ИСТИНА

The photo- and radiation-induced transformations of small molecules at low temperatures play an important role in the interstellar, planetary and atmospheric chemistry. Acetaldehyde has been the subject of extensive investigations because of its simplicity and prevalence, the small size of the system let carry out theoretical studies as well. However, there are virtually no works on the radiolysis of isolated acetaldehyde molecules. In order to elucidate the mechanism of the primary processes involved in the degradation of isolated acetaldehyde molecules in inert ices, we have examined the effect of X-ray irradiation on CH3CHO in various solid noble gas matrices. Basic features of our experimental approach for matrix isolation studies of the radiation-induced processes were described previously [1,2]. Gaseous mixtures of CH3CHO and noble gases (Ar, Kr, or Xe) in typical dilution 1:1000 were slowly deposited onto a KBr plate at the temperatures of 12 – 30 K, depending on the matrix used. Deposited samples were irradiated with X-rays (effective energy ca. 20 keV) at 6 K for 1 – 90 min in order to monitor the dose dependence. Irradiated samples were then annealed for 5 min at different temperatures up to 49 K (depending on the matrix). IR spectra of the samples were recorded using a Bruker TENSOR FTIR-spectrometer (cooled MCT detector, resolution of 1 cm−1, averaging by 144 scans). The FTIR spectra of deposited samples show absorptions of isolated acetaldehyde molecules and trace amounts of matrix-isolated water and carbon dioxide. The main products of radiolysis of isolated CH3CHO include CO, HCO, CH3, CH4, and solvated proton. The relative yield of main products changes markedly while going from Ar to Xe matrix. The formation of C2H3O radicals from isolated acetaldehyde molecules under the action of ionizing radiation is discussed. Experiments with electron scavenger (SF6) were carried out to elucidate the relative role of ionic and neutral excited states. Possible mechanism of the radiation-induced transformations of isolated acetaldehyde is proposed. Increasing temperature (up to 25 K in Kr and 33 K in Xe) in irradiated acetaldehyde/Ng leads to reactions of the thermally mobilized H atoms giving the corresponding annealing-induced products. In addition, the formation of XeH2 was observed in the case of xenon matrix. In summary, we may conclude that the radiation-induced transformations of matrix isolated acetaldehyde molecules and the ratio of the products formed are strongly affected by the matrix used. These results may be important for better understanding of the fate of acetaldehyde in astrochemically relevant ices. References 1. E.V. Saenko, V.I. Feldman, Phys. Chem. Chem. Phys., 2016, 18, 32503. 2. A.V. Kobzarenko et al., Radiat. Phys. Chem., 2012, 81, 1434.