Numerous crucial chemical processes in biology, environmental chemistry, electrochemistry and heterogeneous catalysis happen exclusively at interfaces. This selectivity originates from the very special physical properties of molecular systems in proximity of a phase boundary which leads to the formation of a great variety of distinct molecular structures and dictates their dynamics. The goal of our research is to gain deeper insight into the fundamental processes that govern interface chemistry and improve our understanding of the underlying physics.
To investigate and ultimately understand chemical processes at interfaces, specialized experimental tools are needed that allow for identifying interfacial molecular species, study their structures and follow their dynamics. We therefore develop, refine and employ modern surface spectroscopic techniques based on nonlinear light-matter interactions for the investigation of various molecular interfaces.
In the following list some selected current research projects are presented in more detail.
1) Development of new spectroscopic tools
A key technique in our work is second order vibrational spectroscopy (SFG/DFG) which is in many cases interface specific and can be used as an ultrafast probe of the vibrational fingerprint of interfacial molecular species. While the obtained magnitude spectra report on vibrational resonances important additional information is encoded in the phase of the generated nonlinear signal. With precise knowledge of the phase we can determine absolute molecular orientations, study local field effects and get insight into structural depth profiles (e.g. at liquid interfaces).
An accurate phase measurement of the nonlinear signal is a very challenging experimental task and common spectroscopic approaches often lack the required reliability. We have therefore developed a novel spectrometer design that circumvents most of the typical technical deficiencies and allows us to routinely measure accurate phase-resolved second order spectra from various interfaces.
We are continuously extending our experimental capabilities with the goal to obtain a maximum of spectroscopic information from interfacial systems. These development projects include time resolved techniques such as pump-probe and multidimensional spectroscopies as well as the implementation of phase resolved second order microscopy.
2) Understanding structural depth profiles at liquid interfaces
When we talk about interfaces, we often imagine a well-defined, thin interface layer that separates the two adjoining bulk phases. In many cases, however, this simple picture is far from being accurate e.g. for liquid interfaces. The presence of a phase boundary can here lead to the formation of an extended transition zone in the subsurface region where the local physical properties of the liquid clearly deviate from the properties of the corresponding bulk. These deviations are typically caused by gradients along the surface normal in (i) chemical composition of the liquid (e.g. Ion populations), (ii) structural motives and molecular orientations, and (iii) gradients in the electric potential.
Our goal is to reveal the physical nature of these transition zones and to understand how their properties influence interface chemistry. We therefore investigate the structural depth profiles in various liquids by leveraging phase information from combined SFG and DFG experiments.
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3) Phase sensitive nonlinear vibrational wide-field microscopy
Within the project of investigating spatial phenomena at interfaces we recently added imaging capabilities to our nonlinear vibrational spectroscopy to also resolve lateral sample inhomogeneities. Common SFG microscopes typically suffer from two deficiencies: The first concerns the required tilted sample irradiation which makes the correct imaging highly non-trivial, and the second challenge is the general low signal to noise ratio (S/N) resulting from the weak nonlinear signals.
We have developed a new SFG microscope design that addresses both challenges by employing a sophisticated imaging geometry in combination with a pixel-by-pixel balanced imaging scheme. The successful implementation of this advanced microscope gives us the capability to perform vibrational microscopy experiments on various interesting systems such as biological samples or chiral surfaces.