Ultrafast two-dimensional
infrared (2D-IR) vibrational echo
spectroscopy can probe the fast structural evolution of molecular systems under
thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by
frequency fluctuations of
vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency
correlation function (FFCF), which describes the
frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption
frequencies in the inhomogeneously broadened
absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLSomega(m)) of the 2D-IR spectrum. The CLSomega(m) is obtained by taking slices through the 2D spectrum parallel to the detection
frequency axis (omega(m)). Each slice is a spectrum. The slope of the line connecting the
frequencies of the maxima of the sliced spectra is the CLSomega(m). The change in slope of the CLSomega(m) as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLSomega(m) is
immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1
vibrational transition (positive), and from the 1-2
vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLSomega(m) method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by
solvent background absorption and finite
pulse durations do not affect the determination of the FFCF with the CLSomega(m) method. The CLSomega(m) can also provide information on the
cross correlation between
frequency fluctuations of the 0-1 and 1-2
vibrational transitions.
