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Attosecond Delays in Molecular Photoionization

August 22, 2016

New experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.

Martin Huppert, Hans Jakob Wörner and co-workers report measurements of energy-dependent photoionization delays between the two outermost valence shells of N2O and H2O. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in N2O, whereas the delays in H2O are all smaller than 50 as in the photon-energy range of 20–40 eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in N2O are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to ~ 110 as. The unstructured continua of H2O result in much smaller delays at the same photon energies. Their experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.
Figure 1: Measured and calculated delays between photoelectrons leaving N2O+ in its Ã+ or X+ states, (b) same as (a) for H2O, (c),(d) calculated molecular delays for the two species as defined by Eq. (8), (e) shape resonance of σ symmetry in the photon-energy range of 25–30 eV associated with the Ã+ state of N2O+. The lower surface shows the numerically calculated molecular potential containing electrostatic and exchange interactions. The upper surface shows the total potential, i.e., the sum of the molecular and centrifugal potentials for l = 5. The wave functions of the bound orbital and the shape-resonant state are illustrated by isosurfaces with color-coded signs. The gray arrows represent tunneling of the photoelectron through the barrier.

The work by Huppert et al. is an important milestone in the investigation of molecular photoionization dynamics. Moreover, it complements recent studies on solid surfaces, in which ionization delays can also be attributed to resonance effects involving excited electronic states. Combined with knowledge of molecular orientation, these techniques could provide new insights into previously inaccessible properties of a molecule’s electronic structure including electronic correlations and the localization of electronic wave packets within the molecules.

Reference: Huppert, M., Jordan, I., Baykusheva, D., von Conta, A., and Wörner, H. J. (2016). Attosecond Delays in Molecular Photoionization. Phys. Rev. Lett. 117: 093001. PhysRevLett.117.093001 Huppert-2016 (585 KB)

Also see: Physics Viewpoint
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