Van Der Waals Molecules Research Articles

Photon emission and atomic collision processes in two-phase argon doped with xenon and nitrogen

We present a comprehensive analysis of photon emission and atomic collision processes in two-phase argon doped with xenon and nitrogen. The dopants are aimed to convert the VUV emission of pure Ar to the UV emission of the Xe dopant in the liquid phase and to the near UV emission of the N2 dopant in the gas phase. Such a mixture is relevant to two-phase dark-matter and low-energy neutrino detectors, with enhanced photon collection efficiency for primary- and secondary-scintillation signals. Based on this analysis, we show that the recently proposed hypothesis of the enhancement of the excitation transfer from Ar to N2 species in the two-phase mode is either incorrect or needs assumptions about a new extreme mechanism coming into force at lower temperatures, in particular that of the resonant excitation transfer via the ArN2 compound (van der Waals molecule).

A high volume, batch mode 129Xe polarizer

Numerous designs of optical gas polarizers have been proposed, broadening possible applications of the hyperpolarized gases as contrast agents in magnetic resonance imaging. We present a home–made 129Xe polarizer based on the spin exchange optical pumping method. The polarizer operates under 1bar of the gas mixture (at the maximum temperature of 160°C) in a high volume optical cell (5025cm3). Approximately 100cm3 of 129Xe polarized at 1.50±0.37% is produced in a single cycle of polarization. Operation under standard pressure imposes polarization transfer mainly via van der Waals molecules, resulting in the efficient spin exchange between rubidium and 129Xe atoms. The design, construction and operation of the polarizer are described in details.

Xe(N2)2compound to 150 GPa: Reluctance to the formation of a xenon nitride

The $\mathrm{Xe}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ binary phase diagram was determined at 296 K from the pressure evolution of 14 different concentrations. The properties of $\mathrm{Xe}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ mixtures were characterized using visual observation, Raman spectroscopy, and powder x-ray diffraction. Above 4.9 GPa, the $\mathrm{Xe}({\mathrm{N}}_{2}{)}_{2}$ van der Waals compound is stable and adopts the $\mathrm{MgC}{\mathrm{u}}_{2}$-type Laves phase structure $(Fd\text{\ensuremath{-}}3m)$ with ${\mathrm{N}}_{2}$ molecules orientationally disordered. At 10 GPa, this cubic lattice undergoes a martensitic phase transition into a tetragonal $(I{4}_{1}/amd)$ unit cell. This transition is associated with a partial ordering of the ${\mathrm{N}}_{2}$ molecules, possibly due to the growing ${\mathrm{N}}_{2}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ quadrupole-quadrupole interaction with density. No other phase transition was detected up to 154 GPa, even after heating the sample to 2000 K. Above 30 GPa, a softening of the ${\mathrm{N}}_{2}$ vibron mode with pressure reveals a weakening of the ${\mathrm{N}}_{2}$ intramolecular bond that suggests an electronic redistribution between ${\mathrm{N}}_{2}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ and $\mathrm{Xe}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ entities. These interactions could explain the great stability of the $\mathrm{Xe}({\mathrm{N}}_{2}{)}_{2}$ compound. However, no xenon nitride was observed.

Formation of xenon-nitrogen compounds at high pressure.

Molecular nitrogen exhibits one of the strongest known interatomic bonds, while xenon possesses a closed-shell electronic structure: a direct consequence of which renders both chemically unreactive. Through a series of optical spectroscopy and x-ray diffraction experiments, we demonstrate the formation of a novel van der Waals compound formed from binary Xe-N2 mixtures at pressures as low as 5 GPa. At 300 K and 5 GPa Xe(N2)2-I is synthesised, and if further compressed, undergoes a transition to a tetragonal Xe(N2)2-II phase at 14 GPa; this phase appears to be unexpectedly stable at least up to 180 GPa even after heating to above 2000 K. Raman spectroscopy measurements indicate a distinct weakening of the intramolecular bond of the nitrogen molecule above 60 GPa, while transmission measurements in the visible and mid-infrared regime suggest the metallisation of the compound at ~100 GPa.

Communication: Multiple-property-based diabatization for open-shell van der Waals molecules.

