In
the last ten years, two-dimensional infrared spectroscopy has become an
important technique for studying molecular structures and dynamics. We report
the implementation of heterodyne detected two-dimensional sum-frequency
generation (HD 2D SFG)
spectroscopy, which is the analog of 2D
infrared (2D IR)
spectroscopy, but is selective to noncentrosymmetric
systems such as interfaces. We implement the technique using mid-IR
pulse shaping, which enables rapid scanning, phase cycling, and automatic
phasing. Absorptive spectra are obtained, that have the highest frequency
resolution possible, from which we extract the rephrasing and nonrephasing
signals that are sometimes preferred. Using this technique, we measure the
vibrational mode of CO adsorbed on a polycrystalline Pt
surface. The 2D
spectrum reveals a significant inhomogenous
contribution to the spectral line shape, which is quantified by simulations.
This observation indicates that the surface conformation and environment of CO
molecules is more complicated than the simple “atop” configuration assumed in
previous work. Our method can be straightforwardly incorporated into many
existing SFG
spectrometers. The technique enables one to quantify inhomogeneity, vibrational
couplings, spectral diffusion, chemical exchange, and many other properties
