Two-Dimensional Electronic and Vibrational Spectroscopy

Photon echoes also can be obtained by using only two pulses instead of three. These can be described with the same theoretical formalism by letting the second and third pulses coincide and settingk2 = 3 and 2(0 = [39]. 

Two-dimensional IR (2D-IR) spectra of CO-myoglobin exhibit additional off-diagOTial peaks that are attributable to switching between different conforma-timial states [61]. These peaks are not seen at short times after excitation, but 

11 Pump-Probe Spectroscopy, Photon Echoes and Vibrational Wavepackets 

Two-dimensional IR spectroscopy has been used to examine exciton interactions, internal Stark shifts, and relaxation mechanisms of amide 1 and n transitions in peptides and proteins [57, 59, 82-91]. Spreading the excitatifui energies along a second coordinate can separate components that are difficult to resolve in broad one-dimensional IR spectra. 

Two-dimensional UV spectroscopy has been applied to nucleic acid bases and nucleotides, and holds promise for exploring transfer of vibrational energy and the migration and localization of excitons in DNA [92-96]. These processes are pertinent to understanding how DNA avoids photodamage. 

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