Solid HD Research Articles

Studies of nuclear polarization of hydrogen atoms embedded in solid molecular hydrogen and hydrogen deuteride films

We report on an electron-spin resonance study of nuclear polarization of hydrogen atoms embedded in solid H2 and HD films at temperatures 0.1–1.5 K and in a high magnetic field of 4.6 T. Similar to our previous work [Phys. Rev. Lett. 122, 225301 (2019)], we observed a build-up of the spontaneously ( p ≃ 0.35 ) and highly ( p ≃ 0.75 ) nuclear polarized phases of H atoms in the as-deposited H2 films. However, we did not obtain an enhanced nuclear polarization of H atoms in the regions of their small local concentration. We also failed to observe enhanced nuclear polarization for H atoms in the annealed H2 films as well as in the as-deposited HD sample. These observations tend to support our previous explanation for the build-up of high nuclear polarization due to the formation of weakly-bound triplet H2 molecules.We report on an electron-spin resonance study of nuclear polarization of hydrogen atoms embedded in solid H2 and HD films at temperatures 0.1–1.5 K and in a high magnetic field of 4.6 T. Similar to our previous work [Phys. Rev. Lett. 122, 225301 (2019)], we observed a build-up of the spontaneously ( p ≃ 0.35 ) and highly ( p ≃ 0.75 ) nuclear polarized phases of H atoms in the as-deposited H2 films. However, we did not obtain an enhanced nuclear polarization of H atoms in the regions of their small local concentration. We also failed to observe enhanced nuclear polarization for H atoms in the annealed H2 films as well as in the as-deposited HD sample. These observations tend to support our previous explanation for the build-up of high nuclear polarization due to the formation of weakly-bound triplet H2 molecules.

Tunneling chemical exchange reaction D + HD → D2 + H in solid HD and D2 at temperatures below 1 K

We report on a study of the exchange tunneling reaction D + HD → D2 + H in a pure solid HD matrix and in a D2 matrix with a 0.23% HD admixture at temperatures between 130 mK and 1.5 K. We found that the exchange reaction rates, kexHD ∼ 3 × 10-27 cm3 s-1 in the pure HD matrix, and kexD2 = 9(4) × 10-28 cm3 s-1 in the D2 matrix, are nearly independent of temperature within this range. This confirms the quantum tunnelling nature of these reactions, and their ability to proceed at temperatures down to absolute zero. Based on these observations we concluded that exchange tunneling reaction H + H2 → H2 + H should also proceed in a H2 matrix at the lowest temperatures. On the other hand, the recombination of H atoms in solid H2 and D atoms in solid D2 is substantially suppressed at the lowest temperatures as a result of a decreased probability of resonant tunneling of atoms when they approach each other.

Deuterium and hydrogen tunneling in the hydrogenation of 4-oxocyclohexa-2,5-dienylidene.

4-Oxocyclohexa-2,5-dienylidene is a highly reactive triplet ground state carbene that is hydrogenated in solid H2, HD, and D2 at temperatures as low as 3 K. The mechanism of the insertion of the carbene into dihydrogen was investigated by IR and EPR spectroscopy and by kinetic studies. H or D atoms were observed as products of the reaction with H2 and D2, respectively, whereas HD produces exclusively D atoms. The hydrogenation shows a very large kinetic isotope effect and remarkable isotope selectivity, as was expected for a tunneling reaction. The experiments, therefore, provide clear evidence for both hydrogen tunneling and the rare deuterium tunneling in an intermolecular reaction.

Measurements of spin observables in pseudo-scalar meson photo-production using polarized neutrons in solid HD

A measurement of psuedo-scalar meson photo production from longitudinally polarized solid HD has been carried out with the CLAS at Thomas Jefferson National Accelerator Facility (Jlab) with circularly and linearly polarized photon beams. Its aim is to measure a complete set of spin observables for the neutron simultaneously from the same experiment. As a polarized neutron, deutron in HD was used. Preliminary asymmetries are shown for the π− channel.

