Creation of exciplexes from the charged products of photoionization is considered by means of Integral Encounter Theory. The general kinetic equations of such a reaction following the Weller scheme II are developed. The special attention is given to the particular case of irreversible remote ionization of primary excited electron donor. Kinetics of exciplex formation is considered at fast biexponential geminate transformation of exciplexes in cage that gives way to subsequent bulk reaction of equilibrated reaction products controlled by power law recombination of ions. It is shown that the initial geminate stage of exciplex kinetics is observed only in diffusion controlled regime of the reaction and disappears with increasing mobility of ions in passing to kinetic regime. The quantum yield of exciplexes is studied along with their kinetics.

The Stern-Volmer constant is specified for the luminescence quenched by reversible ionization of excited molecules. The exergonic branch of the Rehm-Weller free energy dependence of this constant is known to be a plateau determined by irreversible ionization being under diffusion control. In the endergonic region the ionization is reversible and competes with the irreversible in-cage recombination of ions and their escape from the cage. At strong Coulomb attraction the latter phenomenon is shown to be negligible compared to the former that determines the shape and location of the descending branch of the Rehm-Weller curve. At weaker Coulomb attraction (at higher solvent polarity), this curve turns down at larger endergonicity. The experimental data obtained in solvents of different polarities are put in order and in full accordance with present theory.

The population and phase relaxation in TLS induced by resonant radiation with stochastic amplitude was studied within the framework of sudden modulation theory. Kinetics and stationary light absorption as well as resonance fluorescence were calculated for light with complex and real amplitude. Although the latter is a very popular model of the former, it was shown to be inappropriate at strong interaction. The real (Rayleigh) light has complex amplitude random in magnitude and phase and changing in time as a Markovian chain, correlated or non-correlated. With respect to atom response to light of medium power, the analytically studied non-correlated field was shown to be similar to strongly correlated, "stochastic" field opened to only numerical inspection.

The excitation quenching by reversible exciplex formation, combined with irreversible but distant electron, transfer, is considered by means of the integral encounter theory (IET). Assuming that the quenchers are in great excess, the set of IET equations for the excitations, free ions, and exciplexes is derived. Solving these equations gives the Laplace images of all these populations, and these are used to specify the quantum yields of the corresponding reaction products. It appears that diffusion facilitates the exciplex production and the electron transfer. On the other hand the stronger the electron transfer is, the weaker is the exciplex production. At slow diffusion the distant quenching of excitations by ionization prevents their reaching the contact where they can turn into exciplexes. This is a screening effect that is most pronounced when the ionization rate is large.

The quenching of fluorescence by reversible bimolecular ionization, followed by reversible exciplex formation from an ion pair (Scheme II) subjected to spin-conversion and subsequent radical-ion recombination/separation, has been studied by means of integral encounter theory (IET) and fitted to the available experimental data. Using the incoherent (rate) model of spin-conversion, the ion recombination to the excited triplet products is also accounted for. All of the results are obtained and shown to be different for the pulse excitation of fluorescence and its stationary detection. The free-energy dependence of all of the calculated properties of the forward and backward electron transfer are specified and compared with the conventional free-energy gap (FEG) law.

The reversible exciplex formation followed by its decomposition into an ion pair is considered, taking into account the subsequent geminate and bulk ion recombination to the triplet and singlet products (in excited and ground states). The integral kinetic equations are derived for all state populations, assuming that the spin conversion is performed by the simplest incoherent (rate) mechanism. When the forward and backward electron transfer is in contact as well as all dissociation/association reactions of heavy particles, the kernels of integral equations are specified and expressed through numerous reaction constants and characteristics of encounter diffusion. The solutions of these equations are used to specify the quantum yields of the excited state and exciplex fluorescence induced by pulse or stationary pumping. In the former case, the yields of the free ions and triplet products are also found, while in the latter case their stationary concentrations are obtained.

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.

The kinetic constants of rhodamine 3B quenching by N,N-dimethyl aniline were extracted from the very beginning of the quenching kinetics, recently studied in a few solvents of different viscosities. They were well fitted with the conventional kinetic constant definition, provided the radial distribution function of simple liquids was ascribed to the reactant pair distribution and the contact electron transfer rate was different in all the cases. This difference was attributed to the chemical anisotropy averaging by the rotation of reactants, which is the faster in solvents of lower viscosity. With the proper choice of a space dependent encounter diffusion, the whole quenching kinetics was well fitted with an encounter theory, using the Marcus [J. Chem. Phys. 24, 966 (1956); 43, 679 (1965)] transfer rate instead of the contact Collins-Kimball [J. Colloid. Sci. 4, 425 (1949)] approximation. Not only the beginning and middle part of the quenching were equally well fitted, but the long time (Markovian) rate constant was also found to be the same as previously obtained. Moreover, the concentration dependencies of the fluorescence quantum yield and the Stern-Volmer constant were specified and await their experimental verification. (c) 2007 American Institute of Physics.

The reversible electron transfer from donor to excited molecule (acceptor of electron) is shown to be the irreversible energy quenching, if it is completed by subsequent irreversible recombination radical-ions which are produced. The Stern-Volmer constant of fluorescence as well as the Markovian rate constant of triplet quenching are calculated analytically, assuming the electron transfer is contact. The multiple Rehm-Weller effect is shown to be peculiar to both constants.

