Solid Neon Research Articles

Electrons Trapped in Solid Neon\u2013Hydrogen Mixtures Below 1, hbox {K}

We report on an electron spin resonance study of electrons stabilized in solid films of neon–hydrogen mixtures. We found that these films are highly porous and may absorb large amount of liquid helium. We observed that free electrons can be stabilized in two different positions: in a pure neon environment and in hbox {H}_{2} clusters formed in the pores of solid neon. It turned out that the presence of the superfluid helium film suppresses the escape of the trapped electrons via diffusion through the pores and stimulates their accumulation in the hbox {H}_{2} clusters even in Ne samples of the best available purity. We propose several possible explanations for this behavior.

First vibrational investigations of N2O–H2O, N2O–(H2O)2, and (N2O)2–H2O complexes from the far to the near-infrared spectral region by neon matrix isolation and ab initio calculations

We present for the first time the investigation of water molecules complexed with dinitrogen monoxide, two abundant molecules in atmosphere, in solid neon using Fourier transform infrared (IR) spectroscopy. We identify at least three complexes from concentration effects, N2O-H2O, N2O-(H2O)2, and (N2O)2-H2O, by observation of new absorption bands close to the monomer fundamental modes from the far to the near IR region. We highlight the presence of isomers for the N2O-H2O complex with the help of theoretical calculations at second order Møller-Plesset (MP2) and coupled-cluster single double triple-F12a/aug-cc-pVTZ levels. The observed frequencies for the N2O-(H2O)2 and (N2O)2-H2O complexes are compared with MP2/aug-cc-pVTZ harmonic data. Anharmonic coupling constants have been derived from the observations of overtones and combination bands.

Far-UV photoluminescence of boron-doped diamond: Cross interaction between boron and diamond

Photoluminescence (PL) spectra of boron-doped diamonds (BDD) were recorded with far-ultraviolet excitation. Upon 175 nm excitation, emissions of two types were observed; a broad emission band A, was recorded about 560 nm, whereas three distinct PL lines were detected at 194.0, 231.5 and 281.0 nm for the first time. The photoluminescence excitation spectra (PLE) of BDD were measured also on monitoring PL lines at 281 and 560 nm; their PLE spectra appear dissimilar. Compared to the absorption spectrum of CVD diamond, we conclude that the energy absorbed by the host diamond became transferred to the dopant boron atom, which subsequently radiated emission band A. According to the emission spectra of atomic boron from photolysis at 180 nm of diborane(6) dispersed in solid neon at 4 K, the PL lines at 194.0, 231.5 and 281.0 nm are due to electronic transitions of boron atoms in the BDD lattice.

Electron spin resonance study of atomic hydrogen stabilized in solid neon below 1 K

The freezing temperature of solid hydrogen may be significantly lowered below its bulk value by encapsulating the sample in a restricted geometry. The authors report on an electron-spin resonance (ESR) study of H atoms, stabilized in highly porous solid mixtures of neon and molecular hydrogen at temperatures below 1 K. The ESR analysis shows that H atoms stabilize into two different substitutional sites of the Ne lattice and into H${}_{2}$ clusters, formed in solid neon. The hydrogen atoms in H${}_{2}$ clusters rapidly recombine upon raising the temperature from 0.1 to 0.3--0.6 K, which is attributed to a solid-liquid transition in the H${}_{2}$ clusters formed in solid Ne.

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 → 3P1,2 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.

Thresholds of photolysis of O2 and of formation of O3 from O2 dispersed in solid neon.

Irradiation of O2 dispersed in solid Ne with ultraviolet light produced infrared absorption lines of O3 and emission lines from atomic O (1D2 → 3P1,2), molecular O2 (A' 3Δu → X 3Σg) and radical OH (A 2Σ+ → X 2ΠI) in the visible and near-ultraviolet regions. The threshold wavelength for the formation of O3 was determined to be 200 ± 4 nm, corresponding to energy 6.20 ± 0.12 eV, which is hence the threshold for dissociation of O2. The thresholds of emission from excited O (1D2), O2 (A' 3Δu) and OH (A 2Σ+) were all observed to be 200 ± 4 nm, the same as for the formation of O3 in this photochemical system. The results indicate that, once O3 was generated, it was readily photolyzed to produce the long-lived atom O (1D2). Further reactions of O (1D2) with O3 produced excited O2 (A' 3Δu); reaction with water yielded radical OH (A 2Σ+). These results enhance our understanding of the evolution of the transformation of oxygen and open a window for the understanding of complicated processes in the solid phase.

