Spectroscopic studies can reveal a wealth of information about the rotational and vibrational behaviour of the constituent molecules of gases and liquids. This 1994 book reviews the fundamental concepts and important models which underpin such studies, dealing in particular with the phenomenon of spectral collapse, which accompanies the transition from rare gas to dense liquid. Throughout, discussion of the various quantum mechanical and semiclassical theories is interwoven with analysis of experimental results. These include data from optical, NMR, ESR and acoustic investigations. The book concludes with a discussion of the latest theories describing the mechanism of rotational diffusion in liquid solutions. This comprehensive review of theoretical models and techniques will be invaluable to graduate students and researchers interested in molecular dynamics and spectroscopy.

The Stern-Volmer constant of fluorescence quenching by reversible intermolecular charge transfer is obtained by means of integral encounter theory. The latter provides the first non-Markovian description of the phenomenon which accounts for the reversibility of excited-state ionization. The forward and backward electron transfers (bimolecular and geminate) are specified by the position-dependent rates of ionization and recombination. Assuming that the conventional free energy gap law is inherent to all of them, a reasonable explanation is given for the famous Rehm and Weller free energy dependence of the Stern-Volmer constant. It requires the production of ions in excited states when forward electron transfer is highly exergonic and implies that the charge recombination occurs not only to the ground but also to the excited triplet state. It is assumed that spin conversion in the radical ion pairs is faster than the geminate recombination.

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The problem of photoinduced donor-acceptor electron transfer in liquid solution is analyzed to obtain an understanding of the relationship between approximate treatments of the role of diffusion in electron transfer, that is, the Collins-Kimball approach, and a detailed analysis of the problem. It is shown why previous analyses of experimental data have yielded distance dependences of electron transfer that are much too long range. From an appropriate fitting of the nonstationary kinetics of donor fluorescence quenching by diffusionassisted electron transfer, the effective radii and the steady-state constants associated with electron transfer are found for a donor-acceptor system studied experimentally in seven solvents with different viscosities. The dependence of diffusion agrees with the one predicted theoretically for electron transfer having a distancedependent transfer rate initially taken to be exponential with distance. In the fast-diffusion limit, the dependence on the rate of diffusion is well approximated by the Collins-Kimball relationship, which permits the kinetic rate constant and the effective radius associated with diffusion-induced quenching to be extracted from the experimental data. The effective radius is then related to the electron transfer rate with arbitrary distance dependence. From this relationship, the tunnelling length for both exponential and Marcus-type rates is obtained from the data analysis, and it is demonstrated that the latter is almost twice as long as the former. For the Marcus transfer rate, it is found that the Marcus parameter β ) 1.2 Å -1 (β ) 2/tunnelling length), which is in accord with previous measurements on a variety of systems. The theoretical analysis presented here resolves the apparent discrepancies between early measurements of very long tunnelling lengths in liquid systems and physically reasonable values of β ≈ 1 Å -1 .

The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate

The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.