Vibrational Förster transfer to hydrated protons.

We have studied the influence of excess protons on the vibrational energy relaxation of the O-H and O-D stretching modes in water using femtosecond pump-probe spectroscopy. Without excess protons, we observe exponential decays with time constants of 1.7 and 4.3 ps for the bulk and anion bound O-D stretch vibrations. The addition of protons introduces a new energy relaxation pathway, which leads to an increasingly nonexponential decay of the O-D stretch vibration. This new pathway is attributed to a distance-dependent long range dipole-dipole (Forster) interaction between the O-D stretching vibration and modes associated with dissolved protons. The high efficiency of hydrated protons as receptors of vibrational energy follows from the very large absorption cross section and broad bandwidth of protons in water. For a proton concentration of 1M we find that Forster energy transfer occurs over an average distance of 4.5 A, which corresponds to a separation of about two water molecules.

Vibrational Förster transfer in ice Ih.

We have studied Förster energy transfer between O-H vibrations in H(2)O/D(2)O ice Ih using femtosecond, two-color, mid-infrared pump-probe spectroscopy. We found that as a result of couplings to nearby O-H stretch modes, the vibrational relaxation time decreases from 480 fs for dilute HDO in D(2)O down to 300 fs for pure H(2)O ice. The anisotropy shows an initial 140 fs decay down to a concentration-dependent end level. This end level for low concentrations can be explained from the limited rotational freedom ( approximately 20 degrees ) of a water molecule in the ice lattice over time scales > 15 ps. The decreasing end levels for higher concentrations of H(2)O result from Forster energy transfer to the next-nearest six O-H groups. No Förster transfer beyond these neighbors is observed. Variation of the ice temperature between 200 and 270 K was found to have negligible effect on the dynamics.

Direct observation of proton transfer in ice Ih using femtosecond spectroscopy.

We studied proton transfer in ice samples containing the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid and the base sodium formate using femtosecond pump-probe spectroscopy. Pump pulses, centered at a wavelength of 400 nm, electronically excited the photoacid molecules which released their protons. These protons subsequently traveled from the photoacid through the ice lattice to the base and were observed as they arrived at the base using the transient absorption of an infrared probe pulse. Both the temperature and concentration dependence of the proton transfer dynamics were modeled using a discrete set of two intrinsic transfer rates, associated with short and long-range proton transfer, respectively. Proton transfer in configurations where the acid and base were separated by up to about two water molecules, was found to occur on a approximately 1 ps time scale for all temperatures (240-270 K). Long range direct proton transfer through water wires of about four water molecules in length was found to occur on a approximately 300 ps time scale at 270 K. This latter process was observed to slow down significantly with decreasing temperature, with an activation energy of approximately 80 kJ/mol.

Structure dynamics of the proton in liquid water probed with terahertz time-domain spectroscopy.

We study the hydration of protons in liquid water using terahertz time-domain spectroscopy and polarization-resolved femtosecond midinfrared pump-probe spectroscopy. We observe that the addition of protons leads to a very strong decrease of the dielectric response of liquid water that corresponds to 19+/-2 water molecules per dissolved proton. This depolarization results from water molecules ( approximately 4) that are irrotationally bound to the proton and from the motion of water (corresponding to the response of approximately 15 water molecules) involved in the transfer of the proton charge.

Hydrogen bond fluctuations of the hydration shell of the bromide anion.

We study the hydrogen bond dynamics of solutions of LiBr and NaBr in isotopically diluted water (2% HDO:D2O) with femtosecond spectral hole-burning spectroscopy. We study the frequency fluctuations of the O-H stretch vibrations of the HDO molecules and observe spectral dynamics with time constants of 0.8 +/- 0.1 ps and 4.3 +/- 0.3 ps. The slow process we assign to the hydrogen bond fluctuations of the O-H...Br- hydrogen bonds of the hydration shell of the Br- anion. We find that the time scale of the hydrogen bond fluctuations of the hydration shell of Br- is independent of the nature of the cation and the concentration.

Molecular reorientation of liquid water studied with femtosecond midinfrared spectroscopy.

The molecular reorientation of liquid water is key to the hydration and stabilization of molecules and ions in aqueous solution. A powerful technique to study this reorientation is to measure the time-dependent anisotropy of the excitation of the O-H/O-D stretch vibration of HDO dissolved in D2O/H2O using femtosecond midinfrared laser pulses. In this paper, we present and discuss experiments in which this technique is used to study the correlation between the molecular reorientation of the water molecules and the strength of the hydrogen-bond interactions. On short time scales (<200 fs), it was found that the anisotropy shows a partial decay due to librational motions of the water molecules that keep the hydrogen bond intact. On longer time scale (>200 fs), the anisotropy shows a complete decay with an average time constant of 2.5 ps. From the frequency dependence of the anisotropy dynamics, it follows that a subensemble of the water molecules shows a fast reorientation that is accompanied by a large change of the vibrational frequency. This finding agrees with the molecular jumping mechanism for the reorientation of liquid water that has recently been proposed by Laage and Hynes.

Water as a molecular hinge in amidelike structures.

The authors have studied the reorientational dynamics of isolated water molecules in a solution of N,N-dimethylacetamide (DMA). From linear spectra, the authors find that the water in this solution forms double hydrogen bond connections to the DMA molecules, resulting in the formation of DMA-water-DMA complexes. The authors use polarization-resolved mid-infrared pump-probe spectroscopy on the water in these complexes to measure the depolarization of three distinct transition dipole moments, each with a different directionality relative to the molecular frame (OH stretch in HDO, symmetric and asymmetric stretch normal modes in H(2)O). By combining these measurements, the authors find that the system exhibits bimodal rotational dynamics with two distinct time scales: a slow (7+/-1 ps) reorientation of the entire DMA-water complex and a fast (0.5+/-0.2 ps) "hinging" motion of the water molecule around the axis parallel to the connecting hydrogen bonds. Additionally, the authors observe an exchange of energy between the two normal modes of H(2)O at a time scale of 0.8+/-0.1 ps and find that the vibrational excitation decays through the symmetric stretch normal mode with a time constant of 0.8+/-0.2 ps.