Photodissociation threshold and emission with 220 nm of icy ethene

The depletion of ethene, C2H4, dispersed in solid neon was measured upon irradiation with ultraviolet light from a synchrotron source. The threshold for the dissociation of solid ethene is wavelength 210 ± 5 nm, corresponding to energy 5.90 ± 0.13 eV. We recorded emission spectra for the transition T → N of ethene dispersed in solid neon, dinitrogen and water, excited at 220 nm; the band origins are 286.5, 289.8 and 290.0 nm, respectively. This investigation of the depletion of icy ethene with UV light enhances our understanding of its photochemistry in astronomical environments. This emission feature of solid ethene can provide a new means of identification for its observation in space.

Monte Carlo simulation of relaxation processes in solid disordered neon under irradiation with photons in the energy range of 4–400 Ry. Role of the cascade decay relaxation processes

The processes in disordered solid neon irradiated with photons in the energy range of 4–400 Ry are studied using detailed Monte Carlo simulations taking into account cascade relaxation of ionized atoms. Trajectories of all secondary electrons and photons inside a spherical zone around the site of initial photoionization are analyzed. Energy absorbed in the zone and its distribution within the zone, distributions of concentrations of secondary inelastic processes, spectra of electrons and photons, and mean numbers of final ions are calculated. Secondary electrons, including those produced by cascade relaxation, are found to be the principal producers of inelastic processes. Cascade relaxation of ionized atoms is shown to be one of the crucial mechanisms in the effect of ionizing radiation on matter.

Photoassisted Homocoupling of Methyl Iodide Mediated by Atomic Gold in Low-Temperature Neon Matrix.

Infrared spectroscopy and density functional theory calculations showed that the gold complexes [CH3-Au-I] and [(CH3)2-Au-I2], in which one and two CH3I molecule(s), respectively, are oxidatively adsorbed on the Au atoms, are formed in a solid neon matrix via reactions between laser-ablated gold atoms and CH3I. Global reaction route mapping calculations revealed that the heights of the activation barriers for the sequential oxidative additions to produce [CH3-Au-I] and [(CH3)2-Au-I2] are 0.53 and 1.00 eV, respectively, suggesting that the reactions proceed via electronically excited states. The reductive elimination of ethane (C2H6) from [(CH3)2-Au-I2] leaving AuI2 was hindered by an activation barrier as high as 1.22 eV but was induced by visible-light irradiation on [(CH3)2-Au-I2]. These results demonstrate that photoassisted homocoupling of CH3I is mediated by Au atoms via [(CH3)2-Au-I2] as an intermediate.

First-principles study of conducting behavior of warm dense neon

The energy gap of solid neon increases with density, which is an opposite density dependency compared to other noble gases. In order to investigate whether this abnormal phenomenon survives in the warm dense region, where the conducting behavior is closely related to the energy gap, we calculated the electrical conductivity of fluid neon for temperatures of 103–105 K and densities of 1.50–10.0 g/cm3 with a first-principles method. Temperature and density dependencies of conductivity in this region were analyzed. The results indicate that the conducting behavior is sensitive to the temperature; there is a significant increase in the direct current (dc) conductivity from 10 000 to 20 000 K. Contrary to other noble gases, we found an abnormal density dependency of dc conductivity, which decreases with increasing density at a given temperature. This phenomenon is due to the elevating localization of electrons and the broadening of the energy gap based on the analyses of charge density distribution and electronic structure under these extreme conditions. Finally, an insulating-conducting fluid phase diagram was constructed using our simulation results, which confirmed the conclusion of the latest experiment results.

Probing the global potential energy minimum of (CH2O)2: THz absorption spectrum of (CH2O)2 in solid neon and para-hydrogen.

The true global potential energy minimum configuration of the formaldehyde dimer (CH2O)2, including the presence of a single or a double weak intermolecular CH⋯O hydrogen bond motif, has been a long-standing subject among both experimentalists and theoreticians as two different energy minima conformations of Cs and C2h symmetry have almost identical energies. The present work demonstrates how the class of large-amplitude hydrogen bond vibrational motion probed in the THz region provides excellent direct spectroscopic observables for these weak intermolecular CH⋯O hydrogen bond motifs. The combination of concentration dependency measurements, observed isotopic spectral shifts associated with H/D substitutions and dedicated annealing procedures, enables the unambiguous assignment of three large-amplitude infrared active hydrogen bond vibrational modes for the non-planar Cs configuration of (CH2O)2 embedded in cryogenic neon and enriched para-hydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 ± 0.3 kJ mol-1 is proposed for the dimerization process. These THz spectroscopic observations are complemented by CCSD(T)-F12/aug-cc-pV5Z (electronic energies) and MP2/aug-cc-pVQZ (force fields) electronic structure calculations yielding a (semi)-empirical value of 13.7 ± 0.3 kJ mol-1 for the dissociation energy D0 of this global potential energy minimum.

