When electrons dance waltzes - Right- and left-handed molecules can be distinguished by short laser pulses

The identification of right- and left-handed molecules is crucial for many applications in chemistry and pharmacy. An international team of researchers (CELIA-CNRS / INRS / Max Born Institute / SOLEIL) has now introduced a new, original and highly sensitive method that allows a much better determination of the handedness of molecules than previous methods. With the help of extremely short laser pulses, the researchers are able to vibrate electrons in molecules and thus determine the direction of rotation of the molecules. The research results appeared in "Nature Physics".

Not only in humans, the question of whether someone is right-handed or left-handed is important. Depending on which hand we use, our fingers enclose an object clockwise or against it. Also in the world of molecules, handedness is of great importance. In molecules, the property of having a preferred handedness is even more important than in humans: for certain substances, depending on whether they are right- or left-handed, may be either toxic or salubrious. Therefore, some medications may only contain either left- or right-handed molecules.

The problem here is to identify and separate right-handed and left-handed molecules, which are otherwise completely identical, according to their "chirality" sense of rotation. Because except when in contact with another chiral substance, they behave completely the same. An international research team has now developed a new method for determining the handedness of molecules with extreme sensitivity.

Since the 19th century it has been known that molecules can be present in different handedness. The best known example is the genetic material, such as human DNA, whose structure corresponds to a clockwise corkscrew. To determine the handedness one usually uses so-called circularly polarized light rays, which have either right- or left-handed electromagnetic fields - like a corkscrew along the propagation axis wound. This chiral light is slightly better or worse absorbed when it encounters molecules with the same or the opposite sense of rotation. However, the effect is small because the wavelength of light is much larger than the atomic distances in molecules. The light "senses" the sense of rotation of the molecules only very slightly.

Fig. 1: An ultra short, circularly polarized laser pulse is followed by electrons of a spiral clockwise or anticlockwise rotation, which depends on the handedness of the molecules. Source: Samuel Beaulieu

With the new method, however, the signal can be increased enormously. "The trick is to irradiate the molecules with a very short laser pulse," says Prof. Olga Smirnova, head of the theory group at the Max Born Institute. Such a pulse is only about a tenth of a trillionth of a second long, and transfers energy to the electrons in the molecule. This stimulates them for a short time to oscillate. Since the electrons are in the right- or left-handed structure of the molecule, their vibration also assumes this sense of rotation.

The oscillation can then be read out with a second laser pulse. This pulse must also be short in order to register the direction of electron movement. He has enough energy to knock the excited electrons out of the molecule. Depending on whether the electrons performed right- or left-handed vibrations, they then either fly out of the molecule in the direction of the laser beam or in the opposite direction.

In experiments at the Center for Intense Lasers and Applications (CELIA) of the University of Bordeaux, it was possible to determine the handedness of the molecules very efficiently, with a 10,000-fold stronger signal than with the commonly used method. In addition, chiral chemical reactions can be initiated and tracked over time. The trick is to provide very short laser pulses with the appropriate frequency. This technology comes from basic physical research and has only recently become available. It may prove to be extremely helpful in other areas where the handedness of molecules plays a role, such as chemical and pharmaceutical research.

Since the identification of the handedness of molecules has been achieved with the new method, the scientists are already thinking about developing a laser separation method for right-handed and left-handed molecules. (Text: Dirk Eidemüller / Forschungsverbund Berlin e.V.)

Original publication

Photoexcitation circular dichroism in chiral molecules

S. Beaulieu, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, A. G. Harvey, F. Légaré, Z. Mašín, L. Nahon, A. F. Ordonez, S. Petit, B. Pons, Y. Mairesse, O. Smirnova, V. Blanchet

Nature Physics 14 (2018) 484-489

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