We derive a new multiple-property-based diabatization algorithm. The transformation between adiabatic and diabatic representations is determined by requiring a set of properties in both representations to be related by a similarity transformation. This set of properties is determined in the adiabatic representation by rigorous electronic structure calculations. In the diabatic representation, the same properties are determined using model diabatic states defined as products of undistorted monomer wave functions. This diabatic model is generally applicable to van der Waals molecules in arbitrary electronic states. Application to locating seams of conical intersections and collisional transfer of electronic excitation energy is demonstrated for O2 - O2 in low-lying excited states. Property-based diabatization for this test system included all components of the electric quadrupole tensor, orbital angular momentum, and spin-orbit coupling.

Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy

Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS2 and ReS2, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS2, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS2. For ReS2, the band gap is direct, and a value of 1.52 and 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS2 forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.

Quantum Dynamics with Spatiotemporal Control of Interactions in a Stable Bose-Einstein Condensate.

Optical control of atomic interactions in quantum gases is a long-sought goal of cold atom research. Previous experiments have been hindered by rapid decay of the quantum gas and parasitic deformation of the trap potential. We develop and implement a generic scheme for optical control of Feshbach resonances which yields long quantum gas lifetimes and a negligible parasitic dipole force. We show that fast and local control of interactions leads to intriguing quantum dynamics in new regimes, highlighted by the formation of van der Waals molecules and localized collapse of a Bose condensate.

Changes in dynamics of the glass-forming pharmaceutical nifedipine in binary mixtures with octaacetylmaltose

Some molecular glass-formers can crystallize in the glassy state, some of which are van der Waals molecules and some are pharmaceuticals. The molecular mechanism responsible for this glass-to-crystal mode of crystallization is of interest to the glass transition research community as well as to the pharmaceutical industry because the effect is detrimental to stability of amorphous form of the drugs stored below the glass transition temperature. Two prominent models have been proposed for the molecular mechanism. In the homogeneous nucleation-based crystallization model, the molecular mechanism is the secondary relaxation, and the other model assumes that the molecular process responsible for crystal growth in the glassy state is from the local molecular motions. Crystal growth requires motion of the entire molecule, and in the glassy state the only such local molecular motion is engendered by the secondary relaxation of the Johari–Goldstein (JG) kind. While the JG secondary relaxation is the crux in the two models of glass-to-crystal growth, it has not been found in the glassy state of the pharmaceuticals studied so far. The examples include 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), indomethacin (IMC) and nifedipine (NIF). In the absence of any evidence of the JG secondary relaxation, the conundrum is that the two models of glass-to-crystal growth cannot be validated. It turns out these pharmaceuticals all have structural α-relaxations with narrow frequency dispersion. Empirically, glass-formers with narrow α-dispersion have JG secondary relaxation with weak relaxation strength, not well separated from the α-relaxation, and hence cannot be resolved. Theoretically, the narrow width of the α-dispersion is due to weak intermolecular coupling. In this article we enhance the intermolecular coupling of NIF by mixing with octaacetylmaltose to enhance the intermolecular coupling of NIF. In this way we have successfully resolved the JG secondary relaxation in the dielectric loss spectra of the NIF component in the glassy state, and validated the two models of glass-to-crystal growth.

Backbone NxH compounds at high pressures.

Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first-principles theoretical structure predictions to investigate mixtures of N2 and H2 up to 55 GPa. Our experiments show the formation of structurally complex van der Waals compounds [see also D. K. Spaulding et al., Nat. Commun. 5, 5739 (2014)] above 10 GPa. However, we found that these NxH (0.5 < x < 1.5) compounds transform abruptly to new oligomeric materials through barochemistry above 47 GPa and photochemistry at pressures as low as 10 GPa. These oligomeric compounds can be recovered to ambient pressure at T < 130 K, whereas at room temperature, they can be metastable on pressure release down to 3.5 GPa. Extensive theoretical calculations show that such oligomeric materials become thermodynamically more stable in comparison to mixtures of N2, H2, and NH3 above approximately 40 GPa. Our results suggest new pathways for synthesis of environmentally benign high energy-density materials. These materials could also exist as alternative planetary ices.

