ИСТИНА

The radiation-induced transformations of small molecules at low temperatures play an important role in the atmospheric, planetary and interstellar chemistry. Methanol is one of the most abundant organic molecules in the interstellar medium [1]. The radiation chemistry of liquid and glassy methanol is well studied [2], but there are virtually no works on the radiolysis of isolated methanol molecules. In order to elucidate the mechanism of the primary processes involved in the degradation of isolated methanol molecules in inert ices, we have examined the effect of X-ray irradiation on CH3OH and CD3OH in various solid noble gas matrices. Basic features of our experimental approach for matrix isolation studies of the radiation-induced processes were described previously [3]. Gaseous mixtures of CH3OH (or deuterium substituted methanol) and noble gases (Ne, Ar, Kr, or Xe) in typical dilution 1:1000 were slowly deposited onto a KBr plate at the temperatures of 7 – 30 K, depending on the matrix used. Deposited samples were irradiated with X-rays (effective energy ca. 20 keV) at 6 K for 1 – 120 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 methanol and trace amounts of methanol-water complexes, methanol clusters, matrix-isolated water and carbon dioxide. The main products of radiolysis of isolated CH3OH are CH2OH, CO, HCO, H2CO, and Ng2H+. The relative yield of CH2OH increased markedly while going from Ne to Xe matrix. It should be noted, that CO appears in all matrices, even at low conversion degree of methanol (less than 5%), and its concentration significantly grows with increasing absorbed dose. We have seen no evidence for trapped CH3O radical in matrices, but there is an indirect sign of its involvement from the experiments with isotopically substituted methanol. Conversion degree of СD3OH after irradiation in noble gas matrices is essentially lower than that for CH3OH. The main products of radiolysis in this case are CD2OH, CD2OD, CO, D2CO, HDCO, Ng2H+. Small amount of HCO also appears, while no absorptions of DCO can be seen after radiolysis. Experiments with electron scavenger (SF6) were carried out to elucidate the relative role of ionic and neutral excited states and possible mechanism of the radiation-induced transformations of isolated methanol is proposed. Increasing temperature (up to 25 K in Kr and 33 K in Xe) in irradiated methanol/Ng leads to reactions of the thermally mobilized H atoms yielding CH2OH and HCO radicals. 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 methanol molecules reveal diverse degradation pathways, which were not found in the radiolysis of glassy methanol. These results may be important for better understanding of the fate of methanol in astrochemically relevant ices. This work was supported by the Russian Science Foundation (project no. 14-13-01266). [1] Charnley S.B., Kress M.E., Tielens A.G.G.M., Millar T.L., Astrophys. J., 448, 232 (1995) [2] Dainton F.S., Salmon G.A., Wardman P., Proc. Roy. Soc. Lond. Ser. A -Math. Phys. Sci., 313, 1 (1969) [3] Kobzarenko A.V., Sukhov F.F., Orlov A.Yu, Kovalev G.V., Baranova I.A., Feldman V.I. Radiat. Phys. Chem., 81, 1434 (2012)