Understanding the correlation between vibrational and electronic properties of molecules at interfaces is important for a variety of chemical interfaces, in particular the dye/titanium dioxide (TiO2) interface in dye-sensitized solar cells. Determining the correlation between molecular structure and its electronic properties as it pertains to its association to a surface will have great implications on determining the ideal properties necessary for effective light harvesting dyes. We demonstrate a technique called heterodyne-detected doubly resonant sum-frequency generation spectroscopy (HD-DRSFG) which effectively measures the vibrational-electronic interactions of N3 dye at the nanoparticulate anatase-TiO2 surface. In this experiment, ultrafast mid-IR and visible pulses excite the sample, where the former is resonant with vibrational transitions and the latter is resonant with an electronic transition. Using heterodyne-detection and small, precise path length changes, we can use a local oscillator to measure the doubly resonant molecular response while scanning the visible pulse along the free-induction decay of the vibration induced by the initial mid-IR pulse. This provides highly resolved spectra with two axes, representing the vibrational and electronic frequencies, which correlate molecular vibrations and structure to electronic transitions that are important for understanding relevant processes associated charge transfer between the molecule and surface. Furthermore the use of heterodyne-detection allows for phase sensitive measurement of the real and imaginary parts of the second order susceptibility of the system which, in addition to polarization experiments, provide detailed orientation information about the dye at the interface.

Probing vibrational-electronic interactions of a sensitizing dye at the TiO2 surface using heterodyne detected doubly resonant sum-frequency generation spectroscopy

249th National Meeting (2015)

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