A pulse sequence for directly measuring the anharmonicities of coupled vibrations: Two-quantum two-dimensional infrared spectroscopy.

We have experimentally demonstrated a pulse sequence for the acquisition of heterodyned two-dimensional infrared (2D IR) spectra that correlates the overtone and combination bands to the fundamental frequencies. The spectra are generated by Fourier transforming the time domain signal that is allowed to evolve during one- and two-quantum coherence times. In this manner, the overtone and combination bands appear along the two-quantum axis, resulting in a direct determination of the diagonal and off-diagonal anharmonicities. To demonstrate this pulse sequence, we have collected two-quantum 2D IR spectra of a ruthenium dicarbonyl complex, extracted the diagonal and off-diagonal anharmonicities, and simulated the spectra using an exciton model. Several polarization conditions are presented that suppress the diagonal or cross peaks and we have used them to improve the accuracy of the measurement.