Photoelectron Holography of Gas-Phase Molecules

Photoelectron holography. A photon of energy  (linear polarized) is absorbed (K-shell) by an atom serving as an emitter of a photoelectron p-wave. The direct wave, Y0, interferes with the object wave, YS, from the scatterer (one of the atoms in the molecule). Such scattering events are summed over all molecular atoms j = 1,…N yielding an intensity distribution I = ïY0 +SiYSï2, which represents a hologram that can be measured over a large portion of photoelectron momentum-space (Krasniqi2010).

Simulated photoelectron holograms.Simulated photoelectron holograms in 2D momentum space (middle panels) for perfectly oriented pFAB molecules for the molecular orientations and detector geometries indicated on the left. The FEL is polarized along the y-axis (top row) and the z-axis (bottom row). The reconstructions of the simulated holograms are shown on the right (Krasniqi2010).

Photoelectron holography (as well as Auger electron holography) is a special case of photoelectron diffraction, where high-kinetic energy electrons are used to record photoelectron holograms that can be directly reconstructed to yield real-space images of the environment surrounding the photoelectron emitter (Barton1988, Fadley2001, Fadley2008). While the technique was developed and is being used in solid state and surface physics, holograms of gas-phase molecules have not been recorded to date, despite the great potential that this method has for directly imaging photochemical reactions of gas-phase molecules in femtosecond pump-probe experiments. Using the photoelectron-photoion-coincidence technique to orient and align gas-phase molecules (see Photoelectron-Photoion Coincidence Experiments with Synchrotron Radiation), we plan to establish photoelectron holography of free gas-phase molecules using synchrotron radiation with the goal of transferring this technique to Free-Electron Lasers in order to perform time-resolved photoelectron holography experiments [Krasniqi2010, Rolles2013].

Own References:

F. Krasniqi, B. Najjari, L. Strüder, D. Rolles, A. Voitkiv, and J. Ullrich, Imaging molecules from within: Ultrafast angström-scale structure determination of molecules via photoelectron holography using free-electron lasers, Phys. Rev. A 81, 033411 (2010).

D. Rolles, Trendbericht Physikalische Chemie 2012: Molekülkino – Experimente mit Freie-Elektronen-Lasern, Nachr. a. d. Chemie 61, 313 (2013).


Other relevant publications in the research field:

J. J. Barton, Photoelectron Holography, Phys. Rev. Lett. 61, 1356 (1988).

C. S. Fadley, M. A. Van Hove, A. Kaduwela, S. Omori, L. Zhao, S. Marchesini, Photoelectron and x-ray holography by contrast: enhancing image quality and dimensionality, J. Phys.: Condens. Matter 13 10517 (2001).

C. S. Fadley, Atomic-level characterization of materials with core- and valence-level photoemission: basic phenomena and future directions, Surf. Interface Anal. 40, 1579–1605 (2008).