Atomic oxygen in crystalline Kr and Xe. II. Adiabatic potential energy surfaces

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.

Modeling of the thermal migration mechanisms of atomic oxygen in Ar, Kr, and Xe crystals

Accommodation and migration of the ground-state (2s²2p^{4 3}P) oxygen atom in the ideal Ar, Kr, and Xe rare gas crystals are investigated using the classical model. The model accounts for anisotropy of interaction between guest and host atoms, spin-orbit coupling, and lattice relaxation. Interstitial and substitutional accommodations are found to be the only thermodynamically stable sites for trapping atomic oxygen. Mixing of electronic states coupled to lattice distortions justifies that its long-range thermal migration follows the adiabatic ground-state potential energy surface. Search for the migration paths reveals a common direct mechanism for interstitial diffusion. Substitutional atoms are activated by the point lattice defects, whereas the direct guest-host exchange meets a higher activation barrier. These three low-energy migration mechanisms provide plausible interpretation for multiple migration activation thresholds observed in Kr and Xe free-standing crystals, confirmed by reasonable agreement between calculated and measured activation energies. An important effect of interaction anisotropy and a minor role of spin-orbit coupling are emphasized.

Photolysis of O2 dispersed in solid neon with far-ultraviolet radiation

Irradiation at 173 or 143 nm of samples of ¹⁶O₂ or ¹⁸O₂ in solid Ne near 4 K produced many new spectral lines in absorption and emission from the mid-infrared to the near-ultraviolet regions. The major product was ozone, O₃, that was identified with its mid-infrared and near-ultraviolet absorption lines. Oxygen atoms were formed on photolysis of O₂ and stored in solid neon until the temperature of a sample was increased to 9 K, which enabled their migration and combination to form O₃ and likely also O₂. O₂ in five excited states and O in two excited states detected through the emission spectra indicate that complicated processes occurred in solid Ne after far-ultraviolet excitation. For the transition ¹D₂ → ³P_1,2 of O, the lifetime was determined to be 5.87 ± 0.10 s; the lifetime of the upper state of an unidentified transition associated with an emission feature at 701.7 nm was determined to be 2.34 ± 0.07 s.

Theoretical investigation of the dynamics of O((1)D→(3)P) electronic quenching by collision with Xe

We present the quantum close-coupling treatment of spin-orbit induced transitions between the (1)D and (3)P states of an atom in collisions with a closed-shell spherical partner. In the particular case of O colliding with Xe, we used electronic structure calculations to compute the relevant potential energy curves and spin-orbit coupling matrix elements. We then carried out quantum scattering calculations of integral and differential quenching cross sections as functions of the collision energy. The calculated differential cross sections for electronic quenching are in reasonable agreement with measurements [Garofalo et al., J. Chem. Phys. 143, 054307 (2015)]. The differential cross sections exhibit pronounced oscillations as a function of the scattering angle. By a semiclassical analysis, we show that these oscillations result from quantum mechanical interference between two classical paths.

Matrix-isolation studies of noncovalent interactions: more sophisticated approaches

Noncovalent interactions are crucial for many physical, chemical, and biological phenomena. Matrix isolation is a powerful method to study noncovalent interactions, including hydrogen-bonded species, and it has been extensively used in this field. However, there are difficult situations, such as in the case of species that are impossible to prepare in the gas phase. In this article, we describe some advanced approaches allowing studies of complexes that are problematic for the traditional methods. Photolysis of a suitable precursor in a matrix can lead to a large concentration of 1:1 complexes, which are otherwise very difficult to prepare (e.g., the H2O···O complex). Photolysis of species combined with annealing can lead to complexes of molecules with mobile atoms (e.g., the same H2O···O complex). Simultaneous photolysis of two species combined with annealing can produce complexes of radicals via reactions of the photogenerated complexes with mobile atoms (e.g., the H2O···HCO complex). Interaction of noble-gas (Ng) hydrides with other species is another topic (e.g., the N2···HArF complex) and very large blue shifts of the H-Ng stretching modes are normally observed for these systems. Complexes and dimers of the higher-energy conformer of formic acid have been prepared by using selective vibrational excitation of the ground-state conformer. The higher-energy conformer of formic acid can be efficiently stabilized in the complexes with strong hydrogen bonding. We also consider some problematic cases when the changes in the vibrational frequencies of the 1:1 complexes are very small (e.g., the phenol···Xe complex) and when the complex formation is prevented by strong solvation in the matrix (e.g., species in solid xenon).

Coinage metal exciplexes with helium atoms: a theoretical study of M*(2L)He(n) (M = Cu, Ag, Au; L = P,D)

The structure and energetics of exciplexes M*((2)L)He(n) (M = Cu, Ag and Au; L = P and D) in their vibrational ground state are studied by employing diffusion Monte Carlo (DMC). Interaction potentials between the excited coinage metals and He atoms are built using the Diatomics-in-Molecule (DIM) approach and ab initio potential curves for the M((2)L)-He dimers. Extending our previous work [Cargnoni et al., J. Phys. Chem. A, 2011, 115, 7141], we computed the dimer potential for Au in the (2)P and (2)D states, as well for Cu and Ag in the (2)D state, employing basis set superposition error-corrected Configuration Interaction calculations. We found that the (2)Π potential correlating with the (2)P state of Au is substantially less binding than for Ag and Cu, a trend well supported by the M(+) ionic radiuses. Conversely, the interaction potentials between a (n - 1)d(9)ns(2 2)D metal and He present a very weak dependency on M itself or the projection of the angular momentum along the dimer axis. This is due to the screening exerted by the ns(2) electrons on the hole in the (n - 1)d shell. Including the spin-orbit coupling perturbatively in the DIM energy matrix has a major effect on the lowest potential energy surface of the (2)P manifold, the one for Cu allowing the formation of a "belt" of five He atoms while the one for Au being completely repulsive. Conversely, spin-orbit coupling has only a weak effect on the (2)D manifold due to the nearly degenerate nature of the diatomic potentials. Structural and energetic results from DMC have been used to support experimental indications for the formation of metastable exciplexes or the opening of non-radiative depopulation channels in bulk and cold gaseous He.

