We have all been familiar since childhood with the fact that our left and right hands are identical in structure but not in shape. They are mirror images of each other. In everyday life, this means that a left-handed glove does not fit on the right hand.
This "handedness" is also a fundamental property of matter: similar to our hands, many molecules exist in two mirror-image versions, which, despite looking confusingly similar, are actually not identical. Chemists call this chirality.
The distinction between right- and left-handed chiral molecules plays an important role in biology, chemistry and the pharmaceutical industry. Many of life's building blocks, such as DNA, amino acids and proteins, are chiral and only occur in a left- or right-handed version. Depending on their handedness, chiral drugs can therefore be effective, ineffective, or even harmful.
Chirality is generally considered to be a structural property. "Recently, however, there has been growing evidence that the adoption of the structural approach is not sufficient to fully understand chiral phenomena," says Hans Jakob Wörner, Professor of Physical Chemistry at ETH Zurich.
What has attracted little research until now is how electrons – the smallest, lightning-fast building blocks of atoms – move differently in chiral molecules, depending on whether they are left- or right-handed. For the first time, a team of researchers led by Wörner has found a way to visualise and manipulate the emission of electrons from chiral molecules in real time. The results have just been published in the journal Nature.
Processes on the attosecond scale
Wörner and his team investigated a fascinating effect that occurs when chiral molecules are irradiated with circularly polarised light – light that rotates in a spiral like a corkscrew. In the very first moments after light excitation, an electron is ejected from the molecule. The main point here is that, depending on the chirality of the irradiated molecule and the direction of rotation of the light, the electron is emitted either in the direction of propagation of the incident light beam or in the opposite direction.
In their study, the researchers not only succeeded in measuring this effect – known as photoelectron circular dichroism, or PECD for short – but also in amplifying it, manipulating it in time and even reversing it.
This measurement was made possible by a unique flash device for electrons that operates with unprecedented precision: it creates circularly polarised attosecond pulses – flashes of light that achieve a temporal resolution of one billionth of a billionth of a second. This is what is needed to observe electron dynamics on their natural attosecond time scale. For the first time, the handedness of electron movements in these light pulses has been detected due to their own direction of rotation.
In combination with a temporally superimposed, also circularly polarised beam of infrared light, the researchers were not only able to measure how soon an electron is ejected from a chiral molecule after light excitation but also to manipulate the direction in which the electron preferentially moves – depending on the sample's chirality, the direction of rotation of the light beams and their phase shift.
Fundamental research with application potential
The findings enable the adoption of a new approach to chirality: "We no longer understand chirality solely as a static feature of molecular structure but also as the dynamic behaviour of electrons in chiral systems," says Meng Han, a former postdoctoral researcher in Wörner's group and first author of the study. Chirality as a controllable electronic phenomenon had previously only been suspected, but was not experimentally accessible due to the lack of the necessary technology.
In the future, the developed attosecond flashes could help determine the chirality of medical agents with greater sensitivity and clarify fundamental questions regarding the origin of chirality in life.
The method also opens up new avenues for the time-resolved studies of chiral processes at the electronic level, potentially leading to advancements in information processing, spintronics, molecular machines and biosensor technology.
