This paper discusses the structural modification of a molecular layer of polymethine dye under the action of laser radiation. Resonance excitation of the all-trans isomer irreversibly alters the spatial orientations and relative concentrations of all the isomeric and aggregated species of the molecules present in the composition of the layer. The influence of the total excitation-energy density on the orientational variation and relative concentrations of the species depends on the lasing regime and the energy density in a single pulse, on the concentration of the all-trans isomer, which determines the amount of absorbed energy in each pulse, and on the total concentration of molecules in the composition of all the species, which affects the heating temperature. A model is proposed for the photostimulated structural modification of a layer of polymethine dye.
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