We have used IR spectroscopy to study the nature of interaction between molecules of the organic dye methylene blue (MB) and the surface of AgCl(I) microcrystals. We recorded the IR absorption spectra of the organic dye methylene blue in the gas phase, adsorbed on the surface of AgCl(I) microcrystals, and also in the form of crystal hydrates. The results obtained allowed us to hypothesize that unsaturated dimethylamino groups and the sulfur atom in the heterocycle of this dye are the most active in the interaction between methylene blue molecules and the surface of AgCl(I) microcrystals. With the help of IR spectra of methylene blue crystal hydrates, we confirmed the significant role of OH groups during adsorption of molecules on the surface of AgCl(I) microcrystals. We suggest the possibility of formation of hydrogen bonds between the anions of the crystal lattice and the unsaturated dimethylamino groups as well as the sulfur atoms in the heterocycle, by formation of hydrogen bonds with the OH groups.Introduction. Development of modern optoelectronics has been determined by the possibilities for effective control of the parameters of optical radiation, including its frequency and intensity [1,2]. This is why there is considerable interest in searching for new materials having nonlinear properties that are exhibited even when the material is exposed to weak light fluxes. Heterogeneous systems based on a number of ionic/covalent crystals with adsorbed organic dye molecules occupy a special position among such media. In such systems, we observe sensitized anti-Stokes luminescence, arising when exposed to optical radiation with wavelengths coinciding with the absorption spectra of the adsorbed dye molecules and with low flux density (10 10 -10 15 photons/sec⋅cm 2 ) [3][4][5][6][7][8]. This phenomenon also occurs in microcrystals of AgCl 0.95 I 0.05 solid solutions with adsorbed methylene blue molecules [7]. However, despite the fact that obviously the anti-Stokes luminescence is two-photon in character, the detailed mechanism by which it arises remains controversial; it has been hypothesized that this luminescence is excited as a result of summing of the electronic excitation energy for two adsorbed dye molecules and its subsequent transfer to the crystal according to a cooperative mechanism [3]. One reason for such a situation is the lack of detailed information in the scientific literature about the nature of the interaction between the dye molecules and the surface of ionic/covalent crystals.At the same time, we should point out a number of attempts to establish the type of interaction between organic dye molecules (spectral sensitizer type) and the surface of silver halide crystals [9-16] using IR spectroscopy. For derivatives of benzothiazole and quinoline adsorbed on the surface of AgCl, AgBr, and AgI crystals, results have been obtained suggesting that interaction of these molecules with the surface involves the appearance of a coordination complex with a silver ion [10]. The considerable difference between t...

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