RIXS : A New Method to Derive Electronegativity from Resonant Inelastic X-ray Scattering


Resonant x-ray Raman spectroscopy takes advantage to explore the ultrafast dynamics and high-energy-resolution decay spectra with long-pulse light sources using the concept of an effective duration time [1-bis] of the scattering process to extract temporal dynamics a posteriori. [1-bis]F. Gel'mukhanov and H. Ågren, Phys. Rep., 312, 87, 1999.
Our activity in this field focus essentially to describe, with help of theoretical models and ab-initio calculations, the role of interference effects occuring in elastic (Thomson/resonant) and Kα processes through Resonant X-ray Raman spectroscopy (RIXS) experiments in chlorinated molecules (HCl, CF3Cl, ...).

Caption: KL emission lines of HCl for different excitation energies around the resonant excitation (2823.5 eV). Comparison between experiments and theory is given.

[1] M. Simon, L. Journel, R. Guillemin, W. C. Stolte, I. Minkov, F. Gel’mukhanov, P. Salek, H. Agren, S. Carniato, R. Taïeb, A. C. Hudson, and D. W. Lindle. Phys. Rev. A 73, 020706 (2006).
[2] Carniato S. et al., Chem. Phys. Lett., 439, 402 (2007).
[3] M. Kavčic, M. Žitnik, K. Bučar, A. Miȟelič, S. Carniato, L. Journel, R. Guillemin, and M. Simon, Phys. Rev. Lett. 105, 113004 (2010).



RIXS : A New Method to Derive Electronegativity from Resonant Inelastic X-ray Scattering

Thanks to experimental measurements and theoretical calculations, we have shown[4], recently, that Polarization-dependent resonant inelastic x-ray scattering (RIXS) can be seen as a new probe of the electronic structure of isolated molecules.
Combining spin-orbit ab-initio calculations and theoretical model taking into account the random molecular orientation effect on the scattering amplitude, we demonstrated [4,5] that Kα emission from gas-phase molecules is sensitive to molecular field, Singlet-Triplet exchange interaction and spin-orbit coupling, through a modulation of the 2pz (and to a lesser extent 2px,y) population in Kα1,2 spin-orbit components with the nature of the chemical bond.
[4] R. Guillemin, S. Carniato, W.C Stolte, L. Journel, R. Taïeb, D.W. Lindle, and M. Simon, Phys. Rev. Lett. 101, 133003 (2008). [5] Experimental and theoretical investigation of molecular field effects by polarization-resolved resonant inelastic x-ray scattering S. Carniato, R. Guillemin, W.C. Stolte, L. Journel, R. Taïeb, D.W. Lindle and M. Simon, Phys. Rev. A 80, 032513/12 (2009).

Direct exciting application is the measurement of atom and group electronegativities. Indeed, based on electron-density analysis of the LUMOs of different substituents bound to chlorine, polarized RIXS provides an electronegativity scale based on charge localization into the LUMO orbital reflecting to the best directly the polarity of the chemical bond.
As a major result, we showed that RIXS gives the most accurate electronegativity value for radicals, without limitations as in Pauling (steric effect), Mulliken/Pearson (geometries of free radical which can be different from the ones of the same species bonded to Chlorine) or XPS (electronegativity derived from XPS are known to be influenced by polarizability and charge transfer) scales and represents the most consistent electronegativity scale coherent with the fundamental ideas of Pauling about the chemical bond, Pauling himself describing electronegativity as the power of an atom in a molecule to attract electrons to itself.
Further extension of this procedure to the determination of group electronegativities in gas phase, solids and even liquids is promising.