Time-resolved nonlinear IR spectroscopies

We do not here describe how these stiU highly specialized techniques work but evocate the kind of resnlts they provide. Most of the bands that have been studied by this new technique 

In addition to measurements of lifetime of these vibrational excited states, time-resolved nonlinear IR could also give precise information on the mechanisms of deexcitation of these states. It could thus be shown that relaxation of the first excited state of modes of water molecules in liquid water was mainly due to resonance interactions of these modes with excited bending modes (65). As a result of the analysis of ID IR spectra shown above, Fermi resonance with bending modes allows the energy of the first excited state of to be transferred to the overtone of the bending band. It offers a fast relaxation path toward vibrational levels of a lower energy. Time-resolved nonlinear IR spectroscopy shows that this process is the main relaxation mechanism of and is at the origin of an unexpected increase of the relaxation time when temperature increases (66, 67). 

These new data convey in addition original information on the structure of water molecules around, for instance, ions. They thus allow the distinguishing of the HjO molecules that establish H-bonds with an anion X from H2O molecules of the bulk. The lifetimes of their first excited state is much longer (2.6 psec in the vicinity of Cl ) than those of HDO molecules dissolved in heavy water (68). It allows the collection of precise information on the acid-base reaction in water, putting into evidence at least three steps with different time constants (69) and enabling measurements of their lifetimes (70). 

---

Despite these problems of saturation of vibrational bands IR spectroscopy, described in the next subsection, has been recently shown to nevertheless remain an especially powerful method to observe H2O molecules. Special recently proposed set-ups can avoid saturation in the whole conventional IR region, thus taking full advantage of the power of IR to study H-bond networks. They are first described, before the contribution of recent time-resolved nonlinear IR spectroscopy is examined. Other methods such as NIR or Raman spectroscopy, which are intrinsically free of this saturation effects can also be used to study the HjO molecule. They are often limited to some specific problems, as they do not display the power of ordinary IR spectroscopy for the study of H-bonds or of H2O molecules and cannot consequently be considered as general methods. They are described in the last subsection of this section on vibrational spectroscopy. 

Time-resolved nonlinear IR spectroscopy is a modem version of ordinary IR spectroscopy examined above. It has been referred to all along the preceding chapters. In Ch. 4 it has been shown to convey information on the dynamics of the surrounding of the studied vibration, in addition to information on this vibration itself, which is the main type of information conveyed by ordinary IR spectroscopy, also named ID IR. This supplementary information appears through the measurements of the lifetime or relaxation time of the studied vibration. Most results obtained up to now with such methods concern liquid water. They have been described in Ch. 4, because they may be applied to any H-bonded system. They have given values of relaxation times of bands of water molecules in liquid water and have thus shown a marked isotopic dependence. Thus the relaxation time of... 

We thus see that, as for the time-resolved nonlinear IR methods, sum-frequency vibrational spectroscopy is a valuable tool to convey information on H-bonds, particularly H-bonds at interfaces. 

Fig. 6.10 Setup for IR pump/SFG probe, time-resolved nonlinear vibrational spectroscopy. Reprinted with permission from Morin et al. (1992). Copyright 1992, American Institute of Physics.

Thus far, we have examined vibrational spectroscopy using IR absorption spectroscopy, what we called in Ch. 3 one photon method , a general type that encompasses most experiments in spectroscopy. There exist, however, other types of spectroscopy to observe vibrations. These are for instance Raman spectroscopy, which is also of a current use in chemical physics and may be considered a routine method. Other less known methods are modem time-resolved IR spectroscopies. All these methods are two-photon or multiphoton spectroscopies. They do not involve a single photon, as in absorption, but the simultaneous absorption and emission of two photons, as in Raman and in other scattering experiments, or the successive absorption(s) and emission(s) of photons that are coherently delayed in time, as in time-resolved nonlinear spectroscopies. By coherently , we assume the optical waves that carry these two photons keep a well-defined phase difference. In this latter type of spectroscopy, we include all modem set-ups that involve time-controlled laser spectroscopic techniques. We briefly sketch the interest of these various methods for the study of H-bonds in the following subsections. 

HCl molecules into Cl and H3O+ ions strongly diluted among H2O molecules, which clearly indicates that protons have been transferred from HCl molecules to water molecules. In this case, the H-bond network is exceptionally dense (liquid water is characterized (Ch. 9) by a number of H-bonds as great as that of covalent bonds), and the whole reaction that leads to dissociation of HCl requires at least two steps, with two different kinds of transfer of protons ionization and diffusion of ions. In this subsection, we examine the first step, which is ionization, before examining diffusion in the next step, and keeping in mind that recent experiments based on nonlinear time-resolved IR spectroscopy have even shown the existence of three steps for this process (5). 

---