the first study that explores the role that the orbital angular momentum (OAM) of free electrons play in the context of photoionization of chiral molecules. This will not only mark the commencement of collaborations aimed to design an attosecond experiment, where the scientists will seek to measure the electron’s OAM and demonstrate their key theoretical results. It will also provide insights on an improved description of the chiral molecule, which at the moment relies on a toy model that captures the essence of the phenomenon, but which does not include important aspects such as the anisotropy of the molecular potential, or recollision scenarios that lead to interesting strong-field phenomena such as high-harmonic generation and light-induced electron diffraction, among others.

As Maciej Lewenstein concludes, “While it is way too early to claim that the particular effect that we found will revolutionize sectors, such as the pharmaceutical sector, we are certain that, at the very least, it will inspire further research into molecular chirality.

Why Chirality is so important
Chiral molecules are ubiquitous in nature, in particular in biological systems, where, for example, most aminoacids and sugars are chiral. Molecular handedness is important because it determines how a chiral molecule reacts with another chiral molecule. Thus, although the left and right handed versions of a chiral molecule look very similar to each other (they are just mirror images of each other, like hands), they can behave rather differently in certain situations. The most striking examples of this occur in the pharmaceutical sector, where chirality has played a fundamental role in the development of drugs, as one handedness may have a positive effect while the opposite handedness may be extremely toxic. Therefore, developing new methods that extend our ability to measure molecular chirality undoubtedly has an impact on society.

Original article

Planas, X. B., Ordóñez, A., Lewenstein, M., & Maxwell, A. S. (2022). Ultrafast Imaging of Molecular Chirality with Photoelectron Vortices. Physical Review Letters, 129(23), 233201. https://doi.org/10.1103/PhysRevLett.129.233201