Research Group Prof. Dr. G. Friedrichs

Sum Frequency Generation

SumFrequencyGeneration

SFG Spectrometer for Ocean Surface Research

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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. 

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

VSFG Spectra of Natural Marine Samples

ScreenSamplingAir-sea interaction takes place at the ocean surface microlayer, which is known to modulate physical, chemical, and biological processes. It is enriched by organic material, flooded by sunlight, inhibited by many organisms, and thus forms a unique reaction environment. In particular, surface active material such as lipids, carbohydrates, and proteinaceous material is enriched at the topmost layer (organic nanolayer), which can be investigated by VSFG spectroscopy. So far, the detailed composition, its molecular structure and seasonal trends of these quantities are largely unknown.

Seasonal changes in the characteristics of the sea surface nanolayer

The topmost part of this microlayer, located at the interface between ocean and atmosphere, is called the sea surface nanolayer. Because of its small vertical dimension and the tiny amount of substance it represents, an analytical tool is needed that is sensitive enough to detect this small amount of substance and selective enough to specifically probe the ocean surface. One tool that serves this purpose is vibrational sum frequency generation spectroscopy (VSFG). We have demonstrated the suitability of VSFG spectroscopy for this purpose [1] by taking samples of the sea surface nanolayer using the screen sampling method (see photo).

SFG Intensity














 

An example of a VSFG spectrum taken on seawater is shown in the Figure. Vibrational spectra of seawater interfaces basically show stretch vibrations of C-H and O-H bonds. The resulting spectra allow conclusions regarding composition and structure of the layer samples, notably the solubility of the surfactants (“wet” or “dry” surfactants) and the abundance of lipid-chain-carrying and saccharide compounds [2].

A time-series of seawater SFG spectra was taken at Boknis Eck Time Series Station (https://www.bokniseck.de) based on on monthly measurements. A seasonal variability has been found that indicates a peak in SFG active surfactant abundance in late summer [3]. This is significantly later than the spring algea bloom showing that nanolayer abundance is not directly related to the phytoplankton concentration. These findings suggest that biochemical and/or photochemical transformation of organic substances is an important factor in ocean nanolayer formation.

[1] Kristian Laß, Joscha Kleber, and Gernot Friedrichs. "Vibrational Sum-Frequency Generation As a Probe for Composition, Chemical Reactivity, and Film Formation Dynamics of the Sea Surface Nanolayer." Limnology and Oceanography: Methods 8 (2010): 216-228. [doi: 10.4319/lom.2010.8.216]

[2] Kristian Laß and Gernot Friedrichs. "Revealing Structural Properties of the Marine Nanolayer From Vibrational Sum Frequency Generation Spectra." Journal of Geophysical Research 116 (2011): C08042. [doi: 10.1029/2010JC006609]

[3] K. Laß, H. Bange, G. Friedrichs, "Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)" Biogeosciences10 (2013) 5325-5334; doi:10.5194/bg-10-5325-2013.
 
Contributing Researchers: G. Friedrichs, H. Bange and (formerly) K. Laß, J. Kleber

Time-Resolved Studies of Organic Monolayer Reactivity

Natural air-water interfaces have an impact on the global climate. Processes such as air-sea gas exchange are mediated by the marine surface microlayer or other surfactant layers. These thin films of organic surface-active substances alter the physical and chemical properties of aqueous aerosols and the ocean surface itself. Moreover, in natural systems all surfaces are subject to chemical aging (e.g., atmospheric oxidation by ozone or OH radicals, photochemistry). Hence, the surface composition, the surfactant characteristics, and the physical properties of natural water-air interfaces are constantly changing.

langmuirtroughVibrational sum-frequency generation (VSFG) spectroscopy is a surface sensitive method, which has been widely used to study the static structure of organic monolayers on a submonolayer scale. To establish VSFG spectroscopy as a quantitative and time-resolved detection tool for surface reaction kinetics at liquid interfaces, we combine VSFG spectroscopy with a Langmuir trough setup. In this configuration, changes of the surface concentration of the surfactants can be followed both by VSFG spectral intensity and surface pressure. Moreover, the 2D phase behavior of the monolayer can be directly related to molecular orientation effects.

sfg_kinetics2_webAs a next step, we investigate the ozonolysis of monolayers of unsaturated fatty acids using a small hemispherical reactor. Fatty acids serve as model system for natural degradation processes of surfactant layers. Here, we try to answer the overarching research question "What determines the reactivity of organic monolayers at the water-air interface?". Recently, the reactor was equipped with a fiber-coupled solar simulator lamp allowing us to investigate the photochemistry of surfactants in future experiments. 

In order to perform quantitative surface concentration measurements, careful calibration of the VSFG signal from the surfactant monolayer has to be performed at different surface concentrations. Relying on this calibration experiments, changes of the surface concentration upon ozone induced surface oxidation or photolysis is measured and the resulting concentration-time profiles are evaluated to extract rate constants for the heterogeneous surface reaction rate constant.

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For example, oleic acid (OA) ozonolysis has been studied as a proxy for unsaturated surfactants in atmospheric aerosols. Our kinetic results are consistent with a reactive ozone uptake coefficient of γ ~ 10−6. This is in agreement with recent monolayer studies based on other surface sensitive techniques, but contradicts older OA droplet experiments reporting γ ~ 10−4. Obviously, many older studies have been biased by bulk reactivity and hence did not measure the surface process alone.

Currently, in order to investigate structure-reactivity relationships for heterogeneous O3 oxidation, we perform systematic studies on several fatty acids with shifted double-bond position in the alkyl chain. Interestingly, a distinct reactivity trend has been found. Further experiments assisted with molecular dynamics simulations will be performed to better asses the roles of surface accommodation (resulting in ozone adsorption layer)s, ozone solubilities, and different ozone permeabilities through monolayers with variable surface densities.

Contributing researchers: A. Dabrowski, F. Lange, G. Friedrichs and (formerly) J. Kleber, K. Laß
 

Properties of Photoswitchable Functionalized Surfaces

Functionalization of surfaces with photochromic molecular switches enables external control of interfacial properties. Consequently, photoswitchable molecular systems have found many applications in nanotechnology, biotechnology, and material sciences.

VSFG-Study of Azobenzene Based SAMs

abswitch1_webAs part of a subproject of the Collaborative Research Center “Function by Switching” (SFB 677), the inherently surface-sensitive Vibrational Sum-Frequency Generation (VSFG) spectroscopy has been applied as a powerful technique to investigate the molecular structure and photoswitching efficiency of functionalized surfaces. Systematic studies on pure and mixed (i.e., “diluted” with simple alkylthiols) self-assembled monolayers (SAMs) help to unravel the effects of steric hindrance and energy dissipation, which are known to degrade the switching efficiency.

VSFG is employed to monitor the morphology and switching properties of azobenzene based SAMs on gold. As a prerequisite for a detailed assessment of the switching efficiency, knowledge of morphological aspects such as possible phase separation, available free volume, and the orientation of the molecular switch is necessary. For example, Methylazobenzene-O-undecanethiol (MeAB) has been used as the molecular switch with the methyl substituent serving as a sensitive VSFG marker to measure the trans-cis switching state of the AB moiety. Upon “dilution” of the surface with different co-ligands, due to the loss of lateral and axial structural order, the SFG spectrum changes significantly.

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Contributing Researchers: Saira Riaz and Gernot Friedrichs

Carbohydrate Structures at the Water/Air Interface