Yu. M. Dubina

Mathematical modeling of the dynamics of photoreactions of a five-level molecule

A mathematical modeling of the time evolution of the populations of the states of a five-level molecule during transformation of resonant monochromatic irradiation and spontaneous emission from the highest-energy state excited by a short pulse of light is performed. The formalism of the optical Bloch equations and quantum theory of radiation are applied a composite system consisting of a molecule and a quantized radiation field. The results of simulation of the evolution of the population of the states of the molecule in the case of spontaneous emission are similar for both of these two approaches, but differ significantly in the case of conversion by the molecule of monochromatic radiation. These differences are the greater, the higher the intensity of resonance Rayleigh scattering or (and) relaxed fluorescence, as a result of which the molecule returns to the initial ground state. An explanation of the nature of these differences is given.

Mathematical modeling of the population dynamics of “dark” states of a three-level system

Mathematical modeling of the population dynamics is performed for states of a three-level system (atom) with a V-type configuration transforming a light pulse. It is assumed that the excited eigenstates of the atom are degenerate and coupled by coherent interaction, one of the states being radiating (radiative), while the other state is nonradiating (“dark”). The population dynamics of atomic states is described on the basis of numerical solutions of equations for the matrix elements of the density operator. The dependence of the efficiency of population of the atomic dark state from the values of the parameters of an irradiation pulse and from the ratio of the period of population oscillations of excited atomic states (caused by their coherent interaction) to the lifetime of the atomic radiating state is determined. Typical examples of the time dependence of the population of states of the atom considered are presented for the cases of irradiation by a short (as compared to the lifetime of the radiating state) sinusoidal light pulse and by a long rectangular light pulse with the resonance carrier frequency.

Simulation of intramolecular dynamics at transformation of two light pulses by molecules of active media of photochemical lasers

Using a numerical solution of a system of equations for matrix elements of file-level model molecule density operator time dependencies of population on its states at different values of two irradiation pulses parameters (the pump pulse and the dump (or probe) pulse) and constants, determining rates of induced radiative transitions of the molecule, and radiative and nonradiative decays of the molecular states are calculated. The values of these constants were taken, in particular, close to these of the corresponding constants of molecules with intramolecular hydrogen bond as the 3-hydroxyflavone (3-HF) molecule. Values of irradiation pulses parameters are obtained, at which after enol-keto photoisomerization, induced by pump pulse, dump pulse induces molecular transition from excited electronic state in keto-form to ground state in this (ldquotautomericrdquo) form. This transition is accompanied by stimulated emission with a frequency of ldquotautomericrdquo fluorescence. Examples of periodic change of intensity of the stimulated emission are given, the frequency of oscillations of fluorescence intensity being proportional to dump pulse intensity.

A THEORY OF LUMINESCENCE OF LIGHT-COLLECTING MOLECULAR SYSTEMS *

Expressions are obtained for the intensity and the quantum yield of the sensitized luminescence of the chromophore that plays the role of a reaction center in the simplest model trichromophore molecular [light‐collecting] antenna which is so constructed and so oriented in space that the irradiation photons coherently excite its other two chromophores‐pigments. The quantum electrodynamics formalism which takes into account the radiative dissipative interaction between the pigments and the reaction center of the antenna was used. The comparative analysis of the obtained expressions with the corresponding expressions for the luminescence of a bichromophore molecular system, differing from the trichromophore antenna by the absence of one of the pigments, has shown that the collective dissipative interaction of the pigments with the reaction center of the antenna can be considered as a highly efficient mechanism of [light collection] in molecular antennas.