Influence of the time of interaction of an alloy based on SmCo5 with low-pressure hydrogen on the phase composition

By the methods of differential thermal analysis and X-ray phase analysis, we have investigated the inter- action of an alloy based on SmCO5 with additives of the SmCO 3 phase under initial hydrogen pres- sures of 0.3, 0.4, 0.5, and 0.6 МРа at temperatures of 713 and 913 K with holding for 2 and 5 h. At 713 K, only the compound SmCO 3 interacts with hydrogen with the formation of unidentified phases. In holding at 913 K under a pressure of 0.3 and 0.4 МРа for τ = 2 and 5 h, the alloy partially dispro- portionates into samarium hydride and cobalt. It has been established that the complete disproportiona- tion occurs in holding at pressures of 0.5 and 0.6 МРа and for τ = 2 and 5 h. In vacuum, at 1223 K, disproportionation products recombine in the initial phases. The use of the hydrogenation-disproportionation-desorption-recombination (HDDR) process for obtaining highly coercive magnetic powders based on Nd 2Fe14B and Sm 2Fe17N x alloys gives positive results (1, 2). Its features were investigated in single-phase compounds of the samarium-cobalt system (3-5), and the improve- ment of their magnetic properties was found. Due to the high thermodynamic stability of samarium-cobalt al- loys, hydrogen under a relatively high pressure and milling in it were used. The conditions and phase composi- tion of HDDR products in commercial alloys based on the compounds SmCO5 (6) and Sm 2Co17 (7) were also studied. In particular, it was shown that an alloy based on SmCO5 disproportionates into samarium hydride and cobalt under a pressure PH2 ≈ 3-5 MPa in conventional and solid hydrogenation- disproportionation (solid HD) at ∼ 853 K. The use of hydrogen of such high pressure is dangerous, and, for this reason, it is important to determine the conditions of disproportionation under a lower pressure. It was shown (8) that, in heating of an alloy based on SmCO5 (containing a SmCO 3 impurity phase) in hydrogen at PH2 = 0.3-0.75 MPa, several phase transformations proceed. By X-ray phase analysis (XRPA), it was established that, at a temperature close to room temperature, hydrides form on the base of phases that are alloy components. At 413 K, the transformation of the SmCO 3 impurity phase occurs, and, at 523 K, hydride of the SmCO5 phase decomposes. At 838 K, the alloy partially disproportionates into SmH 2± x and Co, and at temperatures of 1058 and 1188 K, samarium hy- dride, which formed during disproportionation, decomposes. The influence of holding the alloy-hydrogen sys- tem for 0.5-5 h at a temperature above 1173 K on the interaction products was also found. However, the condi- tions of complete disproportionation of the SmCO5 ferromagnetic phase were not determined. With regard for the fact that the SmCO5 phase partially disproportionates under a low pressure at a temperature of ∼ 838 K and that hydrogen-initiated phase transformations can occur within a long period of time (9), the kinetics of the inter- action in the KS37 alloy-hydrogen system (with a hold for 2 and 5 h) was investigated at 713 and 913 K, i.e., up

The spin-orbit transition of atomic chlorine in solid H2, HD, and D2

Essential to understanding the reaction dynamics of spin-orbit (SO) excited atomic chlorine (2P1/2) with molecular hydrogen is experimental measurements of the SO splitting of Cl in the van der Waals region of the entrance channel to reaction. Here we report high-resolution direct absorption studies of the SO transition (2P1/2<--2P3/2) of atomic chlorine isolated in solid molecular hydrogen (H2, HD, and D2).

Anisotropic interactions between HD molecules

The anisotropic interactions between molecules in the heteronuclear hydrogen solids are discussed with stress on their properties of importance for the orientational and translational lattice dynamics. A specific interaction component for homonuclear species, which originates from crystal-field interactions, is suggested and evaluated, in addition to the well-known similar specific component that originates from the isotropic potential due to the shift of the center of mass respective to the charge distribution center. The energy parameters of both specific interaction components have been accurately evaluated and renormalized to account for zero-point translational vibrations. The dispersion laws of the $J=1$ roton excitation in solid HD and DT have been calculated. The bandwidths of the $J=1$ roton excitation are $1.16\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in HD and $0.59\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in DT.