Mechanism of two types of Na emission observed in sonoluminescence

The sonoluminescence (SL) spectrum of Na atoms revealed that the Na line consists of two components, one of which is a broadened component (broad component) which is shifted from the original D lines, and the other is an unshifted narrow component (narrow component). We spatially separated the continuum, broad, and narrow components by capturing SL images using different optical filters. We also temporally separated these components by measuring SL pulses using respective band-pass filters. The SL image distribution and the timing of the SL pulses were different between the broad and narrow components. The results suggested that the broad and narrow components of Na emission are generated from different bubble populations. The dependences of SL spectra on ultrasonic frequency and dissolved rare gas (He, Ne, Ar, Kr, and Xe) were also investigated. It is concluded that the broad component and a blue satellite peak at 558 nm originate from van der Waals molecules composed of Na and rare-gas atoms. The narrow component was predicted to occur under temperature conditions at bubble collapse higher than that for the broad component.

Structure and stability of solid Xe(H2)n.

Mixtures of xenon and molecular hydrogen form a series of hexagonal, van der Waals compounds at high pressures and at 300 K. Synchrotron, x-ray, single crystal diffraction studies reveal that below 7.5 GPa, Xe(H2)8 crystallizes in a P3̄m1 structure that displays pressure-induced occupancy changes of two pairs of xenon atoms located on the 2c and 2d sites (while the third pair on yet another 2c site remains fully occupied). The occupancy becomes 1 at the P3̄m1 to R3 transition and all the xenon atoms occupy the 3d sites in the high-pressure structure. These pressure-induced changes in occupancy coincide with volume changes that maintain the average Xe:H2 stoichiometry fixed at 1:8. The synchrotron x-ray diffraction and Raman measurements show that this unique hydrogen-bearing compound that can be synthesized at 4.2 GPa and 300 K, quenched at low temperatures to atmospheric pressure, and retained up to 90 K on subsequent warming.

Systematic T1 improvement for hyperpolarized 129xenon

The spin–lattice relaxation time T1 of hyperpolarized (HP)-129Xe was improved at typical storage conditions (i.e. low and homogeneous magnetic fields). Very long wall relaxation times T1wall of about 18h were observed in uncoated, spherical GE180 glass cells of ∅=10cm which were free of rubidium and not permanently sealed but attached to a standard glass stopcock. An “aging” process of the wall relaxation was identified by repeating measurements on the same cell. This effect could be easily removed by repeating the initial cleaning procedure. In this way, a constant wall relaxation was ensured. The Xe nuclear spin-relaxation rate 1/T1Xe–Xe due to van der Waals molecules was investigated too, by admixing three different buffer gases (N2, SF6 and CO2). Especially CO2 exhibited an unexpected high efficiency (r) in shortening the lifetime of the Xe–Xe dimers and hence prolonging the total T1 relaxation even further. These measurements also yielded an improved accuracy for the van der Waals relaxation for pure Xe (with 85% 129Xe) of T1Xe–Xe=(4.6±0.1)h. Repeating the measurements with HP 129Xe in natural abundance in mixtures with SF6, a strong dependence of T1Xe–Xe and r on the isotopic enrichment was observed, uncovering a shorter T1Xe–Xe relaxation for the 129Xe in natural composition as compared to the 85% isotopically enriched gas.

Pressure-induced chemistry in a nitrogen-hydrogen host-guest structure.

New topochemistry in simple molecular systems can be explored at high pressure. Here we examine the binary nitrogen/hydrogen system using Raman spectroscopy, synchrotron X-ray diffraction, synchrotron infrared microspectroscopy and visual observation. We find a eutectic-type binary phase diagram with two stable high-pressure van der Waals compounds, which we identify as (N2)6(H2)7 and N2(H2)2. The former represents a new type of van der Waals host-guest compound in which hydrogen molecules are contained within channels in a nitrogen lattice. This compound shows evidence for a gradual, pressure-induced change in bonding from van der Waals to ionic interactions near 50 GPa, forming an amorphous dinitrogen network containing ionized ammonia in a room-temperature analogue of the Haber-Bosch process. Hydrazine is recovered on decompression. The nitrogen-hydrogen system demonstrates the potential for new pressure-driven chemistry in high-pressure structures and the promise of tailoring molecular interactions for materials synthesis.

Pressure-induced metallization of dense (H₂S)₂H₂ with high-Tc superconductivity.

