Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source

January 17, 2017

The mystery of ultrafast reactions

As reported by researchers from the biophotonics group led by Professor Jean-Pierre Wolf in Geneva and the ultrafast spectroscopy group led by Professor Hans Jakob Wörner in Zurich in the latest edition of the journal Science, they have refined an X-ray-based measuring technology, known as X-ray absorption spectroscopy, to achieve a time resolution of 20 femtoseconds (2 x 10^-14 s). This has allowed the scientists to observe how structural changes take place in the space occupied by the electrons – the molecular orbitals – of two highly fluorinated compounds. The molecules themselves – carbon tetrafluoride (which contains four fluorine atoms) and sulphur hexafluoride (which contains six fluorine atoms) – also adopted a different shape as the chemical bonds broke due to the movement of the electrons. For example, carbon tetrafluoride spontaneously lost a fluorine atom after being ionized and transformed from a tetrahedron into a flat, triangular carbon molecule with three fluorine atoms. This method allows element-specific measurement of carbon-containing molecules and their reactions, which are relevant to ozone depletion in the atmosphere.

Figure 2. Transient-absorption spectroscopy at the carbon K-edge. (A) An intense near-infrared pulse induces single ionization of CF₄ to CF₄⁺ which is unstable in its electronic ground state and dissociates into CF₃⁺ + F. The sequence of geometries is taken from a calculated minimum-energy reaction path (see text for details). (B) Absorbance A(t) = ln[I₀/I(t)] as a function of the SXR-NIR time delay. Negative time delays correspond to the SXR pulse preceding the NIR pulse. The intensity axis, as well as the color scale, are linear. The standard deviation of this data set amounts to 4%. The calculated stick spectrum in the inset has been shifted by -2.5 eV. (C) Orbital diagram illustrating selected transitions, as obtained from TDDFT/LB94 calculations.

News sites reporting on the publication:

• Science et Avenir
• Tribune de Genève
• Analytik News
• Der Standart
• pro-physik.de
• ETH News
• 20min (Geneve)

Also see the UniGE Presse Release (48 KB) with a link to the movie.

Reference: Pertot, Y., C. Schmidt, M. Matthews, A. Chauvet, M. Huppert, V. Svoboda, A. von Conta, A. Tehlar, D. Baykusheva, J.-P. Wolf and H. J. Wörner (2017). Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source. Science. (10.1126/science.aah6114) Pertot-2017 (1.13 MB)

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