SciPost Thesis Link
|Title:||Femtosecond vibrational dynamics in hydrogen-bonded systems|
|As Contributor:||(not claimed)|
|Degree granting institution:||University of Amsterdam|
|Supervisor(s):||Ad Lagendijk; Huib J. Bakker|
This thesis is concerned with the dynamical behavior of vibrational excitations in hydrogen-bonded systems. In particular, we have investigated the dynamics of the OH-stretch mode in water, ice, and hydrogen-bonded ethanol clusters. The dynamics of the OH-stretch mode supplies important information on the structure and dynamics of the hydrogen bonds in these systems, because the OH-stretch vibration and the hydrogen bond are strongly coupled. The method used in our experiments is that of time-resolved pump- probe spectroscopy: an intense resonant pump pulse excites a fraction of the OH-groups from the ground (v=0) state to the first excited (v=1) state, and a second, delayed probing pulse monitors the vibrational dynamics. By detecting the probe pulse in a polarization-resolved manner, the orientational dynamics of the vibrational excitation can be monitored as well. We found that in hydrogen-bonded ethanol clusters, excitation of the OH-stretch mode leads to a fast (subpicosecond) predissociation of the hydrogen bond. The predissociation lifetime decreases with increasing hydrogen-bond strength, as a consequence of the stronger coupling between the OH-stretch mode and the hydrogen bond. The polarization-resolved measurements show that the OH-stretch excitation is rapidly delocalized over the ethanol molecules in the hydrogen-bonded cluster. The strong coupling of the OH-stretch mode and the hydrogen bond in water leads to several remarkable features in the vibrational dynamics of this liquid. We found that the excitation of the OH-stretch mode in dilute solutions of HDO in D2O leads to a dynamic Stokes shift of the OH-stretch frequency. This Stokes shift is a consequence of the fact that the minima of the hydrogen-bond potentials in the v=0 and v=1 states occur at different positions. Upon excitation from the v=0 to the v=1 state, the hydrogen bond is initially in a non-equilibrium position and the subsequent relaxation (contraction) to its equilibrium position in the v=1 state causes a dynamic Stokes shift of the OH-stretch frequency. The coupling between the OH-stretch mode and the hydrogen-bond leads to an approximately linear relation between the hydrogen-bond strength and the OH-stretch frequency. This means that by tuning the center frequency of the infrared pulses used in our experiments to a particular OH-stretch frequency, we can in principle selectively study subensembles of water molecules with a specific hydrogen-bond strength. In this way, we were able to study the influence of hydrogen-bonding on the orientational dynamics of HDO molecules dissolved in D2O. It was found that the orientational relaxation of a water molecule can occur on two distinct time scales, depending on the local hydrogen-bond structure. On the basis of our measurements, we propose a model for the hydrogen-bond strength dependence of the orientational relaxation constant. In this model, we also take into account the dynamic Stokes shift of the OH-stretch vibration that occurs upon excitation to the v=1 state. It is found that the model accurately describes both our pump-probe measurements and the low-frequency dielectric response function of liquid water. As a consequence of the strong coupling between the OH-stretch mode and hydrogen bond, energy can be transferred very efficiently from the first excited state of the OH-stretch mode to the hydrogen-bond mode. The vibrational lifetime in water is observed to become longer with increasing temperature, in contrast to what is generally observed. This can be explained by the fact that the average coupling strength between the OH-stretch and hydrogen-bond mode decreases with increasing temperature. It is well known that in pump-probe experiments, the observed transients are strongly influenced by coherent coupling between the pump and probe pulses. This effect is well understood and has been described quantitatively in many studies. If the pump and probe pulses have the same center frequency, the coherent coupling leads to an additional contribution to the pump-probe signal, that can easily be mistaken for a spectral hole in a transient pump-probe spectrum. Using a third-order perturbation expansion of the density matrix, we show that the “spectral holes” in the OH-stretch bands of water and ethanol, which were recently reported by Laenen et al., are in fact such coherent coupling contributions to the pump-probe signal, and have nothing to do with the vibrational dynamics of the OH-stretch vibration. Finally, we have presented the first observation of an incoherent vibrational photon echo. Compared to coherent photon-echo spectroscopy, the method of incoherent photon echoes has the advantage of experimental simplicity: since the time resolution is determined by the coherence time rather than the duration of the light pulses used in a photon-echo experiment, a good time-resolution can be obtained using relatively long pulses. We demonstrate this in a vibrational photon-echo experiment with sub picosecond time resolution using pulses with a duration of 20 picoseconds. It is only since a few years that it is possible to generate the short and intense mid- infrared pulses needed for femtosecond time-resolved vibrational spectroscopy. In this short period, femtosecond spectroscopy on the OH-stretch mode has revealed many new aspects of the complex dynamics of hydrogen-bonded systems such as liquid water. There is no doubt that in the future, as femtosecond vibrational time-resolved spectroscopy will be used to investigate even more complicated systems, such as surfaces, adsorbates, and proteins, many other unexpected properties of matter will be revealed.