The high pressure structures, metallization, and superconductivity of recently synthesized H2-containing compounds (H2S)2H2 are elucidated by ab initio calculations. The ordered crystal structure with P1 symmetry is determined, supported by the good agreement between theoretical and experimental X-ray diffraction data, equation of states, and Raman spectra. The Cccm structure is favorable with partial hydrogen bond symmetrization above 37 GPa. Upon further compression, H2 molecules disappear and two intriguing metallic structures with R3m and Im-3m symmetries are reconstructive above 111 and 180 GPa, respectively. The predicted metallization pressure is 111 GPa, which is approximately one-third of the currently suggested metallization pressure of bulk molecular hydrogen. Application of the Allen-Dynes-modified McMillan equation for the Im-3m structure yields high Tc values of 191 K to 204 K at 200 GPa, which is among the highest values reported for H2-rich van der Waals compounds and MH3 type hydride thus far.

The , and potentials of NaNe from a combined analysis of optical collision and spectroscopic data

We present new measurements of differential cross sections for the optical collision process for an extended range of optical detunings. A combined analysis of the present scattering results and the earlier NaNe laser-spectroscopic data by Lapatovich et al (1980 J. Chem. Phys. 73 5419) for optical transitions A-X and B-X is performed using a coupled channels approach in both cases. We obtain the Born–Oppenheimer potentials for the ground state and the excited states and as well as the diagonal and off-diagonal spin-orbit functions of the NaNe van der Waals molecule for a wide range of atomic distances.

Tuning the Shear Modulus of Single-Walled Carbon Nanotube by Feeding Van Der Waals Molecules

Torsional buckling of single-walled carbon nanotubes filled with light weight molecular via molecular dynamics is reported. The model accounts for the deformation of CNTs, and interactions among gas molecules; between gas and carbon atoms. The effect of particle loading is predicted to significantly change CNT’s critical torsional moment and stiffness. This is therefore an approach by which the torsional mechanical properties and oscillation frequencies of carbon nanotubes may be tuned. Importantly, the predicted changes in torsional siffness are unique relative to conventional linear elastic materials and are indicative of nonlinear oscillations due to nonlinear mechanical effects. CNTs subjects to large deformations reversibly switch into different morphological patterns. Each shape change corresponds to an abrupt release of energy and a singularity in the stress-strain curve. At higher torsional angle, van der Waals (VDW: He, Ar, H2) molecules reveal a stability effect on carbon nanotubes.

New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system

AbstractThe application of pressure to materials can reveal unexpected chemistry. Under compression, noble gases form stoichiometric van der Waals (vdW) compounds with closed-shell molecules such as hydrogen, leading to a variety of unusual structures. We have synthesised Kr(H2)4 for the first time in a diamond-anvil high-pressure cell at pressures ≥5.3 GPa and characterised its structural and vibrational properties to above 50 GPa. The structure of Kr(H2)4, as solved by single-crystal synchrotron X-ray diffraction, is face-centred cubic (fcc) with krypton atoms forming isolated octahedra at fcc sites. Rotationally disordered H2 molecules occupy four different, interstitial sites, consistent with the observation of four Raman active H2 vibrons. The discovery of Kr(H2)4 expands the range of pressure-stabilised, hydrogen-rich vdW solids, and, in comparison with the two known rare-gas-H2 compounds, Xe(H2)8 and Ar(H2)2, reveals an increasing change in hydrogen molecular packing with increasing rare gas atomic number.

Hyperfine-frequency shifts of alkali-metal atoms during long-range collisions

Collisions with chemically inert atoms or molecules change the hyperfine coupling of an alkali-metal atom through the hyperfine-shift interaction. This interaction is responsible for the pressure shifts of the microwave resonances of alkali-metal atoms in buffer gases, is an important spin interaction in alkali-metal--noble-gas van der Waals molecules, and is anticipated to enable the magnetoassociation of ultracold molecules such as RbSr. An improved estimate is presented for the long-range asymptote of this interaction for Na, K, Rb, and Cs. To test the results, the change in hyperfine coupling due to a static electric field is estimated and reasonable agreement is found.

A Fragile Union Between Li and He Atoms

The LiHe van der Waals molecule has been produced and spectroscopically detected and its binding energy found to only suffice to stabilize a single rotationless vibrational state.

Spectroscopic detection of the LiHe molecule.

We use cryogenic helium buffer-gas cooling to form large densities of lithium atoms in a high-density helium gas, from which LiHe molecules form by three-body recombination. These weakly bound van der Waals molecules are detected spectroscopically, and their binding energy is measured from their equilibrium thermodynamic properties.