Relativistic coupled-cluster and density-functional studies of argon at high pressure

The equation of state $P(V,T)$ for solid argon is determined by the calculation of accurate static and vibrational terms in the free energy. The static component comes from a quantum theoretical many-body expansion summing over all energetic contributions from two-, three-, and four-body fragments treated with relativistic coupled cluster theory, while the lattice vibrations are described at an anharmonic level including two- and three-body forces as well as temperature effects. The dynamic part is calculated within the Debye and Einstein approximation, as well as by a more accurate phonon treatment where the vibrational motions in the lattice are coupled. Our results are in good agreement with room-temperature high-pressure experimental data up to $\ensuremath{\sim}20$ GPa. In the 20--100 GPa pressure range, however, we see considerable deviations between experiment and theory, perhaps indicating missing four-body contributions (beyond the quadruple dipole terms included here), missing five and higher-body effects, and the need to go beyond the coupled cluster singles-doubles with perturbative triples treatment in such higher-body forces. This contrasts with the results for solid neon, where excellent agreement has been achieved taking only two- and three-body forces into account [P. Schwerdtfeger and A. Hermann, Phys. Rev. B 80, 064106 (2009)]. We demonstrate that the phase transition from fcc to hcp cannot account for the large discrepancies observed. Density functional calculations give very mixed results in the high-pressure region, but some functionals such as optB88-vdW (proposed by Lundqvist and co-workers) describe the many-body forces in argon reasonably well over the range of pressures investigated. Theoretical investigations of the heavier rare gas solids reaching experimental accuracy in the high-pressure regime therefore remain a significant challenge.

First infrared investigations of OCS-H2O, OCS-(H2O)2, and (OCS)2-H2O complexes isolated in solid neon: Highlighting the presence of two isomers for OCS-H2O.

For the first time, complexes involving carbonyl sulfide (OCS) and water molecules are studied by FTIR in solid neon. Many new absorption bands close to the known fundamental modes for the monomers give evidence for at least three (OCS)n-(H2O)m complexes, noted n:m. With the help of theoretical calculations, two isomers of the 1:1 complex are clearly identified. Concentration effects combined with a detailed vibrational analysis allow for the identification of transitions for the 1:1, 1:2, and 2:1 complexes. Anharmonic coupling constants have been derived from the observations of overtones and combinations.

Identification of cyc-B3H3 with Three Bridging B–H–B Bonds in a Six-Membered Ring

Irradiation of samples of diborane(6), B2H6 and B2D6, separately and together, dispersed in solid neon near 4 K with tunable far-ultraviolet light from a synchrotron yielded new infrared absorption lines that are assigned to several carriers. Besides H, B, BH, BH2, BH3, B2, B2H2, and B2H4, previously identified, a further species is assigned on the basis of quantum-chemical calculations of vibrational wavenumbers and intensities to be cyc-B3H3 (D3h, singlet state) in several isotopic variants, which feature three bridging B–H–B bonds in a six-membered ring.

Monte Carlo modeling and optimization of buffer gas positron traps

Buffer gas positron traps have been used for over two decades as the prime source of slow positrons enabling a wide range of experiments. While their performance has been well understood through empirical studies, no theoretical attempt has been made to quantitatively describe their operation. In this paper we apply standard models as developed for physics of low temperature collision dominated plasmas, or physics of swarms to model basic performance and principles of operation of gas filled positron traps. The Monte Carlo model is equipped with the best available set of cross sections that were mostly derived experimentally by using the same type of traps that are being studied. Our model represents in realistic geometry and fields the development of the positron ensemble from the initial beam provided by the solid neon moderator through voltage drops between the stages of the trap and through different pressures of the buffer gas. The first two stages employ excitation of N2 with acceleration of the order of 10 eV so that the trap operates under conditions when excitation of the nitrogen reduces the energy of the initial beam to trap the positrons without giving them a chance to become annihilated following positronium formation. The energy distribution function develops from the assumed distribution leaving the moderator, it is accelerated by the voltage drops and forms beams at several distinct energies. In final stages the low energy loss collisions (vibrational excitation of CF4 and rotational excitation of N2) control the approach of the distribution function to a Maxwellian at room temperature but multiple non-Maxwellian groups persist throughout most of the thermalization. Optimization of the efficiency of the trap may be achieved by changing the pressure and voltage drops and also by selecting to operate in a two stage mode. The model allows quantitative comparisons and test of optimization as well as development of other properties.

