ABSTRACT
One of the interesting 2D spectroscopies is doubly resonant SFG, where two different incident field frequencies w1 and w2 are tuned to be in resonances with two different electronic or vibrational transitions. Then, the measured SFG signal field amplitude or intensity becomes a function of the two frequencies w1 and w2. For instance, if w1 and w2 are close to two vibrational transition frequencies wa and wb, the SFG spectrum in a two-dimensionally displayed frequency space would exhibit a peak at w w1 = a and w w2 = b. It was shown that such a cross-peak when w wa b≠ results from mechanical and/or electric anharmonicity-induced couplings, which are critically dependent on the structure and dynamics of molecules in general. This will be discussed in a later chapter. The SFG spectroscopy involves two radiation-matter interactions. However, there are many different types of 2D spectroscopic methods, much like the cases of various linear spectroscopic methods that are different from one another by the involved effective radiation-matter interactions chosen from the list in Scheme 3.1.
