Femtosecond Dynamics of Hydrogen Bonds in Liquid Water: A Real Time Study

Abstract

A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5 -4.4 mm spectral region, at a 10 mJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH · · · O motions are "photographed" in real time. [S0031-9007(98)08298-2] PACS numbers: 82.20.Wt, 78.47. + pElementary chemical processes in gases and liquids take place on time scales ranging from 10 fs to 10 ps. It took ultrafast laser technology over 20 years to reach these scales. An important breakthrough was accomplished when Zewail and his colleagues, applying laser spectroscopic methods, succeeded in following the photochemical dissociation of gaseous iodine cyanide (ICN) in real time [1], producing in this way a "photographic" picture of the process. A number of related studies were published later; see, e.g., . Although essential, this was only the first step; thermally activated chemical reactions in liquids necessitate further studies. They require the availability of femtosecond infrared sources, the knowledge of statistical mechanics of nonlinear optical processes, and that of large scale quantum molecular dynamics simulations.Published work on femtosecond dynamics of thermally activated processes mainly refers to an isotopic variety of liquid water, the diluted solution HDO͞D 2 O. The following directions of research were explored: (i) Pumpprobe experiments were realized by Graener, Seifert, and Laubereau at 10 ps [5] and by Laenen, Rauscher, and Laubereau at 1 ps [6] time scales. Hole burning of the OH-stretching band was observed for the first time and a population relaxation time of 1.5 ps was inferred. (ii) Another midinfrared pump-probe study of HDO dissolved in D 2 O, realized with polarization resolution at 250 fs time scales, was published by Woutersen, Emmerichs, and Bakker [7]. The orientational relaxation of HDO was found to be biexponential, with time constants of 0.7 and 13 ps. (iii) Theoretical work on this subject, mainly due to Bratos and Leicknam [8,9], employed the correlation function approach of statistical mechanics of nonlinear optical processes. A method was proposed to study OH · · · O motions of water in real time.The purpose of this paper is to realize the experiment suggested in Ref.[9], "photographing" H-bond motions in this way. The principle of the experiment is as follows.Its key ingredient is the well-known relation between the OH-stretching frequency and the length of an H bond; it is described in Refs. [10,11] and is illustrated in Fig. 1. Pumping the HDO͞D 2 O solution at a given frequency V 1 thus results in selecting the OH · · · O bonds of a specified length; an alternative statement is that the pump pulse generates a coherently excited wave ...

Watching Hydrogen Bonds Break: A Transient Absorption Study of Water

“…The solution to the coupled rate equations for the population, N p , of photoproduct is (5) The long-time solution to eq 5 is Q = fN e (T w =0). The T w = 0 solution is 0.…”

Two-Dimensional Infrared Spectroscopy of Isotope-Diluted Low Density Amorphous Ice

“…Nonlinear vibrational spectroscopy, such as two dimensional infrared spectroscopy (2D IR) [26][27][28][29][30][31][32][33], is an excellent tool to monitor the local structure and dynamical properties of the hydrogen bonds in water. These experiments make use of the fact that the vibrational frequency of the OH stretch vibration is highly correlated with the OO distance of a pair of hydrogen bonded water molecules [34][35][36][37][38]. That is, a shorter and therefore stronger hydrogen bond results in a larger red shift of the OH stretch vibration.…”

Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy

“…Static x-ray spectroscopy has revealed quantitative information on local atomic structure 4,5 whereas time-resolved vibrational spectroscopy has produced information on timescales of structural dynamics as short as 50fs. 6,7,8,9 Time-resolved vibrational spectroscopy has concentrated on the infrared spectrum of HOD in D 2 O and H 2 O 10,11,12,13,14,15,16 and to a lesser extent on pure H 2 O, 17,18 limited by the (sub-)micron absorption length. The latter also poses a challenge for x-ray absorption spectroscopy.…”