Research Group Prof. Dr. G. Friedrichs

SFG Spectrometer for Ocean Surface Research


Sum frequency generation (SFG) spectroscopy makes possible the investigation of composition, structure, and reactivity of organic monolayers on water samples. The photograph displays the SFG spectrometer (right) operated by a picosecond laser system (left). Short pulses of light in the visible (green arrow) and infrared (red arrow) spectral range are used for nonlinear generation of sum frequency photons (blue arrow) at the water/air interface - provided that the frequency of the infrared laser pulse is resonant to a vibrational transition of the molecule present at the air-water interface.

A sum-frequency generation (SFG) spectrometer is used to study the structure and reactivity of the organic monolayer present at air-water interfaces. In SFG spectroscopy, two high-intensity laser pulses, an infrared (IR) and a visible (VIS) laser beam, are spatially and temporally overlapped on a surface or interface - in our case the water/air interface. In a second-order nonlinear optical process, two photons of the incident beams combine to form a new photon exhibiting the sum of the frequencies (or energies) of the two incident photons. This nonlinear process can be visualized by an incident IR photon inducing a vibrational excitation of a surface species and a simultaneous Raman transition initiated by the VIS photon. As a second-order nonlinear optical phenomenon, the overall SFG process can only take place in non-centrosymmetric media. As both the air above the surface and the bulk liquid phase below the surface are centrosymmetric in average, SFG exclusively takes place at the interface. From this it follows that SFG is inherently surface sensitive and hence SFG spectroscopy is ideally suited to monitor surface films. Moreover, SFG is species selective (measurement of vibrational spectra) and orientation selective (polarization dependence of SFG signal).

SFG Spectroscopy PrincipleSFG Spectroscopy: Principle

Our SFG spectrometer is operated by the outputs of a picosecond Nd:YAG laser (EKSPLA PL2241A) pumped OPG/OPA system with difference frequency generation unit (PG401/DFG2-10P). The narrow-bandwidth (3.5 cm-1) scanning SFG spectrometer can be used to measure IR vibrational spectra down to wavenumbers of 1000 cm-1. Current research of the group is concerned with the application of the SFG methodology in the field of marine sciences. Test spectra obtained with seawater samples are shown below: next to the OH stretching bands at wavelengths of 3000-3700 cm-1, which are typical for the hydrogen bonding network of water surfaces, we also observe strong CH-stretch vibrations clearly indicating the presence of a long alkyl-chain containing organic monolayer at the seawater surface. 


Left Figure: ps-Laser Performance. Right Figure: SFG Experiment.