Ultrafast 2D-IR vibrational echo spectroscopy: A probe of molecular dynamics

Ultrafast two dimensional infrared (2D-IR) vibrational echo spectroscopy has emerged as a powerful method for the study of molecular dynamics under thermal equilibrium conditions occuring on ultrafast time scales. Here, we describe experimental details of 2D-IR vibrational echo spectroscopy including the experimental setup, pulse sequence, heterodyne detection, and extraction of the mainly absorptive part of the 2D-IR spectrum. As an experimental example, the measurements of the hydrogen bond dynamics of neat water and water in a high concentration of NaBr solution are presented and compared. The experiments are performed on OD stretching vibration of dilute HOD in H₂O to eliminate contributions from vibrational excitation transport. A new experimental observable for extracting dynamical information the yields the frequency-frequency correlation function is presented. The observable is the inverse of the center line slope (CLS) of the 2D spectrum, which varies from a maximum of 1 to 0 as spectral diffusion proceeds. The CLS is the inverse of the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis associated with the first radiation field-matter interaction. Comparisons of the dynamics obtained from the data on water and the concentrated NaBr solutions show that the hydrogen bond dynamics of water around ions are much slower than in bulk water.