Vibrational population relaxation in liquids

Oxtoby D W 1981 Vibrational population relaxation in liquids Photoselective Chemistry Part 2 (Advances in Chemical Physics 47) ed J Jortner, R D Levine and S A Rise (New York Wiley) pp 487-519... 

Oxtoby DW. Vibrational population relaxation in liquids. In Jortner J, Levine RD, Rice SA, eds. Photoselective Chemistry Part 2. Vol. 47. New York Wiley, 1981 487-519. 

In this chapter we review experimental and theoretical studies of vibrational population relaxation in liquids. This review is complementary to our previous article in the same series, which treated vibrational phase relaxation (dephasing) in liquids due to vibrationally elastic interactions. A number of reviews have appeared recently on related subjects vibrational relaxation in solid matrices has been covered elsewhere, and several reviews have been devoted to experimental studies of picosecond time-scale relaxation processes in liquids. Diestler has recently reviewed theoretical studies of vibrational relaxation in liquids and solids the focus of the present article is rather different from that of Diestler. 

Oxtoby, D. W. 1981, Vibrational population relaxation in liquids . Adv. Chem. Phys. 47, 487. 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

The Molecular Mechanisms Behind the Vibrational Population Relaxation of Small Molecules in Liquids... 

Vibrational population relaxation plays a fundamental role in many physical and chemical processes in liquids. In liquid-state photochemical reactions there is often competition between relaxation and reaction and the observed reaction rates and yields can therefore be quite sensitive to solute-solvent interactions. There has also been considerable recent interest in the possible development of liquid state chemical lasers based on the long vibrational lifetimes observed in simple liquids. For both these prob-... 

Certain features of light emission processes have been alluded to in Sect. 4.4.1. Fluorescence is light emission between states of the same multiplicity, whereas phosphorescence refers to emission between states of different multiplicities. The Franck-Condon principle governs the emission processes, as it does the absorption process. Vibrational overlap determines the relative intensities of different subbands. In the upper electronic state, one expects a quick relaxation and, therefore, a thermal population distribution, in the liquid phase and in gases at not too low a pressure. Because of the combination of the Franck-Condon principle and fast vibrational relaxation, the emission spectrum is always red-shifted. Therefore, oscillator strengths obtained from absorption are not too useful in determining the emission intensity. The theoretical radiative lifetime in terms of the Einstein coefficient, r = A-1, or (EA,)-1 if several lower states are involved,... 

Ultrafast vibrational spectroscopy offers a variety of techniques for unraveling the microsopic dynamics of hydrogen bonds occurring in the femto- to picosecond time domain. In particular, different vibrational couplings can be separated in nonlinear experiments by measuring vibrational dynamics in real-time. Both coherent vibrational polarizations and processes of population and energy relaxation have been studied for a number of hydrogen bonded systems in liquids [1],... 

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... 

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