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The principle of two-dimensional vibrational spectroscopy
The basic idea behind multi-dimensional
vibrational spectroscopy is to apply the elegant methods originally
developed in NMR to study nonlinear excitations of nuclear spin
transitions to nonlinear optical excitations of vibrational
transitions. These vibrational transitions can be resonantly driven by
excitation in the infrared (IR) wavelength region. Two-dimensional
vibrational (2D-IR) spectroscopy can be used to study both structure
(by measuring couplings between vibrations) and structural fluctuations
(by measuring relaxation processes).
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Couplings between
vibrations can be measured by means of 2D-IR pump-probe spectroscopy
(see picture). We use an intense pump pulse with frequency v_pump to
excite a particular vibrational mode, and a probing pulse to
investigate the response of the molecule at a different frequency
v_probe. The response is measured as a function of both frequencies
v_pump and v_probe. When two vibrations are coupled, exciting mode A
leads to a response at the frequency of mode B, which is observed as an
off-diagonal peak at (v_B, v_A) in the 2D spectrum. The intensity of
the off-diagonal peaks reflects the strength of the coupling, which in
turn contains information about the molecular conformation. More
precisely, the distance and orientation of the two vibrating chemical
bonds can be derived from the observed off-diagonal peak intensity and
anisotropy (details can be found in
the articles below).
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