Vibrational population relaxation experimental studies

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. 

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... 

Electronic relaxation in different excited vibronic levels corresponding to the same electronic configuration can be experimentally studied, provided that, as mentioned above, (1) single vibrational levels within the initial electronic state are populated and (2) the excited molecule decays nonradiatively on a timescale much shorter than the mean time between deactivating collisions or by other means such as infrared fluorescence [115]. For typical polyatomic molecules in the gas phase, a narrow-band optical excitation pulse (as small as 1 and shorter relative to the genuine decay times wiH result in the selection of a single vibronic state. U nder these conditions,... 

Three important papers, published at about the same time in 1966, demonstrated very dramatically the usefulness of lasers in the measurement of molecular energy transfer. The first of these, by DeMartini and Ducuing [137], reports a study of vibrational relaxation in normal H2 using stimulated Raman scattering. The experimental arrangement is shown in Figure 3.16. Radiation from a -switched ruby laser was focused onto a pressure cell of H2 gas at room temperature to produce about IO16 vibrationally excited H2 molecules in a period of about 20 nsec. This excess population distribution... 

More importantly, a molecular species A can exist in many quantum states in fact the very nature of the required activation energy implies that several excited nuclear states participate. It is intuitively expected that individual vibrational states of the reactant will correspond to different reaction rates, so the appearance of a single macroscopic rate coefficient is not obvious. If such a constant rate is observed experimentally, it may mean that the process is dominated by just one nuclear state, or, more likely, that the observed macroscopic rate coefficient is an average over many microscopic rates. In the latter case k = Piki, where ki are rates associated with individual states and Pi are the corresponding probabilities to be in these states. The rate coefficient k is therefore time-independent provided that the probabilities Pi remain constant during the process. The situation in which the relative populations of individual molecular states remains constant even if the overall population declines is sometimes referred to as a quasi steady state. This can happen when the relaxation process that maintains thermal equilibrium between molecular states is fast relative to the chemical process studied. In this case Pi remain thermal (Boltzmann) probabilities at all times. We have made such assumptions in earlier chapters see Sections 10.3.2 and 12.4.2. We will see below that this is one of the conditions for the validity of the so-called transition state theory of chemical rates. We also show below that this can sometime happen also under conditions where the time-independent probabilities Pi do not correspond to a Boltzmann distribution. 

Other experimental techniques have been used to study the very fast relaxation of dye molecules in solution. Ricard and Ducuing studied rhodamine molecules in various solvents and observed vibrational rates ranging from 1 to 4 ps for the first excited singlet state. Their experiment consisted of two pulses with a variable delay time between them the first excites molecules into the excited state manifold and the second measures the time evolution of stimulated emission for different wavelengths. Ricard found a correlation between fast internal conversion and vibrational relaxation rates. Laubereau et al. found a relaxation time of 1.3 0.3 ps for coumarin 6 in CCI4. They used an infrared pulse to prepare a well-defined vibrational mode in the ground electronic state, and monitored the population evolution with a second pulse that excited the system to the lowest singlet excited state, followed by fluorescence detection. 

The first experimental observation of slow vibrational relaxation goes actually back to the early experiments of Vegard, and several other examples were noted in the laboratories of Broida, Robinson, and other investigators. " Systematic studies of vibrational relaxation rates, however, only became possible with the advent of tunable dye lasers, permitting selective population of excited vibronic levels of the guest molecules. 

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