Coherent two-dimensional optical spectroscopy.

Two-dimensional (2D) optical spectroscopy utilizing multiple ultrafast coherent laser pulses have been used to study protein structure and dynamics, femtosecond solvation dynamics, hydrogen-bonding dynamics, solute-solvent complexation, and excitation migration process in a photosynthetic light harvesting complex. Due to dramatic advent of recent laser technology, femtosecond laser systems operating in infrared and visible frequency range have been commercially available so that we have seen a wide range of applications utilizing such ultrafast nonlinear spectroscopic techniques. Most of the conventional linear spectroscopies, though they are still extremely useful for studying structural and dynamic properties of complex molecules, can only provide highly averaged information. Therefore, novel spectroscopic techniques with much higher information content has been sought and tested continuously. In NMR spectroscopy, such efforts led to developing a variety of 2D NMR techniques such as NOESY (Nuclear Overhauser Enhancement Spectroscopy) and COSY (correlation spectroscopy) methods among many others, and they have been extensively used to study structural and dynamical properties of proteins in solution. In Figure 1, a direct analogy between 2D vibrational spectroscopy and 2D NMR is depicted. In the case of 2D NMR, one can measure the nuclear spin-spin coupling constant that carries information on the 3D molecular structure. Similarly, the 2D vibrational spectroscopy can provide information on the vibrational coupling constant between two spatially separated vibrational modes and the 2D electronic spectroscopy has been found to be useful for estimating the magnitudes of electronic coupling constant between two optical chromophores. Despite that the interaction mechanism between two different vibrational (electronic) degrees of freedom differs from that between two nuclear spins, the coupling strengths in both cases are sensitively dependent on the 3D molecular structure (Fig. 2). Although the optical analogs of 2D NMR do not provide an atomic resolution structure of complex molecules, optical domain multi-dimensional spectroscopy has certain advantages because of the dramatic gain in time resolution (~subpicosecond scale) possible and because of the ability to directly observe and quantify the couplings between quantum states involved in molecular dynamical processes. For the