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O: Fachverband Oberflächenphysik
O 87: Solid-liquid interfaces: Structure, spectroscopy
O 87.7: Vortrag
Donnerstag, 12. März 2026, 16:30–16:45, TRE/PHYS
Beyond the Electric Dipole Approximation: Electric and Magnetic Multipole Contributions Reveal Biaxial Water Structure from SFG Spectra at the Air-Water Interface — •Louis Lehmann1, Maximilian R. Becker1, Lucas Tepper1, Alexander P. Fellows2, Álvaro Díaz Duque2, Martin Thämer2, Martin Wolf2, and Roland R. Netz1 — 1Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany — 2Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
Sum-frequency generation (SFG) analysis has been hindered by the lack of quantitative predictions for higher-order multipole contributions. Magnetic dipole and electric quadrupole terms, though determined by bulk properties, appear in all SFG spectra and obscure connections to interfacial structure. We present a simulation-based framework that predicts all multipole contributions and provides depth-resolved spectra for precise localization of features. Applied to the air-water interface, it reproduces experimental spectra across polarization combinations in both bending and stretching regions. Higher-order multipoles are essential: in the bending band, electric and magnetic dipoles are comparable, while the quadrupole dominates; in the OH-stretch region, the quadrupole accounts for the 3600/cm shoulder. Isolating the true second-order electric-dipole susceptibility reveals pronounced biaxial ordering of interfacial water and enables reliable interfacial structure determination from SFG spectra.
Keywords: Sum Frequency Generation Spectroscopy; Air Water; Multipole Expansion; Spectroscopy; Interfaces