Biochemical reactions are very sensitive to the spatial conformation of interacting molecules. Some drugs owe their effectiveness to only one of the enantiomers, while the other can be very toxic.

However, enantiomers are very difficult to distinguish by chemical or physical methods. Certainly, they absorb right- or left-circularly-polarized light differently, in which the electromagnetic field "rotates" clockwise or counter-clockwise during its propagation. Making one "turn of the clock" by traversing one wavelength (1 µm), its rotation is therefore very small at the scale of a single molecule (200 nm) with extremely fine differences in absorption between enantiomers.

The researchers were able to greatly amplify the chirality signal by probing the molecules using ultra-short femtosecond (10^-15 s) circularly polarized laser pulses that carry electrons in a high-energy state. Like a nut on a bolt, these electrons are propagated along a spiral whose direction of propagation depends on the chirality of their original molecule. Then they are photoionized with a second linearly polarized laser pulse and ejected from the molecules in two distinct directions.

This method applicable to all chiral molecules, called photoexcitation circular dichroism, is of interest to those domains where chirality is a central issue such as chemistry, biochemistry, catalysis and pharmaceuticals.

This work is the result of a collaboration between the Institut national de recherche et de sécurité, the Centre lasers intenses et applications (CNRS, Université de Bordeaux, CEA), the Max Born Institute (Germany), the SOLEIL synchrotron, and the Iramis.