Regensburg 2025 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 91: Focus Session Chemical Imaging for the Elucidation of Molecular Structure I (joint session O/BP)

O 91.4: Vortrag

Donnerstag, 20. März 2025, 16:30–16:45, H24

Elucidating the Composition, Order, and 3D Molecular Orientation of Thin Films with Phase-Resolved Sum-Frequency Generation Microscopy — •Alexander Fellows, Ben John, Martin Wolf, and Martin Thämer — Fritz-Haber-Institute, Berlin, Germany

The vast majority of molecular interfaces have highly heterogeneous structures, ranging across all length-scales. These manifest as variations in density, composition, and molecular packing structure, all of which are critical in controlling the macroscopic properties and functional behaviour of the films. While various chemical imaging techniques can access many of these important structural details, characterising their relative order and specific packing arrangements represents a formidable challenge.

Here, we present a chemical imaging approach based on phase-resolved sum-frequency generation (SFG) microscopy. By probing molecular vibrations, this technique achieves molecular recognition and thus is sensitive to the local composition and density. Furthermore, through its symmetry selection rules, output SFG signals are dependent on absolute molecular orientations. This hence allows it to distinguish different molecular conformations and characterise the amount of orientational order in the system. Finally, with an azimuthal-scanning approach, the in-plane and out-of-plane signal contributions can be separated, allowing the 3D molecular orientations to be elucidated. By applying SFG imaging to model lipid monolayers, we gain an unprecedented overview of their hierarchical packing structures.

Keywords: Sum-Frequency Generation; Vibrational Spectroscopy; Molecular Imaging; Thin Films; Lipid Membranes