Polarization plays a central role in the measurement and interpretation of 2D IR spectra. In standard pump–probe spectroscopies, polarization has been used for many years to measure the rotational times of molecules or eliminate rotational motion from dynamics measurements. The polarization dependence of the diagonal peaks provide the same capabilities, but polarization can do much more in 2D and 3D spectroscopies. Recall Fig. 4.11 from Chapter 4, which is a schematic of a rephasing 2D IR spectrum with each peak labeled by its respective Feynman pathway. The objective of using polarization in 2D IR spectroscopy is to enhance or suppress particular Feynman pathways based on the relative angles of the transition dipoles. Selection is possible because each Feynman pathway has a different ordering of quantum states (e.g. j →j →i →i versus j →i → j →i). Thus, the ordering of polarized pulses in a pulse sequence will scale one pathway differently from another, thereby altering the intensities and phases of the diagonal and cross-peaks. By measuring these effects, the relative angles between transition dipoles can be measured [69, 190, 202]. Angles are an extremely insightful tool for monitoring the structures of molecules, perhaps more so than actual couplings. In fact, properly polarized pulses can actually eliminate the diagonal peaks from the 2D IR spectra [201], thereby better resolving the cross-peaks, which is illustrated in Fig. 5.1.