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Abstract:
Since its development in the 1980s the second-order nonlinear optical technique of vibrational sum frequency generation spectroscopy (SFG) evolved into a versatile tool to detect and characterize molecules at interfaces. To obtain complete information on the molecules’ absolute orientation and to disentangle the interference of a possible non-resonant substrate’s response which distorts the spectral line shape, however, phase-resolved measurements need to be applied. In such heterodyned SFG experiments the signal pulse which is generated by the nonlinear interaction between two incoming short pulses and a sample subsequently interferes with a so called local oscillator (LO) pulse. To yield accurate results, relative phase stability between the signal and LO as well as a precise control of their individual timings must be warranted. The simultaneous fulfillment of this two requirements so far have restricted heterodyned SFG experiments to the study of solid/air and liquid/air interfaces. This thesis presents a way to overcome these limitations by integrating a timing control scheme into a collinear high accuracy phase-resolved SFG spectrometer. The versatility of this approach is tested at the solid/liquid interface and extended to potential dependent measurements since among others understanding of electrochemical processes at the electrode/electrolyte interface will be critical in the development of more efficient batteries and fuel cells to tackle the challenges presented by the climate crisis. The obtained heterodyned SFG spectra allow an insight into the parameters that influence the non-resonant substrate’s response, such as an applied potential bias and the presence of specifically and non-specifically adsorbed ions and molecules. In addition a method is presented how to use phase-resolved SFG spectra to determine the phase of the local field Fresnel factors, which so far had to be modeled. The heterodyned spectra are compared to their homodyned analogs to discuss which new information in fact can be obtained and what limits still need to be overcome.
