Resonance IR: A Coherent Multidimensional Analogue of Resonance Raman

This work demonstrates the use of triply resonant sum frequency (TRSF) spectroscopy as a "resonance IR" analogue to resonance Raman spectroscopy. TRSF is a four-wave-mixing process where three lasers with independent frequencies interact coherently with a sample to generate an output at their triple summation frequency. The first two lasers are in the infrared and result in two vibrational excitations, while the third laser is visible and induces a two-quantum anti-Stokes resonance Raman transition. The signal intensity grows when the laser frequencies are all in resonance with coupled vibrational and electronic states. The method therefore provides electronic enhancement of IR-active vibrational modes. These modes may be buried beneath solvent in the IR spectrum and also be Raman-inactive and therefore inaccessible by other techniques. The method is presented on the centrosymmetric complex copper phthalocyanine tetrasulfonate. In this study, the two vibrational frequencies were scanned across ring-breathing modes, while the visible frequency was left in resonance with the copper phthalocyanine tetrasulfonate Q band, resulting in a two-dimensional infrared plot that also reveals coupling between vibrational states. TRSF has the potential to be a very useful probe of structurally similar biological motifs such as hemes, as well as synthetic transition-metal complexes.