Interface-specific ultrafast two-dimensional vibrational spectroscopy.

Surfaces and interfaces are omnipresent in nature. They are not just the place where two bulk media meet. Surfaces and interfaces play key roles in a diversity of fields ranging from heterogeneous catalysis and membrane biology to nanotechnology. They are the site of important dynamical processes, such as transport phenomena, energy transfer, molecular interactions, as well as chemical reactions. Tools to study molecular structure and dynamics that can be applied to the delicate molecular layers at surfaces and interfaces are thus highly desirable. The advent of multidimensional optical spectroscopies, which are the focus of a special issue of Accounts of Chemical Research, and in particular of two-dimensional infrared (2D-IR) spectroscopy has been a breakthrough in the investigation of ultrafast molecular dynamics in bulk media. This Account reviews our recent work extending 2D-IR spectroscopy to make it surface-specific, allowing us to reveal the structure and dynamics of specifically interfacial molecules. 2D-IR spectroscopy provides direct information on the coupling of specific vibrational modes. Coupling between different modes can be resolved and quantified by exciting a particular mode at a specific frequency and probing the effect of the excitation on a different mode at a different frequency. The response is thus measured as a function of two frequencies: the excitation and the probe frequency, which provides a two-dimensional vibrational spectrum. When two vibrational modes are coupled, this will give rise to the intensity in the off-diagonal part of the 2D-IR spectrum. The intensity of the cross-peak is determined by the strength of the coupling between the two modes, which, in turn, is determined by molecular conformation. One can therefore relate the 2D-IR spectrum to the molecular structure. By delaying pump and probe pulses relative to one another, one can obtain additional information about conformational fluctuations. The surface-specific 2D-IR approach presented here combines the virtues of 2D-IR with the surface specificity and sub-monolayer sensitivity of vibrational sum frequency generation (SFG). We demonstrate its application on a self-assembled monolayer of a primary alcohol on water. It allows for the elucidation of different contributions to the coupling between the different interfacial methyl and methylene stretching modes. Although the surface 2D-IR technique presented here is conceptually closely related to its bulk counterpart, it is shown to have distinct characteristics, owing to the preferential alignment of molecules at the interface and the strict selection rules of the SFG probing scheme. We present an analytic theoretical framework that incorporates these effects and present simulations on instructive examples as well as on the alcohol monolayer. Overall, these results illustrate the potential of extending 2D-IR spectroscopy to the investigation of surface molecular dynamics.