A theoretical analysis is presented of the problem of how distance-dependent electron transfer in photoinduced forward electron transfer followed by geminate backward electron transfer in liquid solution is re ̄ ected in the viscosity dependence of the magnetic ® eld e€ ect (MFE) on the e ciency of free radical formation u ce in such reactions. The stochastic Liouville equation formalism is employed to model the reaction behaviour of distance-distributed, triplet-born radical pairs (RPs) undergoing free di€ usion, distanceand spin-dependent backward electron transfer, coherent and incoherent spin evolution in the ps time domain. In comparison with real systems the spin situation is simpli® ed by reducing it to a two state S , T 0 problem, yet it is parametrized in a way that allows sensible comparison of the results with those of recent experiments. It is predicted that the MFE on u ce exhibits characteristic minima in the MFE versus viscosity curves, and it is veri® ed in detail that this feature is peculiar to the di€ usional model with distance-dependent electron transfer, i.e. cannot be reproduced with the simpler (`exponential’ ) RP model employing distance-independent rate constants. Thus, the MFE versus viscosity curves are established as a genuine ® ngerprint of distance-dependent electron transfer. The theoretical results compare favourably with recent experimental results obtained with Ru complex/methylviologen RPs.

We show that irreversibility of quasi-resonant bimolecular ionization in solution is due to geminate and/or bimolecular recombination of radical ions into the triplet state of neutral products. Spin conversion in the radical ion pair, which can be a limiting stage of their geminate recombination, is included in the integral encounter theory of the phenomenon. Both the triplet and charge separation quantum yields are calculated in the contact approximation and their free energy dependence is specified.

The irreversible inter-molecular energy transfer in solutions, acting as a mechanism of fluorescence quenching, is studied at an arbitrary strength of the pumping light. Using powerful methods of an original Integral Encounter Theory, we prove that the fluorescent quantum yield obeys the generalized Stern–Volmer law, linear in concentration of the unexcited energy acceptors. In contact approximation we specify how this fraction depends on the total concentration of acceptors at different lifetimes of excitations and an arbitrary excitation rate. The Stern–Volmer constant which is a function of the same parameters, either increases or decreases with light intensity depending on how fast is the decay of excited acceptor. These non-linear effects provide additional information about the energy exchange mechanism and its parameters.

The most general theoretical model of charge separation, after binary ionization in solutions, is developed. The model includes the contact creation of the exciplexes during encounters of neutral reactants and their generation due to association of solvent separated ion pairs. The recombination of singlet ion pairs to the ground state and triplet ion pairs to the triplet excited state of the reactants are also included. The spin conversion in ion pairs is considered as a stochastic process which affects the quantum yields of reaction products. Assuming contact recombination of ions, two alternative schemes of exciplex production are considered separately and analytical solutions for all the quantum yields have been found.

Reversible transfer of photogenerated short-lived excitations to energy acceptors with relatively stable excited states was considered. When it occurs in the course of bimolecular encounters in liquid solutions, it leads to energy conservation for a longer time. The conventional rate description of such a process even within the most sophisticated non-Markovian differential encounter theory is shown to be impossible due to divergency of the rate constants. The only alternative is the original integral encounter theory. It provides a proper kinetic description of delayed luminescence during encounter via energy donor as well as quantum yield of energy trapping by stable products (ions) escaped from encounters. The reasons are revealed why the usual reduction of integral kinetic theory to the differential one is inappropriate for either contact or remote reversible energy transfer between metastable states. The energy trapping via reversible ionization of the excited state is an extreme case that may be approached only within integral theory by means of which the ion accumulation kinetics and charge separation quantum yield were obtained.

Preface. 1. Gases. 2. Radiation. 3. Crystals. 4. Liquids. 5. Thermodynamics. Bibliography. Index.

We present a unified theory of binary photoionization due to remote electron transfer followed by geminate charge recombination to neutral radicals via contact proton transfer. The kinetics of the accumulation and separation of ions and radicals in a non-polar solution were studied numerically and a simple analytical model was used for the interpolation and qualitative interpretation of the results obtained for normal and inverted ionization regions. The time evolution of the charge distribution was used to calculate the number of ion-radicals outside the Onsager radius which are available for FT-EPR registration. The same distribution was used to estimate the broadening of the EPR spectra due to a dipole-dipole magnetic interaction in a radical pair which decreases with their separation.

The non-Markovian encounter theory is reformulated and used to find the quantum yields and kinetics of reversible energy transfer. Assuming a short range interaction between the particles, the kinematic approximation is employed to determine the integral kinetic equations and following from them differential equations of encounter theory. The differential variant of the theory is shown to be less appropriate since it fails to describe delayed luminescence during encounter: The reaction constant diverges with time when the acceptor of energy decays slower than the energy donor. On the contrary, integral encounter theory is very suitable for calculating the quantum yields and dissipation kinetics at any ratio between decay times.

Two different approaches to the velocity relaxation in dense fluid are compared in the limiting case common for both and the relationship between their major parameters is found.