Formation of HXeO in a xenon matrix: Indirect evidence of production, trapping, and mobility of XeO (1 1Σ+) in solid Xe

IR spectroscopy, laser induced fluorescence (LIF), and thermoluminescence (TL) measurements have been combined to monitor trapping, thermal mobility, and reactions of oxygen atoms in solid xenon. HXeO and O(3) have been used as IR active species that probe the reactions of oxygen atoms. N(2)O and H(2)O have been used as precursors for oxygen atoms by photolysis at 193 nm. Upon annealing of matrices after photolysis, ozone forms at two different temperatures: at 18-24 K from close O ...O(2) pairs and at approximately 27 K due to global mobility of oxygen atoms. HXeO forms at approximately 30 K reliably at higher temperature than ozone. Both LIF and TL show activation of oxygen atoms around 30 K. Irradiation at 240 nm after the photolysis at 193 nm depletes the oxygen atom emission at 750 nm and reduces the amount of HXeO generated in subsequent annealing. Part of the 750 nm emission can be regenerated by 266 nm and this process increases the yield of HXeO in annealing as well. Thus, we connect oxygen atoms emitting at 750 nm with annealing-induced formation of HXeO radicals. Ab initio calculations at the CCSD(T)/cc-pV5Z level show that XeO (1(1)Sigma(+)) is much more deeply bound [D(e) = 1.62 eV for XeO --> Xe+O((1)D)] than previous calculations have predicted. Taking into account the interactions with the medium in an approximate way, it is estimated that XeO (1(1)Sigma(+)) has a similar energy in solid xenon as compared with interstitially trapped O((3)P) suggesting that both possibly coexist in a low temperature solid. Taking into account the computational results and the behavior of HXeO and O(3) in annealing and irradiations, it is suggested that HXeO may be formed from singlet oxygen atoms which are trapped in a solid as XeO (1(1)Sigma(+)).

A neutral xenon-containing radical, HXeO

We report an open-shell species containing xenon, HXeO ((2)Sigma), prepared by UV photolysis of H(2)O/Xe or N(2)O/HBr/Xe solid mixtures at 7 K and subsequent thermal mobilization of oxygen atoms at >/=30 K. The H-Xe stretching absorption of HXeO in solid Xe is at 1466.1 cm(-1), and it shifts to 1070.3 cm(-1) upon deuteration. The extensive ab initio calculations indicate that HXeO is intrinsically stable, owing to significant ionic and covalent contributions to its bonding. The formation of HXeO ((2)Sigma) radicals in these experiments suggests extensive stabilization and thermal mobility of singlet ((1)D) oxygen atoms in solid Xe and holds promises for the stability of the HKrO and HArO species.

Modeling of the Three-Body Effects in the Neutral Trimers in the Quartet State by ab initio Calculations. H3, Na3, and Na2B

The Na2B, Na3, and H3 trimers in the lowest quartet states were studied by ab initio methods, using both the supermolecular approach and the intermolecular Møller-Plesset perturbation theory. Partitioning of the nonadditive contribution into the orientational two-body part and the genuine three-body part was proposed. The lowest quartet state of the Na3 trimer and all the three lowest quartet states of the Na2B trimer are bound, and the forms of these clusters are essentially determined by two-body forces. In the case of the Na2B trimer the orientational two-body nonadditivity proved to be crucial. In addition, in the title metal trimers, in the region of the van der Waals minima, the genuine nonadditivity is very important, and amounts to 30% in Na2B and up to 70% in Na3. The leading nonadditive term is the triple-exchange Heitler-London exchange term. For triangular arrangements it considerably enhances the total stabilization. The single-exchange term and the SCF deformation play only a secondary role. The dispersion nonadditivity is negligible. The isotropic part of the basis set superposition error (BSSE) is large and must be corrected by the counterpoise method. The anisotropic contribution to BSSE is practically negligible.

Concentration dependence of the spectroscopic and photochemical properties of atomic and molecular oxygen in argon matrices

Vacuum ultraviolet (VUV) excitation (200-100 nm) and visible emission (300-650 nm) spectra of O2 imbedded in Ar matrices at different concentrations are presented. At 0.1 and 0.2% concentrations a linear increase in the intensities of the excitation and emission spectra is observed. At these concentrations, photolysis of O2 is found to be negligible. At higher concentrations (0.5, 1 and 2%) the normalized intensities of the excitation and emission spectra of O2 decrease. With increasing concentration of O2 the permanent photolysis of O2 increases, which does not correlate, based on the excitation spectra of O in Ar, to the production of isolated O atoms. It has been shown that this anomalous behavior should be due to the formation of van der Waals dimers and oligomers at higher concentrations of O2 that upon photolysis produce ozone.