Tunneling chemical reactions D+H2→DH+H and D+DH→D2+H in solid D2–H2 and HD–H2 mixtures: An electron-spin-resonance study

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid HD-H2 and D2-H2 mixtures were studied in the temperature range between 4 and 8 K. These reactions were initiated by UV photolysis of DI molecules doped in these solids for 30 s and followed by measuring the time course of electron-spin-resonance (ESR) intensities of D and H atoms. ESR intensity of D atoms produced by the photolysis decreases but that of H atoms increases with time. Time course of the D and H intensities has the fast and slow processes. The fast process, which finishes within approximately 300 s after the photolysis, is assigned to the reaction of D atom with one of its nearest-neighboring H2 molecules, D(H2)n(HD)(12-n) --> H(H2)(n-1)(HD)(13-n) or D(H2)n(D2)(12-n) --> H(HD)(H2)(n-1)(D2)(12-n) for 12 > or = n > or = 1. Rate constant for the D + H2 reaction between neighboring D atom-H2 molecule pair is determined to be (7.5 +/- 0.7) x 10(-3) s(-1) in solid HD-H2 and (1.3+/-0.3) x 10(-2) s(-1) in D2-H2 at 4.1 K, which is very close to that calculated based on the theory of chemical reaction in gas phase by Hancock et al. [J. Chem. Phys. 91, 3492 (1989)] and Takayanagi and Sato [J. Chem. Phys. 92, 2862 (1990)]. This rate constant was found to be independent of temperature up to 7 K within experimental error of +/-30%. The slow process is assigned to the reaction of D atom produced in a cage fully surrounded by HD or D2 molecules, D(HD)12 or D(D2)12. This D atom undergoes the D + DH reaction with one of its nearest-neighboring HD molecules in solid HD-H2 or diffuses to the neighbor of H2 molecules to allow the D + H2 reaction in solid HD-H2 and D2-H2. The former is the main channel in solid HD-H2 below 6 K where D atoms diffuse very slowly, whereas the latter dominates over the former above 6 K. Rate for the reactions in the slow process is independent of temperature below 6 K but increases with the increase in temperature above 6 K. We found that the increase is due to the increase in hopping rate of D atoms to the neighbor of H2 molecules. Rate constant for the D + DH reaction was found to be independent of temperature up to 7 K as well.

Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium

The solid hydrogen compounds D2, HD and H2 remain quantum molecular solids up to pressures in the 100 GPa range. A remarkable macroscopic consequence is the existence of a pressure-induced broken symmetry phase transition, in which the molecules go from a spherical rotational state to an anisotropic rotational state. Theoretical understanding of the broken symmetry phase structure remains controversial, despite numerous studies. Some open questions concern the existence of long- or short-range orientational order; whether a strong isotopic shift on the transition pressure should be assigned to the nuclear zero-point motion or to quantum localization; and whether the structures are cubic, hexagonal or orthorhombic. Here we present experimental data on the structure of the broken symmetry phase in solid D2, obtained by a combination of neutron and X-ray diffraction up to 60 GPa. Our data are incompatible with orthorhombic structures predicted by recent theoretical works. We find that the broken symmetry phase structure is incommensurate with local orientational order, being similar to that found in metastable cubic para-D2.

Entropy-Driven Reentrant Phase Transitions in Even-J/Odd-J Mixtures of Linear Rotors

Depending on the parity of the rotational quantum number J, solid hydrogens exhibit either pressure-driven broken symmetry phase (BSP) transitions (even-J species – para-hydrogen (p-H2), and ortho-deuterium (o-D2)) or usual order-disorder phase transitions (odd-J species – o-H2 and p-D2) with the phase transition temperature increasing monotonically with pressure from the phase transition point at zero pressure. At the same time, for solid HD the BSP phase transition line displays a minimum, indicating that the disordered phase is reentrant. In this work a model of quantum linear rotors is used to study characteristic features of the P–T phase diagrams for ortho–para mixtures in solid H2 and D2. We developed a mean-field theory of even-J – odd-J mixtures of quantum linear rotors on a 3D lattice. Two limiting cases are considered: mixtures at thermodynamic equilibrium, where the conversion time is small or comparable with the thermalization time, and the opposite case, frozen mixtures, when the conversion time is large compared with other relevant times. We found that for all equilibrium linear rotor systems — the even-J – odd-J mixtures — the reentrant behavior of the phase transition lines is an entropy-driven phenomenon, as was previously found for the all-J system. Experimentally, conditions to find the reentrant BSP transition line are most favorable for H2 mixtures, whereas the frozen monotonic phase lines should be the case for D2 even-J – odd-J mixtures. These results may therefore be particularly useful for understanding the anomalous transition region of the BSP transition documented for D2 mixtures.

