Fedor F. Sukhov

Formation and decay of transient xenon dihydride resulting from hydrocarbon radiolysis in a xenon matrix

The formation and decay of transient xenon dihydride upon annealing xenon-hydrocarbon systems irradiated with fast electrons at 15 K was monitored by FTIR spectroscopy. It is concluded that XeH2 is a chemically identical species, which can be generated in different ways. The two bands of these species (positioned at 1181 and 1166 cm−1) show essentially different behaviour with annealing temperature. Evidence for a chemical reaction of xenon dihydride with trans-2-butene occurring in a xenon matrix at 60–70 K was obtained. The addition of electron scavengers results in a dramatic decrease in the XeH2 yield. It is suggested that xenon dihydride results from H atoms rather than from charged precursors.

Isotopic effect on thermal mobility of atomic hydrogen in solid xenon

We have studied thermal mobility of atomic hydrogen in solid Xe using decomposition of water molecules as a source for hydrogen atoms. The formation of various isotopomers of HXeH and HXeOH is monitored at temperatures from 37 to 42 K by using infrared absorption spectroscopy, and the activation energy of this diffusion-controlled process is found to be ∼110 meV. Most importantly, the different mobility for hydrogen isotopes is demonstrated, H being faster than D, and the difference between the corresponding activation energies is estimated to be ∼4 meV. The electron paramagnetic resonance measurements of the thermal decay of H atoms and OH radicals show that the formation of HXeH and HXeOH is controlled by hydrogen mobility. The modeling of thermally activated jumps of hydrogen atoms in a relaxed Xe lattice agrees reasonably with experiment with respect to the isotopic effects but it underestimates the jump rate.

Infrared absorption and electron paramagnetic resonance studies of vinyl radical in noble-gas matrices

Vinyl radicals produced by annealing-induced reaction of mobilized hydrogen atoms with acetylene molecules in solid noble-gas matrices (Ar, Kr, and Xe) were characterized by Fourier transform infrared and electron paramagnetic resonance (EPR) spectroscopies. The hydrogen atoms were generated from acetylene by UV photolysis or fast electron irradiation. Two vibrational modes of the vinyl radical (nu7 and nu5) were assigned in IR absorption studies. The assignment is based on data for various isotopic substitutions (D and 13C) and confirmed by comparison with the EPR measurements and density-functional theory calculations. The data on the nu7 mode is in agreement with previous experimental and theoretical results whereas the nu5 frequency agrees well with the computational data but conflicts with the gas-phase IR emission results.

Direct visualization of the H-Xe bond in xenon hydrides: xenon isotopic shift in the IR spectra.

IR spectra of xenon hydrides (HXeCCH, HXeCC, and HXeH) obtained from different xenon isotopes ((129)Xe and (136)Xe) exhibit a small but detectable and reproducible isotopic shift in the absorptions assigned to H-Xe stretching (by 0.17-0.38 cm(-1)). To our knowledge, it is the first direct experimental evidence for the H-Xe bond in HXeY type compounds. The shift magnitude is in good agreement with quantum-chemical calculations.

Radiation-induced degradation of alkane molecules in solid rare gas matrices

Abstract The radiation-induced degradation of heptane molecules in solid argon and xenon matrices at 15 K was studied using low-temperature IR spectroscopy. The total radiation-chemical yield of the destruction of heptane molecules in argon (mole ratio 500:1) was estimated to be 1.4 molecule per 100 eV. Methane, vinyl- and trans-vinylene-type olefins, and allyl-type radicals were identified among the main radiolysis products in both matrices. The C-C bond rupture is favoured in argon probably due to formation of excited heptane cations in the hole transfer in this matrix. An indication of the radical cation trapping was obtained in a xenon matrix containing an electron scavenger (Freon-113). The mechanism of the radiation-induced degradation of alkane molecules and the fate of the primary cations in rigid inert media are discussed.

