Polarization-angle-scanning two-dimensional spectroscopy: Application to dipeptide structure determination

Coherent two-dimensional optical spectroscopy based on a heterodyne-detected stimulated photon echo measurement technique requires four ultrashort pulses whose pulse-to-pulse delay times, wavevectors, and frequencies are experimentally controllable variables. In addition, the polarization directions of the four radiations can also be arbitrarily adjusted. We show that the polarization-angle-scanning two-dimensional spectroscopy can be of effective use to selectively suppress either all the diagonal peaks or a cross-peak in a given two-dimensional spectrum. Theoretical relationships between the transition dipole vectors of a given pair of coupled modes or quantum transitions and the polarization angle configuration making the corresponding cross-peak vanish are established. Here, to shed light into the underlying principles of the polarization-angle-scanning two-dimensional spectroscopy, we considered the amide I vibrations of various isotope-labeled dipeptide conformers and show that one can selectively suppress a cross-peak by properly controlling the polarization angle of a chosen beam among them. Once the relative directions of the amide I transition dipole vectors are determined using the polarization-angle-scanning technique theoretically proposed here, they can serve as a set of constraints for determining structures of model peptides. The present work demonstrates that the polarization-controlled two-dimensional vibrational or electronic spectroscopy can provide invaluable information on intricate details of molecular structures.