CPL finds numerous applications ranging from modern displays and RealD 3D cinema technology, to circular dichroism of magnetic materials or chiral macromolecules in structural biology and allows addressing the fascinating origin of the homochirality of life. For the first time, on the PLEIADES beamline, circularly polarized light¹ (CPL) was used to probe nuclear dynamics of highly excited neutral molecules, which dissociate within a few femtoseconds (10^-15 s) only after absorption of the X-ray photon.

The highly excited species SF6 F1s^-1a^*_1g experience electronic relaxation by emitting a valence electron², a so-called “Auger electron”. However, since the fragmentation process is very fast, the ejection of the Auger electron may occur from already dissociated fragments.

As it has been observed before using linearly polarized light, one is able to distinguish Auger electrons ejected from the fragment-atoms moving either towards or away from the electron detector due to the small differences in their kinetic energies. This effect, called Auger-Doppler by analogy with acoustical Doppler effect³, is observed for the first time using CPL despite the uniform distribution of the dissociating chemical bonds induced by the CPL.

Based on the present results, it will be possible to use the circular dichroism in Auger-Doppler profiles as an efficient probe of the photoionization dynamics as well as of the molecular structure and chemical environment even in achiral species⁴.

Figure : - Left panel: Schematic diagram of the ultrafast dissociation and formation of atomic-like Auger lines observed in Resonant Auger Spectroscopy (RAS). - Right panel: Schematic diagram of the Auger-Doppler effect observed in RAS due to emission of the electrons from the forward/backward travelling core-excited atomic fragments.

 


Reference:

O. Travnikova, J.-C. Liu, A. Lindblad, C. Nicolas, J. Södreström, V. Kimberg, F. Gel'mukhanov, C. Miron
Phys. Rev. Lett 105, 233001 (2010) 

¹Polarized light - polarization is a characteristic of electromagnetic waves, such as light, which propagate themselves in a straight line by vibrating vertically, horizontally or circularly. Using polarizing filters, it is possible to select one direction of vibration rather than another. When illuminated, certain molecules have the property of changing the direction of polarization of the light.

²Valence electrons - electrons located on the outermost layers of the atom that are involved in the chemical bonds between atoms.

³Doppler effect - frequency shift of an acoustic or electromagnetic wave between emitted and received signals when the distance between the transmitter and receiver varies with time. Although this physical phenomenon is generally denoted as the Doppler effect, the term "Doppler-Fizeau effect" is reserved for light waves.

⁴Achiral species – molecules for which right- and left-handed forms cannot be determined, and thus sometimes called ambidextrous, rather like socks that have never been worn.