The dynamics of water are examined using ultrafast IR stimulated vibrational echo correlation spectroscopy. The OD hydroxyl stretch of HOD in H2O is probed with 45-fs pulses that have sufficient bandwidth (>400 cm-1) to span the entire broad spectrum. High-quality 2D correlation spectra are obtained having the correct phase relations across the broad hydroxyl band. The correlation spectra are found to evolve on multiple time scales. The time evolution of the vibrational echo correlation spectrum reflects the structural evolution of the hydrogen bond networks. The extended vibrational lifetime of the OD hydroxyl stretch of HOD in H2O facilitates the measurement of hydrogen bond dynamics for longer times than possible in previous studies of the OH stretch. Molecular dynamics simulations/electronic structure calculations are used to obtain the time correlation functions (TCF) for two water models, TIP4P and SPC/E. The TCFs are inputs to full time-dependent diagrammatic perturbation theory calculations, which yield theoretical correlation spectra. Quantitative comparison with the data demonstrates that the two water models somewhat overemphasize the fast fluctuations in water and do not contain a slow enough component to account for the slowest fluctuations. Fits to the data using a phenomenological triexponential TCF yield a slowest component of ∼2 ps, and TIP4P and SPC/E have slowest components of <1 ps. The TCF obtained from the water models and the triexponential TCF reproduce the linear absorption line shape equally well, but all miss to some extent the asymmetric “wing” on the low-energy side of the line. Therefore, the time dependence of the vibrational echo correlation spectra provides a good test for the TCF, but the absorption spectrum does not.

Hydrogen bond dynamics are explicated with exceptional detail using multidimensional infrared vibrational echo correlation spectroscopy with full phase information. Probing the hydroxyl stretch of methanol-OD oligomers in CCl4, the dynamics of the evolving hydrogen bonded network are measured with ultrashort (<50 fs) pulses. The data along with detailed model calculations demonstrate that vibrational relaxation leads to selective hydrogen bond breaking on the red side of the spectrum (strongest hydrogen bonds) and the production of singly hydrogen bonded photoproducts.

Hydrogen bond population dynamics are extricated with exceptional detail using ultrafast (Ͻ50 fs) infrared multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved infrared pump-probe experiments performed on the hydroxyl stretch of methanol-OD oligomers in CCl 4. Hydrogen bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calculations demonstrate that vibrational relaxation leads to hydrogen bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy ͑red͒ side of the hydroxyl stretch spectrum, the spectral region that is associated with the strongest hydrogen bonds. Frequency resolved pump-probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump-probe spectra, in conjunction with assignments made possible through the correlation spectra, it is demonstrated that the residual ground state and photoproduct of hydrogen bond breaking are prepared near their thermal equilibrium distribution. The spectrum of the hydrogen bond breaking photoproduct and the residual ground state approach the steady-state temperature difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale.

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