The ocean surface layer, flooded by sunlight, forms an environment of rich photochemistry and photobiology. The gas exchange between ocean and atmosphere happens through the ocean surface, often inhibited by the presence of an organic surfactant layer. Such ocean nanolayers are regularly found directly at the sea-air interface. Moreover, soluble surfactants and particulate matter including microorganisms are present in the sea surface microlayer. This layer comprises about the uppermost millimeter of the water column, hence there is only few of this substance available. Furthermore, exclusive separation of nanolayer matter still poses a challenge. The common basic principle is a spatial and temporal overlap of intense, pulsed laser beams on a surface. An important step of optimization was the implementation of a tunable mid-IR laser in SFG experiments during cluster phase 1. By scanning the IR wavelength, this approach yields a surface-sensitive vibrational spectroscopy technique (vibrational sum frequency generation, VSFG). Our own research span the first successful application of VSFG spectroscopy for characterization of the sea surface nanolayer. Lab model systems as well as field samples taken at Boknis Eck time series station have been analyzed. Based on a detailed analysis of the obtained spectra, we also were able to gain information regarding amount of natural surface layer surfactants, their solubility behavior and interaction with the aqueous phase.
Several entities representing or featuring interfaces are found in the oceanic water column. Examplesare: The ocean surface, Biofilms, Microorganisms, Dust particles, rocks, Aerosol particles, Transparent expolymer particles (TEP). Processes taking place at such surfaces largely govern exchange processes in the ocean system (e. g. aerosol oxidation,22, TEP coagulation,23, gas exchange,1 etc.). In addition, in some surface VSFG spectroscopy experiments we observed phenomena that raise the question whether particles actually might participate in the signal generation process and their influence needs to be taken into account for spectra interpretation In the past years, we have gained considerable experience in the application of VSFG spectroscopy on flat-surface water samples with the focus on sea-surface nanolayers. Together with the technique of sum frequency scattering (SFS), which just has been established as a new feature of the VSFG setup of the Friedrichs workgroup, this opens up the possibility to assess vibrational features on a wide range of surfaces throughout the water column. Furthermore, the laser system available allows for the generation of tunable visible light down to a wavelength of 420 nm. This paves the way, with very little extra technical effort, also for the application of second harmonic generation and scattering to the samples in question. The additional information available from these techniques will allow for the detection of characteristic electronic resonances from chromophores in organic matter at the surface or within scattering particles. Examples for such chromophores are molecules such as chlorophyll and characteristic aromatic ring systems from humic material. If applied together with the already established VSFG spectroscopy, this opens up a much improved perspective for the identification of surface nanolayer compounds. The combination of all these techniques will provide for a spectroscopical “swiss army knife” (term coined by Geiger13) for examination of interfacial entities in seawater, ranging from two-dimensional macroscopic interfacial layers down to different types of particles found below the actual surface in the microlayer and the water column.
