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

Abstract

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 occurring 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 that 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.

Hydrogen bond dynamics in aqueous NaBr solutions

“…The structural evolution (frequency changes) is reflected in the changes of peak shape of the 2D IR spectra. The 2D IR vibrational echo experiments have been reviewed recently (23,32,33).…”

“…Here, we used the center line slope (CLS) method (23,35). The CLS method is a new approach for extracting the FFCF from the T w dependence of the 2D IR spectra that is accurate and much simpler to implement numerically than iterative fitting methods with calculations of all response functions based on time-dependent diagrammatic perturbation theory.…”

“…The CLS method is a new approach for extracting the FFCF from the T w dependence of the 2D IR spectra that is accurate and much simpler to implement numerically than iterative fitting methods with calculations of all response functions based on time-dependent diagrammatic perturbation theory. Furthermore, the CLS provides a more useful quantity to plot for visualizing differences in the dynamics as a function of the NaBr concentration than a series of full 2D IR spectra (23,35). In the CLS method, frequency slices through the 2D IR spectrum parallel to the axis at various m values are projected onto the axis (35).…”

“…The Lorentzian contribution to the 2D IR spectrum also has contributions from lifetime (T 1 ) and orientational relaxation (T or ). The total homogeneous (Lorentzian) contribution, T 2 , is given by 1/T 2 ϭ 1/T* 2 ϩ 1/(2T 1 ) ϩ 1/(3T or ), where T 2 is determined from the CLS combined with the linear FT IR absorption spectrum (23,35), and T 1 and T or are measured by polarizationselective IR pump-probe experiments. The total homogeneous line width is ⌫ ϭ 1/( T 2 ).…”

“…By examining such highly concentrated NaBr solutions, all of water molecules are placed in close proximity to ions, and therefore, the influence of water-ion interactions is not diluted out by large quantities of bulk water that are not affected by the ions. Some limited initial vibrational echo experiments on highly concentrated salt solutions have been performed (22,23). These experiments showed that the dynamics of water in concentrated salt solutions differ…”