A discharge investigation of hydrogen and deuterium atom formation, and parahydrogen and orthodeuterium reconversion.

Hydrogen is flowed through a mild tesla-coil discharge and trapped at 3.8 K: New infrared absorptions of H2 are induced by interaction with trapped H atoms and H- anions. High purity parahydrogen and orthodeuterium samples are 1%-9% reconverted depending on the discharge pressure and recombination of atoms. Annealing the solid samples to 7 K reveals growth in p-H2 induced by o-H2, which shows that H atom recombination produces thermal nuclear spin populations. Similar results are found in discharge experiments with HD and on annealing solid HD. The observed increase in induced HD absorption by J = 1, H2 and D2 molecules formed on recombination gives approximately 1% for the H[D] atom concentration in our solid HD samples.

Infrared Spectra of H2 Molecules Near H Atoms Trapped in Solid H2

Laser-ablation irradiation of normal hydrogen during condensation at 3.8 K produces new IR absorptions in p-H2 and o-H2 cage molecules at 4151.8 and 4143.4 cm-1 that are redshifted 1.0 and 3.3 cm-1, respectively, from induced IR absorptions for each nuclear spin isomer in solid n-H2. Assignment to perturbed p-H2 and o-H2 molecules, respectively, are confirmed by experiments with p-H2-enriched samples. Deuterium experiments give rise to analogous bands for o-D2 and p-D2 at 2985.2 and 2982.4 cm-1, respectively, that are redshifted 1.6 and 2.2 cm-1 from the absorptions induced for each nuclear spin isomer in solid n-D2. However, investigations with solid HD give a single, sharp new line at 3621.7 cm-1, redshifted by 3.0 cm-1 from the sharp induced solid HD absorption. Comparable intensity half-lives recently observed for H atom electron spin resonance signals in solid H2 suggest trapped H atoms as the origin of substantial induced IR intensity in adjacent H2 molecules in the solid. The H atom−H2 molecule interaction is stronger for o-H2 than for p-H2 owing to anisotopic charge distribution for o-H2 that is lacking in p-H2.

Isotope Effects of Solid Hydrogenic Pellet Ablation

The isotope effects of ablation processes in fusion plasma forfive combinations of solid isotopic hydrogenic pellets H2,HD, D2, DT, and T2 have been studied for thefirst time to our knowledge. The results show that the modificationscaused by isotope effects for pellet erosion speeds range from 1 forhydrogen pellet down to 0.487 for tritium pellet and are not negligiblein ablation rate calculations. These effects can lead to deeper massdeposition and improved core fuelling efficiency.

Absorption intensities of the multipole-field-induced double transitions involving a homonuclear-heteronuclear pair of hydrogen molecules in condensed phase

Theoretical expressions are derived for the integrated absorption coefficients of various zero-phonon double transitions involving a homonuclear-heteronuclear pair of hydrogen molecules in condensed phase. The formulas given in this paper can be applied to transitions in the absorption spectra of solid parahydrogen (or orthodeuterium, or solid HD, HT, etc.) matrices that contain a low concentration of impurity molecules which do not possess a permanent dipole moment (e.g., ${\mathrm{N}}_{2}).$

Absorption intensities of the multipole-field-induced zero-phonon transitions in solid HD, HT, and DT

Closed-form theoretical expressions are derived for the integrated absorption coefficient of various zero-phonon single and double transitions in heteronuclear isotopic variants of hydrogen in condensed phase. Theoretical analysis predicts a different kind of double transition in the spectra of solid HD, HT, and DT, where the rotational energy of one molecule changes by three quanta and the rotational energy of another molecule simultaneously changes by at least three quanta. The expressions for double transitions given in this paper may, for example, be applied to double transitions involving para-${\mathrm{H}}_{2}$--HD or ortho-${\mathrm{D}}_{2}$--HD pairs.

