Absorption spectroscopy of heavy alkaline earth metals Ba and Sr in rare gas matrices--CCSD(T) calculations and atomic site occupancies

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.

Imaging individual barium atoms in solid xenon for barium tagging in nEXO

Double-β-decay involves the simultaneous conversion of two neutrons into two protons, and the emission of two electrons and two neutrinos; the neutrinoless process, although not yet observed, is thought to involve the emission of the two electrons but no neutrinos. The search for neutrinoless-double-β-decay probes fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct particles. Double-β-decay detectors are large and expensive, so it is essential to achieve the highest possible sensitivity with each study, and removing spurious contributions ('background') from detected signals is crucial. In the nEXO neutrinoless-double-β-decay experiment, the identification, or 'tagging', of the ¹³⁶Ba daughter atom resulting from the double-β decay of ¹³⁶Xe provides a technique for discriminating background. The tagging scheme studied here uses a cryogenic probe to trap the barium atom in a solid xenon matrix, where the barium atom is tagged through fluorescence imaging. Here we demonstrate the imaging and counting of individual barium atoms in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser irradiates an individual atom, the fluorescence persists for about 30 seconds before dropping abruptly to the background level-a clear confirmation of one-atom imaging. Following evaporation of a barium deposit, the residual barium fluorescence is 0.16 per cent or less. Our technique achieves the imaging of single atoms in a solid noble element, establishing the basic principle of barium tagging for nEXO.

Ab initio interaction potentials of the Ba, Ba+ complexes with Ar, Kr, and Xe in the lowest excited states

The complexes of the Ba atom and Ba⁺ cation with the rare gas atoms Ar, Kr, and Xe in the states associated with the 6s → 5d, 6p excitations are investigated by means of the multireference configuration interaction techniques. Scalar relativistic potentials are obtained by the complete basis limit extrapolation through the sequence of aug-cc-pwCVnZ basis sets with the cardinal numbers n = Q, T, 5, combined with the suitable effective core potentials and benchmarked against the coupled cluster with singles, doubles, and non-iterative triples calculations and the literature data available for selected electronic states. Spin-orbit coupling is taken into account by means of the state-interacting multireference configuration interaction calculations performed for the Breit-Pauli spin-orbit Hamiltonian. The results show weak spin-orbit coupling between the states belonging to distinct atomic multiplets. General trends in the interaction strength and long-range anisotropy along the rare gas series are discussed. Vibronic spectra of the Ba and Ba⁺ complexes in the vicinity of the ¹S → ¹P° and ²S → ²P° atomic transitions and diffusion cross sections of the Ba(¹S₀, ³D_J) atom in high-temperature rare gases are calculated. Comparison with available experimental data shows that multireference calculations tend to underestimate the interaction strength for excited complexes.

Luminescence of Atomic Barium in Rare Gas Matrices: A Two-Dimensional Excitation/Emission Spectroscopy Study

A detailed characterization is made of the distinct sites occupied by atomic barium isolated in the three rare gas hosts Ar, Kr, and Xe in excitation scans extracted from the recorded total 6s6p ¹P₁ → (6s)²¹S₀ fluorescence. Extensive use has been made of two-dimensional excitation/emission (2D-EE) spectroscopy to achieve a comprehensive characterization for the wide variety of sites present in the Ba/RG matrix systems. The 2D-EE technique has proved to be a very powerful method to probe the effects of strong intersite reabsorption when extensive spectral overlap occurs between emission and resonance 6s6p ¹P₁ ← (6s)²¹S₀ absorption of barium atoms occupying multiple sites. Two-dimensional excitation/emission scans have also been used in this study to monitor the effects of sample annealing and thereby identify the thermally stable sites of isolation. Sites of the same type occupied by atomic barium in the three host solids are identified in resolved excitation spectra and are associated on the basis of the observed matrix shift versus host polarizability. Following site associations, the photophysical properties of each matrix site were characterized revealing that the Stokes shift was greatest in the blue site, smallest for the violet site, and intermediate for the green site. The emission temperature dependences and excited state lifetimes were recorded, indicating that measured radiative lifetimes of 4-5 ns were in good agreement with the gas phase value of 8.4 ns when corrected for the effective field of the solids. The only exception to this was the blue site in Ba/Xe, where a nonradiative quenching channel exists even at 9.8 K that competes effectively with the nanosecond fluorescence. An unusual, asymmetric 2 + 1 excitation band has been recorded for atomic barium in the three rare gas hosts in addition to the threefold split, Jahn-Teller bands typically observed for P ← S absorptions of matrix-isolated metal atoms. Possible assignments of the sites responsible for these band shapes are made on the basis of recent spectral simulations obtained from molecular dynamics calculations on the Ba/Xe system.

An investigation of the sites occupied by atomic barium in solid xenon-A 2D-EE luminescence spectroscopy and molecular dynamics study

A detailed characterisation of the luminescence recorded for the 6p ¹P₁-6s ¹S₀ transition of atomic barium isolated in annealed solid xenon has been undertaken using two-dimensional excitation-emission (2D-EE) spectroscopy. In the excitation spectra extracted from the 2D-EE scans, two dominant thermally stable sites were identified, consisting of a classic, three-fold split Jahn-Teller band, labeled the blue site, and an unusual asymmetric 2 + 1 split band, the violet site. A much weaker band has also been identified, whose emission is strongly overlapped by the violet site. The temperature dependence of the luminescence for these sites was monitored revealing that the blue site has a non-radiative channel competing effectively with the fluorescence even at 9.8 K. By contrast, the fluorescence decay time of the violet site was recorded to be 4.3 ns and independent of temperature up to 24 K. The nature of the dominant thermally stable trapping sites was investigated theoretically with Diatomics-in-Molecule (DIM) molecular dynamics simulations. The DIM model was parameterized with ab initio multi-reference configuration interaction calculations for the lowest energy excited states of the Ba⋅Xe pair. The simulated absorption spectra are compared with the experimental results obtained from site-resolved excitation spectroscopy. The simulations allow us to assign the experimental blue feature spectrum to a tetra-vacancy trapping site in the bulk xenon fcc crystal-a site often observed when trapping other metal atoms in rare gas matrices. By contrast, the violet site is assigned to a specific 5-atom vacancy trapping site located at a grain boundary.

Interaction potentials and transport properties of Ba, Ba+, and Ba2+ in rare gases from He to Xe

A highly accurate, consistent set of ab initio interaction potentials is obtained for the title systems at the coupled cluster with singles, doubles, and non-iterative triples level of theory with extrapolation to the complete basis set limit. These potentials are shown to be more reliable than the previous potentials based on their long-range behavior, equilibrium properties, collision cross sections, and transport properties.