Double C-H bond activation of acetylene by atomic boron in forming aromatic cyclic-HBC2BH in solid neon.

The organo-boron species formed from the reactions of boron atoms with acetylene in solid neon are investigated using matrix isolation infrared spectroscopy with isotopic substitutions as well as quantum chemical calculations. Besides the previously reported single C-H bond activation species, a cyclic-HBC2BH diboron species is formed via double C-H bond activation of acetylene. It is characterized to have a closed-shell singlet ground state with planar D2h symmetry. Bonding analysis indicates that it is a doubly aromatic species involving two delocalized σ electrons and two delocalized π electrons. This finding reveals the very first example of double C-H bond activation of acetylene in forming new organo-boron compounds.

Conformational Landscape of the 1/1 Diacetyl/Water Complex Investigated by Infrared Spectroscopy and ab Initio Calculations.

The complexes of diacetyl with water have been studied experimentally by Fourier transform infrared (FTIR) spectroscopy coupled to solid neon matrix and supersonic jet, and anharmonic ab initio calculations. The vibrational analysis of neon matrix spectra over the 100-7500 cm-1 infrared range confirms the existence of two nearly isoenergetic one-to-one (1/1) diacetyl-water S1 and S2 isomers already evidenced in a previous argon matrix study. A third form (S3) predicted slightly less stable ( J. Mol. Mod. 2015 , 21 , 214 ) is not observed. The correct agreement obtained between neon matrix and anharmonic calculated vibrational frequencies is exploited in several cases to derive band assignments for the vibrational modes of a specific isomer. Thereafter, theoretical xij anharmonic coupling constants are used for the attribution of combination bands and overtones relative to the 1/1 dimer. Finally, the most stable isomer of the one-to-two (1/2) diacetyl-water complex is identified in the OH stretching region of water on the grounds of comparison of experimental and calculated vibrational shifts between water dimer and the three most stable 1/2 isomers.

Ultraviolet and Infrared Spectra of Diboron in Solid Neon at 4 K

Apart from products H, B, BH, BH2 and BH3 identified from their emission spectra in the UV/Vis region, photolysis of diborane(6) dispersed in solid neon at 4 K with far-ultraviolet light from a synchrotron led to observation of absorption line (0,0) of the electronic transition A 3 Σu- ←X 3 Σg- of B2 at 326.39 nm. Absorption lines (1,0) of 11 B2 , 11 B10 B and 10 B2 were recorded at 316.63, 316.40 and 316.15 nm, respectively. ΔG1/2 of state A 3 Σu- for 11 B2 , 11 B10 B and 10 B2 in solid neon are accordingly derived to be 945, 968 and 993 cm-1 , respectively. Weak lines (0,1) of 11 B2 at 29586 cm-1 and of 11 B10 B at 29560 cm-1 , corresponding to 1042±30 and 1068±30 cm-1 for vibrational modes in the electronic ground state, were recorded in emission. An absorption line recorded at 1066.5±0.5 cm-1 in infrared spectra after photolysis of either B2 H6 in Ne or B2 D6 with D2 in Ne is thus attributed to 11 B10 B.

Longitudinal Spin Relaxation of Optically Pumped Rubidium Atoms in Solid Parahydrogen

We have grown crystals of solid parahydrogen using a single closed-cycle cryostat. We have doped the crystals with rubidium atoms at densities on the order of 10^{17} cm^{-3} and used optical pumping to polarize the spin state of the implanted atoms. The optical spectrum of the rubidium atoms shows larger broadening than previous work in which the rubidium was implanted in solid argon or neon. However, the optical pumping behavior is significantly improved, with both a larger optical pumping signal and a longer longitudinal relaxation time. The spin relaxation time shows a strong dependence on orthohydrogen impurity levels in the crystal, as well as the applied magnetic field. Current performance is comparable to state-of-the-art solid state systems at comparable spin densities, with potential for improvement at higher parahydrogen purities.

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6.

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.