Advanced SFG Spectrometer for Ultrafast Vibrational and Electronic Analysis of Interfaces

• Subject Area: Physical Chemistry
• Term: Funded in 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 574594370

Interfaces play a crucial role in many colloids and interface-controlled materials and can be used to control material properties at the molecular level. However, there are typically only a few methods available to characterize fluid interfaces - such as solid/liquid or liquid/gas interfaces - at the molecular scale. This is where the spectrometer for time-resolved and electronic sum-frequency generation (SFG) spectroscopy is to be used, helping to elucidate the composition, structure, and dynamics of interfaces at the molecular level. The focus of the work lies on photo-switchable interfaces and interfaces at which photochemical reactions can occur upon optical excitation. These can include, for example, surfaces with plasmonic nanostructures. The dynamics of such interfaces will be investigated on the one hand using time-resolved SFG spectroscopy, and on the other hand, the electronic structure of the interfaces will be further characterized using double-resonant SFG spectroscopy. Interfaces can be modified, for example, with photo-switchable amphiphiles or their aggregates in combination with other compounds such as polyelectrolytes or photosensitizers. These systems are used to control inherently interface-dominated materials with light, leveraging structureproperty relationships from the interface to the macroscopically visible material, thereby enabling control at the molecular level. In addition to studying the molecular structure of interfaces, the spectrometer will also be used to investigate fundamental physicochemical questions concerning the dynamics of molecules in different geometries. This spans, single molecules in bulk liquid, aggregates such as micelles in solution, as well as the lateral arrangement and dynamics of molecules at interfaces. To this end, the multifunctional spectrometer will be used in two key modes: (1) In the bulk, photo-switchable molecules will be studied using transient absorption spectroscopy. (2) At interfaces, various forms of SFG spectroscopy will be applied - ranging from pump-probe SFG to double-resonant SFG spectroscopy. For this, a pulsed high-power laser system with a high repetition rate and ultrashort femtosecond pulses, along with several conversion stages, is required to generate three pulses: a picosecond pulse with narrow spectral bandwidth, tunable in the visible to near-IR spectral range, and two additional frequency-tunable ultrashort laser pulses. One of these pulses will serve as the pump pulse for excitation, and another will be used for the SFG experiment at wavelengths in the mid-infrared spectral range.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: SFG-Spektrometer für ultraschnelle vibronische und elektronische Analyse von Grenzflächen
• Instrumentation Group: 5700 Festkörper-Laser
• Applicant Institution: Universität Münster
• Leader: Professor Dr. Björn Braunschweig