Magnetic circular dichroism and the Jahn-Teller effect in the 2S .fwdarw. 2P transition of sodium and lithium atoms isolated in xenon matrixes

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.

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.

Magnetic circular dichroism and absorption spectra of phosphinidene in noble-gas matrices

Electronic magnetic circular dichroism and absorption spectra are reported for the A 3pi(i) <-- X 3sigma- transitions of phosphinidene (PH) isolated in Ar, Kr, and Xe matrices at cryogenic temperatures (approximately 1.4-20 K) and over a range of magnetic field strengths (0-5 T). The results are analyzed by the method of moments, and parameters are extracted by fitting the experimental data to a model in which the A 3pi(i) term is split by spin-orbit (SO) coupling interactions, while the X 3sigma- term is split by spin-spin and higher-order SO coupling. The analysis indicates that, unlike the equivalent imidogen (NH) systems, ground-state PH radicals isolated in noble-gas matrices do not behave as free rotors. Trends in excited-state SO coupling constants are attributed to the external heavy-atom effect and guest-host orbital mixing. It is tentatively concluded that librational amplitudes of the guest radical decrease in the order Ar > Xe > Kr, probably as a consequence of competition between stronger guest-host interactions and larger matrix sites in heavier hosts.

Moment Analysis Method As Applied to the 2S → 2P Transition in Cryogenic Alkali Metal/Rare Gas Matrices

The moment analysis method (MA) has been tested for the case of 2S --> 2P ([core]ns1 --> [core]np1) transitions of alkali metal atoms (M) doped into cryogenic rare gas (Rg) matrices using theoretically validated simulations. Theoretical/computational M/Rg system models are constructed with precisely defined parameters that closely mimic known M/Rg systems. Monte Carlo (MC) techniques are then employed to generate simulated absorption and magnetic circular dichroism (MCD) spectra of the 2S --> 2P M/Rg transition to which the MA method can be applied with the goal of seeing how effective the MA method is in re-extracting the M/Rg system parameters from these known simulated systems. The MA method is summarized in general, and an assessment is made of the use of the MA method in the rigid shift approximation typically used to evaluate M/Rg systems. The MC-MCD simulation technique is summarized, and validating evidence is presented. The simulation results and the assumptions used in applying MA to M/Rg systems are evaluated. The simulation results on Na/Ar demonstrate that the MA method does successfully re-extract the 2P spin-orbit coupling constant and Landé g-factor values initially used to build the simulations. However, assigning physical significance to the cubic and noncubic Jahn-Teller (JT) vibrational mode parameters in cryogenic M/Rg systems is not supported.

The absorption and excitation spectroscopy of matrix-isolated atomic manganese: Sites of isolation in the solid rare gases

This study collects information from absorption and luminescence excitation spectra recorded for Mn atoms isolated in the solid rare gases Ar, Kr, and Xe and presents an analysis of the site occupancy, based on the polarizabilities of the rare gases and the observed spectral shifts. Two thermally stable sites of isolation exist for atomic Mn in solid Ar and Kr, while a single thermally stable site is present in Mn/Xe. Site occupancy assignments are based on the application of a polarizability model to the z (6)P(5/2)<--a (6)S(5/2); z (8)P(5/2)<--a (6)S(5/2), and y (6)P(5/2)<--a (6)S(5/2) electronic transitions of atomic Mn. From an analysis of the observed RG matrix-to-gas phase energy shifts for P<--S type transitions, this model allows the association of certain site types occupied by metal atoms in the rare gas solids. The required condition being a linear dependence of the matrix shifts with rare gas polarizability for those metal atoms "trapped" in a particular site type. Application of the polarizability model in conjunction with trends observed in site dominance, established a connection between the blue sites in Ar and Kr and the single site in Xe. Use of the known MgRG ground state bond lengths facilitated an identification of the sites of Mn atom isolation assuming the transference of the known MgRG bond lengths to the MnRG systems. Substitutional site occupancy of atomic Mn is assigned to the blue sites in Ar and Kr and the single site in Xe, while tetra-vacancy site occupancy is assigned to the red sites in Ar and Kr. Consistent with these assignments, Mn atoms in solid Ar show a preference for trapping in tetra-vacancy sites whereas in solid Kr, single substitutional sites are preferred and in Xe, this is the only site observed.