Vibrational energy relaxation , liquid modes

Aside from the difficulties at the upper and lower ends of the liquid s vibrational band, the INM ideas do seem to work and to work quantitatively. The ability of the liquid-mode concept to account for the absolute magnitude of the vibrational friction, including the factor of 2 difference between liquid and supercritical CO2 solvents, is worth noting (52). But does this success mean that vibrational energy relaxation is really a collective process To answer this question, we need to carry out precisely the kind of mechanistic investigation we discussed in Section II.C. 

We next briefly discuss a second liquid phase chemical process, namely, the vibrational energy relaxation of high-frequency solute normal modes [33],... 

Molecular motions (rotation, translation, and vibration) of a water molecule also turn out to be quite different from those of other common liquids. Here all the six unique features of an individual water molecule outlined in Chapter 1 manifest themselves in diverse ways. As we discuss below, not only is the mechanism of displacements of individual water molecules different, but the collective dynamics and dynamical response of bulk water are also different. For example, the rotational motion of an individual water molecule contains a surprising jump component and vibrational energy relaxation of the O—H mode involves a cascading effect mediated by anharmonicity of the bond. These motions are reflected in many important processes such as electrical conductivity, solvation dynamics, and chemical reactions in aqueous medium. 

To focus on the question of surface effects on vibrational energy relaxation rate, without the complications of intramolecular energy flow, Benjamin and coworkers studied the vibrational relaxation of a diatomic solute molecule (single vibrational mode) at various liquid/vapor and liquid/liquid interfaces." " The solute is modeled using the Morse potential ... 

Recent experimental studies on interference effects in solution, and on collisional vibrational energy transfer between molecules in solution, provide some insight into the molecular time scales of these relaxation events. For example [171], the time scale for transfer of population to die vibrational modes in liquid CH3OH is on thd order of 5 to 15ps [172], Further, studies of the preparation of coherent superpositions of states in solution show that phase coherences of molecules exist in solution for time scales greater than 100 fs [173, 174], -- i... 

Spin-lattice relaxation (time TJ results from dissipation of the excited-state energy among vibrational modes of the matrix. In mobile liquids, vibrational fluctuations are spread over a very wide frequency range. This configuration decreases the probability of spin-lattice coupling. As a result, Tl is long and thus makes a negligible contribution to the line width. 

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). 

Figure 11.13 Time-resolved pump-probe vibrational relaxation of a C—H stretch vibration in liquid acetonitrile at room temperature detected by (Raman) emission from the excited state. In the isolated molecule such a v = 1 stretch mode will relax on the nanosecond time scale. The experiment maps out the energy transfer pathways in the liquid. The essential point is that the relaxation is by solvent-aided intramolecular transfer, as discussed in the text and indicated by arrows. Energy dumping into the liquid is far slower, requiring about 260ps (adapted from Deak etal. (1998) see also Iwaki and DIott (2001), Wang eta/. (2002)).

---