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 thermal migration of the first- and second-row atoms in Ar, Kr and Xe crystals

Trapping and temperature-induced migration (TIM) of the first- and second-row atoms A from H to Ne in the face-centered cubic rare gas RG = Ar, Kr and Xe crystals are investigated within the classical crystal model parameterized by the empirically modified pairwise potentials. New ab initio coupled cluster A-RG potentials computed in a uniform way for all the atoms A are used to represent the atom-crystal interactions. Absolute and relative stabilities of the substitutional and interstitial trapping sites, their structures, interstitial migration pathways, related activation energies and rough estimates of the TIM rates are obtained. The isotropic model, which neglects non-zero atomic electronic orbital momentum, reveals that migration of interstitial atoms along the network of conjugated fcc octahedral voids is the generic case for atomic mobility. Anisotropic interactions with a crystal inherent to P-state atoms B, C, O and F are accounted for using the non-relativistic diatomics-in-molecule method. Depending on its sign, interaction anisotropy can alter the structures of interstitial trapping sites and transition states remarkably. This, in turn, can dramatically affect the TIM rates. Comparison with reliable experimental data available for oxygen and hydrogen indicates a systematic overestimation of the measured activation energies, by 30% at worst. A comprehensive literature review accomplished for other atoms reveals a lack of information on the TIM processes and rates, though makes it possible to verify a part of the present results on the trapping site energies and structures.

An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C₂H₅OH and CH₃CD₂OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH₃˙) and formyl (HCO˙) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of "hot" ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of α-hydroxyethyl radicals (CH₃˙CHOH or CH₃˙CDOH(D) in the cases of C₂H₅OH and CH₃CD₂OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH₃CD₂O˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH₃˙CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.

Radiation-induced transformations of acetaldehyde molecules at cryogenic temperatures: a matrix isolation study

Acetaldehyde is one of the key small organic molecules involved in astrochemical and atmospheric processes occurring under the action of ionizing and UV radiation. While the UV photochemistry of acetaldehyde is well studied, little is known about the mechanism of processes induced by high-energy radiation. This paper reports the first systematic study on the chemical transformations of CH₃CHO molecules resulting from X-ray irradiation under the conditions of matrix isolation in different solid noble gases (Ne, Ar, Kr, and Xe) at 5 K. CO, CH₄, H₂CCO, H₂CCO-H₂, C₂H₂⋯H₂O, CH₂CHOH, CH₃CO˙, CH₃˙, HCCO˙, and CCO were identified as the main radiolysis products. The dominant pathway of acetaldehyde degradation involves C-C bond cleavage leading to the formation of carbon monoxide and methane. The second important channel is dehydrogenation resulting in the formation of ketene, a potentially highly reactive species. It was found that the matrix significantly affected both the decomposition efficiency and distribution of the reaction channels. Based on these observations, it was suggested that the formation of the methyl radical as well as vinyl alcohol and the C₂H₂⋯H₂O complex presumably included a significant contribution of ionic pathways. The decomposition of acetyl radicals under photolysis with visible light leading to the CH₃˙-CO radical-molecule pair was observed in all matrices, while the recovery of CH₃CO˙ in the dark at 5 K was found only in Xe. This finding represents a prominent example of matrix-dependent chemical dynamics (probably, involving tunnelling), which deserves further theoretical studies. Probable mechanisms of acetaldehyde radiolysis and their implications for astrochemistry, atmospheric chemistry and low-temperature chemistry are discussed.

Stable axially symmetric atomic impurity in an fcc solid-Ba in rare gases

Closed-shell metal atoms in rare gas solids tend to occupy highly symmetric polyhedral crystal sites, as follows from the generic triplet Jahn-Teller splitting of the S → P excitation bands and complies with the isotropic nature of the dispersion forces. Atypical 2 + 1 Jahn-Teller splitting inherent to axially symmetric sites observed recently for Ba atoms has been therefore interpreted as the defect accommodation. By modeling the structure, stability, and spectra of the Ba atom in the face-centered cubic rare gas crystals, we identify thermodynamically stable crystal site of axial C_3v symmetry that explains experimental observations. We also demonstrate the dramatic effect of the interaction anisotropy on the trapping site structure and stability for an excited P-state atom. Our results provide strong evidence for stable axially symmetric accommodation of isotropic impurity in a close-packed lattice.

Accommodation of a dimer in an Ar-like lattice: exploring the generic structural motifs

A global optimization strategy is applied to Lennard-Jones models describing the stable trapping sites of a dimer in the face-centered cubic Ar-like lattice. Effective volumes of the trapping sites, quantified as the number of host atoms dislodged from the lattice, are mapped onto the parameter space defined by the strength and range of the dimer interaction potentials. The two models considered differ in the host-guest interaction and give very different maps that reflect the effect of local lattice relaxation. A hierarchical complete-linkage clustering technique is applied to identify generic structural types of the dimer accommodations. The dominant types found and enlisted maintain the symmetry of the isolated dimer and possess high tetrahedral and octahedral symmetry of the host environment with respect to the dimer atoms or center and can be roughly classified as the "whole" or "per atom" dimer accommodations. The results are compared to the analysis of the analogous model for trapped atoms and realistic model for trapped alkaline-earth metal dimers. Implications for matrix isolation spectroscopy are discussed.

Reaction mechanisms in swelling clays under ionizing radiation: influence of the water amount and of the nature of the clay mineral

Picosecond pulse radiolysis experiments performed on natural swelling clays evidence a fast trapping of electrons in the layers of the material.

Photolabile xenon hydrides: a case study of HXeSH and HXeH

The photo-induced transformations of HXeSH and HXeH under the action of IR and visible light have been studied using FTIR spectroscopy. The xenon hydrides were produced by the X-ray induced decomposition of H2S and its isotopomers in a solid xenon matrix at 7.5 K followed by thermal annealing at the temperatures up to 45 K. Selective IR-induced photodissociation of HXeSH at 3500-2500 cm(-1) was attributed to vibrational excitation of the 3ν(H-Xe) mode. The IR-photodecomposed HXeSH molecules can be almost quantitative recovered below 22 K with very small effective activation energy (~20 meV) indicating local character of this process. Analysis of the photoactivity of xenon hydrides in the visible region revealed previously unknown absorptions for HXeSH (in the region of 400-700 nm) and HXeH (above 700 nm). The decomposition of HXeH occurs due to both direct photolysis and reactions of "hot" H atoms produced from the photodissociation of HXeSH. The efficiency of thermal recovery for both xenon hydrides after photolysis with visible light was found to be dependent on the excitation wavelength, which was explained by the effect of photon energy on spatial distribution of the dissociation fragments.