Stabilisation and reactions of aliphatic radical cations produced by fast electron irradiation in solid argon matrices

Stabilisation and reactions of radical cations produced by irradiation of various aliphatic molecules (ethers and carbonyl compounds) in argon matrices in the presence of electron scavengers at 10–16 K were studied by EPR. It was found that the relative yield of trapped radical cations depended strongly on the chemical structure of ionised organic molecules. In particular, the radical cations of tetrahydrofuran and acetone are trapped with high yields at large dilution (above 1000: 1). Acetaldehyde gives both primary radical cations and fragmentation products (methyl radicals). No sign of trapping of the primary radical cations was obtained in the case of methyl tert-butyl ether; instead of this, methyl radicals are produced with high yield. Specific deuteration of the methoxy group shows that methyl radicals originate from the tert-butyl moiety. The secondary reactions of ionised molecules occurring upon irradiation of organic molecules in argon matrices were attributed to high exothermicity of the positive hole transfer from argon to solute molecules (large “IP gap”). The striking difference in the yields of primary radical cations is explained by the effect of molecular structure on intramolecular relaxation of excess energy. The implication of the obtained results for basic radiation chemistry of organic molecules in solids is discussed.

An ESR study of benzene radical cation in an argon matrix: evidence for favourable stabilization of 2B1g rather than 2B2g state

Abstract Benzene radical cations were generated in an argon matrix by fast electron irradiation at 16 K. The ESR spectrum measured immediately after irradiation results probably from the mixture of the two distorted states; however, it converts irreversibly to the spectrum of the 2 B 1 g state upon annealing. The latter spectrum exhibits major hyperfine coupling with four of six protons (the isotropic coupling constant was estimated to be 0.64 mT). The role of basic Jahn–Teller distortion and matrix-assisted effects for benzene radical cations trapped in low-temperature matrices is discussed.

Hydrogen atoms in solid xenon: Trapping site structure, distribution, and stability as revealed by EPR studies in monoisotopic and isotopically enriched xenon matrices

Trapping and decay of hydrogen atoms generated by fast electron irradiation of solid xenon doped with small hydrogen-containing molecules (acetylene, water) were studied by EPR using monoisotopic (136)Xe matrix (I = 0) and highly isotopically enriched (129)Xe matrix (I = 12). It was found that more than 99% of H atoms observed by EPR are initially trapped in the octahedral interstitial trapping sites, whereas initial population of the substitutional trapping sites is very small (less than 1%). The (129)Xe hyperfine coupling tensor parameters for major trapping site were determined from direct measurements in a (136)Xe matrix doped with small amount of (129)Xe: A(0) ((129)Xe) = -92.1 MHz and B((129)Xe) = -22 MHz. Final proof for the trapping site structure was obtained from comparison between experiment and simulation for the highly enriched (129)Xe matrix. The mean interspin distance of approximately 4 nm was estimated from the EPR signal linewidth in a (136)Xe matrix, the hydrogen atom loss upon irradiation being negligible at low doses. Decay of trapped H atoms occurring at 38-45 K leads to population (or creation) of metastable traps of lower symmetry.

Effect of matrix and substituent on the electronic structure of trapped benzene radical cations

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B1g and 2B2g states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B2g-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

The radiation-induced chemistry in solid xenon matrices

The paper presents an overview of recent studies of the radiation-chemical transformations of guest molecules in solid xenon induced by fast electrons and x-ray irradiation. Specific features of the experimental approach based on the combination of matrix isolation IR and EPR spectroscopy are briefly outlined (with a particular emphasis on monoisotopic and isotopically enriched xenon matrices). The results reveal rich and diverse radiation-induced chemistry in solid xenon, which is considered in the following major aspects: (1) matrix-induced and matrix-assisted transformations of the primary guest radical cations; (2) production and dynamics of hydrogen atoms; (3) formation of xenon hydrides. Finally, preliminary results on the radiation-induced generation of oxygen atoms and ions in solid xenon are presented.