On the orientational relaxation of HDO in liquid water

We use femtosecond mid-infrared pump-probe spectroscopy to study the orientational relaxation of HDO molecules dissolved in H 2 O. In order to obtain a reliable anisotropy decay we model the effects of heating and correct for these effects. We have measured the reorientation time constant of the OD vibration from 2430 to 2600 cm −1 , and observe a value of 2.5 ps that shows no variation over this frequency interval. Our results are discussed in the context of previous experiments that have been performed on the complementary system of HDO dissolved in D 2 O.

Chemical Physics, 2000

The reorientational motion of the molecules in liquid water is investigated by measuring the anisotropy decay of the O±H stretching mode of HDO dissolved in D 2 O using femtosecond mid-infrared pump±probe spectroscopy. We observe that the anisotropy shows a non-exponential decay with an initial fast component of which the amplitude increases with increasing lengths of the O±HÁ Á ÁO hydrogen bond. The experimental results can be accurately described with a model in which the dependence of the reorientation rate on the hydrogen-bond length and the stochastic modulation of this length are accounted for. It is found that the O±H group of a water molecule can only reorient after the O±HÁ Á ÁO hydrogen bond has suciently lengthened. As a result, the eective rate of reorientation of the molecules in liquid water is determined by the rate at which the length of the hydrogen bonds is modulated. Ó

Physical Review Letters, 1999

We present the first experimental observation of a vibrational dynamic Stokes shift. This dynamic Stokes shift is observed in a femtosecond pump-probe study on the OH-stretch vibration of HDO dissolved in D 2 O. We find that the Stokes shift has a value of approximately 70 cm 21 and occurs with a time constant of approximately 500 femtoseconds. The measurements can be accurately described by modeling the hydrogen bond in liquid water as a Brownian oscillator. PACS numbers: 78.47. + p, 78.30.Cp For hydrogen-bonded O-H · · · O systems, the OHstretch frequency n OH is strongly correlated to the hydrogen-bond length R O-H···O . In isolated (gasphase) hydrogen-bonded complexes, the difference between the hydrogen-bond potentials in the y OH 0 and y OH 1 states leads to Franck-Condon progressions in the n OH spectra with a typical value for n OH of 3600 cm 21 and a typical spacing n O···H of 200 cm 21 . In the condensed phase, the hydrogen-bond mode is strongly damped by interaction with bath modes, resulting in a smooth and broad n OH absorption band. If the hydrogen-bond mode can still be described with displaced potential-energy curves, excitation of the OH-stretch mode will be followed by relaxation (contraction) of the hydrogen bond to its equilibrium position in the y OH 1 state. This should lead to a dynamic Stokes shift of the n OH frequency of the excited molecule, in close analogy with the dynamic Stokes shift observed in electronic transitions of fluorescent probe molecules in liquid solution .

The Journal of Physical Chemistry A, 2001

We report the first observation of coherent vibrational dynamics in a hydrogen bond obtained by femtosecond nonlinear measurements in the mid-infrared spectral range. The results provide the first experimental evidence in the time domain for the anharmonic coupling of slow and fast vibrational motions in a hydrogen bond. We show that the absorption band of the high-frequency hydrogen stretching (O-H/O-D) vibration is modulated by a coherent oscillatory motion corresponding to a periodic variation of the hydrogen bond length and its strength. This mechanism, which is of general relevance for hydrogen bonds, is connected with underdamped low-frequency vibrationssin contrast to the assumption of overdamped nuclear motions in most theoretical treatments.

Physical chemistry chemical physics : PCCP, 2012

We report the energy relaxation of the OH stretch vibration of HDO molecules contained in an HDO:D(2)O water bridge using femtosecond mid-infrared pump-probe spectroscopy. We found that the vibrational lifetime is shorter (~630 ± 50 fs) than for HDO molecules in bulk HDO:D(2)O (~740 ± 40 fs). In contrast, the thermalization dynamics following the vibrational relaxation are much slower (~1.5 ± 0.4 ps) than in bulk HDO:D(2)O (~250 ± 90 fs). These differences in energy relaxation dynamics strongly indicate that the water bridge and bulk water differ on a molecular scale.

Chemical Physics Letters, 2003

Vibrational energy relaxation (VR) of the OH stretch m OH and bend d H 2 O in water is studied by the mid-IR pump with anti-Stokes Raman probe technique. The broad m OH band in water consists of two inhomogeneously broadened subbands. VR in the larger red-shifted subband m R OH , with T 1 ¼ 0:55 ps, is shown to occur by the mechanism m OH ! d H 2 O (1/3) and m OH ! ground state (2/3). VR in the smaller longer-lived blue-shifted subband m B OH , with T 1 ¼ 0:75 ps, occurs by the mechanism m OH ! ground state. The bending fundamental d H 2 O decays directly to the ground state with T 1 ¼ 1:4 ps.

Nature chemistry, 2013

The ability of liquid water to dissipate energy efficiently through ultrafast vibrational relaxation plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The vibrational couplings that govern energy relaxation in H2O remain difficult to characterize because of the limitations of current methods to visualize inter- and intramolecular motions simultaneously. Using a new sub-70 fs broadband mid-infrared source, we performed two-dimensional infrared, transient absorption and polarization anisotropy spectroscopy of H2O by exciting the OH stretching transition and characterizing the response from 1,350 cm(-1) to 4,000 cm(-1). These spectra reveal vibrational transitions at all frequencies simultaneous to the excitation, including pronounced cross-peaks to the bend vibration and a continuum of induced absorptions to combination bands that are not present in linear spectra. These observations provide evidence for strong mixing of int...

Nature, 2021

Perturbation Raman spectroscopy has underscored the hydrogen bond (O:H-O or HB) cooperativity and polarizability (HBCP) for water, which offers a proper parameter space for the performance of the HB and electrons in the energy-space-time domains. The O-O repulsive coupling drives the O:H-O segmental length and energy to relax cooperatively upon perturbation. Mechanical compression shortens and stiffens the O:H nonbond while lengthens and softens the H-O bond associated with polarization. However, electrification by an electric field or charge injection, or molecular undercoordination at a surface, relaxes the O:H-O in a contrasting way to the compression with derivation of the supersolid phase that is viscoelastic, less dense, thermally diffusive, and mechanically and thermally more stable. The H-O bond exhibits negative thermal expansivity in the liquid and the ice-I phase while its length responds in proportional to temperature in the quasisolid phase. The O:H-O relaxation modifies the mass densities, phase boundaries, critical temperatures and the polarization endows the slipperiness of ice and superfluidity of water at the nanometer scale. Protons injection by acid solvation creates the H↔H anti-HB and introduction of electron lone pairs derives the O:⇔:O super-HB into the solutions of base or H 2 O 2 hydrogen-peroxide. The repulsive H↔H and O:⇔:O interactions lengthen the solvent H-O bond while the solute H-O bond contracts because its bond order loss. Differential phonon spectroscopy quantifies the abundance, structure order, and stiffness of the bonds transiting from the mode of pristine water to the perturbed states. The HBCP and the perturbative spectroscopy have enabled the dynamic potentials for the relaxing O:H-O bond. Findings not only amplified the power of the Raman spectroscopy but also substantiated the understanding of anomalies of water subjecting to perturbation.

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 D 2 O/H 2 O 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.