Rotation of O2 molecules in solid D2 and HD: An electron spin resonance study

X-band electron spin resonance (ESR) spectroscopy has been applied to the study of molecular rotation of O2 molecules in isotopic solid hydrogen, D2 and HD. ESR signal of the O2 molecules in hindered rotational states has been observed, and its pressure dependence has been measured up to 19 MPa. Although molar volume of solid hydrogen decreases, the O2 rotation has become less hindered with the increase in pressure and isotope substitution from HD to D2. These effects of pressure and isotope substitution suggest that the potential barrier for the O2 rotation is mainly produced by distortion of O2 cages in D2 and HD. Since solid hydrogen becomes less compressible with the increase in pressure and the isotope substitution from HD to D2, the O2 rotation becomes less hindered in the less distorted O2 cages.

Sputtering of Isotopes of the Solid Hydrogens by Ions

Sputtering of the solid hydrogens by light ions has shown isotope effects which are greater than for any other solid targets. These hydrogenic solids are unique because of the extreme volatility and because the first step of the electronic sputtering process is identical for all hydrogenic solids. The sputtering for protons in the energy range from 5 to 10 keV can be qualitatively described by an electronic spike of cylindrical geometry. The sputtering yield of solid tritium has been evaluated on the basis of results for solid H2, D2 and HD.

Rovibrational matrix elements of multipole moments of the HD, HT and DT molecules

Rovibrational matrix elements of the multipole moments Q ℓ of HD, HT and DT for ranks 2 ≤ ℓ ≤ 11 have been computed. Since the present calculations have been performed with the nuclear centre of mass (rather than the geometric centre) as the origin, the computations had to include even as well as odd values of ℓ. The results are used to correlate the absorption intensities of the zero-phonon single transitions in solid HD to theory.

Measurement of the resonant d(mu)t molecular formation rate in solid HD.

Measurements of muon-catalyzed dt fusion ( d(mu)t-->4He + n + mu(-)) in solid HD have been performed. The theory describing the energy dependent resonant molecular formation rate for the reaction (mu)t + HD-->[(d(mu)t)pee](*) is compared to experimental results in a pure solid HD target. Constraints on the rates are inferred through the use of a Monte Carlo model developed specifically for the experiment. From the time-of-flight analysis of fusion events in 16 and 37 microg x cm(-2) targets, an average formation rate consistent with 0.897+/-(0.046)(stat)+/-(0.166)(syst) times the theoretical prediction was obtained.

Controlling factors of tunneling reactions in solid hydrogen at very low temperature

The recent studies on tunneling reactions of our group are auto-reviewed. The local structure around reactants, the new temperature effect, and the impurity effect are pointed out as important controlling factors of tunneling reactions in the solid phase. The distances between H(D) atoms and H 2(HD, D 2) molecules in solid hydrogen and solid argon were estimated by ESR, electron nuclear double resonance (ENDOR), and electron spin echo (ESE). The new temperature effects on tunneling reaction were observed in a reaction D+HD→D 2+H in solid HD. A mechanism of a vacancy-assisted tunneling reaction has been proposed to account for the temperature effect. The strange temperature dependence of a tunneling electron-transfer-reaction H 2 −+H 2→H 2+H 2 − was explained in terms of the phonon-scattering effect and the impurity effect on the tunneling reaction. The rate constant for a tunneling reaction H+ p-H 2→ p-H 2+H in solid para -H 2 (p -H 2) decreases with the increase in the concentration of ortho-H 2 ( o-H 2). The results were explained by the model that the orientational defects by o-H 2 molecules affect the tunneling reaction H+ p-H 2. A tunneling reaction at very low temperature gives a surprising example in control of a reaction that a small amount of energy as such 2 cal mol −1 can affect the rate of a reaction. The tunneling reaction in the solid phase, which can be considered as a multidimensional tunneling phenomenon, is affected significantly by the condition surrounding reactants.