G. Gallot

Frequency-dependent vibrational population relaxation time of the OH stretching mode in liquid water

Abstract The vibrational population relaxation time has been measured across the inhomogeneously broadened OH stretching mode (ν3) of a 0.5% solution of HDO in D2O. Relaxation times vary from 0.5 ps at low frequency (3270 cm−1) to 1.0 ps at high frequency (3600 cm−1). This variation is attributed to a V–V transfer mechanism between ν3 and 2ν2.

Transient absorption of vibrationally excited water

We study the spectral response of the transition between the first and the second excited state of the O–H stretch vibration of HDO dissolved in liquid D2O with two-color femtosecond mid-infrared spectroscopy. The spectral response of this transition differs strongly from the fundamental absorption spectrum of the O–H stretch vibration. In addition, excitation of the O–H stretch vibration is observed to lead to a change of the hydrogen-bond dynamics of liquid water. We show that both these observations can be described with a refined quantum-mechanical version of the Lippincott–Schroeder model for hydrogen-bonded OH⋯O systems.

Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.

Non-monotonic decay of transient infrared absorption in dilute HDO/D2O solutions

Abstract Transient infrared absorption of diluted HDO/D 2 O solutions is measured with 150 fs laser pulses. The signal intensity decays in a heavily non-exponential way, and its variation is often non-monotonic; no quantum beats are observed. These effects are ascribed to the simultaneous presence of solvent and population dynamics in low lying vibrational states, and to a partial cancellation of the bleach and absorption intensities.

Infra-Red Spectra of Hydrogen Bonded Systems: Theory and Experiment

Infra-red spectroscopy of hydrogen bonded systems is shortly reviewed. Both experimental and theoretical aspects of the problem are covered, but the accent is put on the theory. The first part of the paper refers to the classical, whereas its second part covers the time-resolved spectroscopy of the OH stretching band. Modern theories of vibrational band shapes are presented; they are all based on the density matrix approach of statistical mechanics and employ the correlation function formalism. Using this technique, molecular dynamics of hydrogen bonds can be monitored up to time scales of the order of a few tens of femtoseconds.

Real Time Visualization of Atomic Motions in Dense Phases

It has always been a dream of physicists and chemists to follow temporal variations of molecular geometry during a chemical reaction in real time, to “film” them in a way similar as in the everyday life. Unfortunately, chemical events take place on tiny time scales comprised between 10 fs and 100 ps, approximately. Visualizing atomic motions thus remained a dream over two centuries. This is no longer true today, consequence of an immense instrumental development the last decades. Two methods are particularly important. The first of them is ultrafast optical spectroscopy employing the recently developed laser technology. In his breakthrough work A. Zewail was able to show how can this method be used to follow the photoelectric dissociation of gaseous ICN in real time[1,2]. It has later been applied to several other problems, and particularly so to visualize OH..O motions in liquid water[3,4]. Unfortunately, visible light interacts predominantly with outer shell rather than with deeper lying core electrons that most directly indicate molecular geometry. It is thus difficult to convert spectral data into data on molecular geometry. The second method refers to time resolved x-ray diffraction and absorption. As x-rays interact predominantly with deeply lying core electrons which are tightly bonded to the nuclei, converting x-ray data into data relative to molecular geometry is, in principle at least, much more straightforward than in optical spectroscopy. Unfortunately, pulsed x-rays techniques are technically very demanding.

Dynamics of the Hydrogen-Bond in Liquid Water

Intermolecular O-O movement in the fundamental electronic state is fully resolved for the first time for the hydrogen bond in liquid water and compared with a theoretical model. Strongly varying energy relaxation times are also measured and interpreted in terms of a V-V transfer model.