Ortho-para Transition Research Articles

The ortho-para transition, confinement and self-diffusion of H2 in three distinct carbide-derived carbons by quasi- and inelastic neutron scattering

Microporous carbon materials are promising for hydrogen storage due to their structural variety, high specific surface area, large pore volume and relatively low cost. Carbide-derived carbons are highly valued as model materials because their porous structure is fine-tuned through the choice of the precursor carbide and the synthesis route. This study investigates H2 adsorption in three carbide derived carbons with well-defined pores and pore size distributions with quasi- and inelastic neutron scattering methods. Concerning previous studies, a wider neutron energy transfer window is investigated, and a detailed quantitative evaluation of the graphitic structure is presented. The graphitic structure of the carbon is shown to influence the speed of the ortho-to-para transition of H2. Namely, the ortho-para transition was the slowest in carbon derived from TiC, which also had the smallest average stacking size of graphene layers. The possibility to inhibit the ortho-para transition in cryo-adsorption devices is sought after to mitigate the evaporation of H2 during storage. In addition, the self-diffusion of H2 in different timescales is detected in carbon derived from Mo2C, demonstrating the usefulness of obtaining data in a wide energy window.

Detection of Ortho-Para Transition in Molecules

Detection of Ortho-Para Transition in Molecules

Millimeter-Wave Spectroscopy of S2Cl2: A Candidate Molecule for Measuring Ortho–Para Transition

S2Cl2 is a candidate for the observation of ortho-para transition. To estimate the ortho-para mixing in a hyperfine-resolved rotational state, pure rotational transitions were measured by millimeter-wave (mm-wave) spectroscopy using two different experimental set-ups. The transitions from the term value around 20 K was measured with a supersonic jet and those around 200 K were measured with a dry ice cooled gas cell. Several hundred peaks were assigned for the naturally abundant S2(35)Cl2 and S2(35)Cl(37)Cl isotopic species and the rotational molecular parameters including the fourth-order and sixth-order centrifugal distortion constants were determined. The hyperfine structures were partly resolved in some Q-branch transitions, which were well described with the hyperfine constants determined by FTMW spectroscopy in the centimeter-wave region. With the new rotational constants determined in our study and the previous hyperfine constants, it will be possible to obtain a more reliable ortho-para mixing ratio and to narrow down the possible candidate transitions in the mm-wave region for the observation of ortho-para transition.

Ortho-para transition of interstitialH2andD2in Si

The ortho-para transition of interstitial ${\text{H}}_{2}$ in Si has been observed in a recent Raman study [M. Hiller et al., Phys. Rev. Lett. 98, 055504 (2007); M. Hiller et al., Phys. Rev. Lett. 99, 209901 (2007)]. In order to address issues that are difficult to study by Raman spectroscopy, we have performed IR-absorption experiments to further investigate this ortho-para transition. We find that when a Si sample containing ${\text{H}}_{2}$ that has been equilibrated at room temperature is subsequently held at 77 K, the $3618.4\text{ }{\text{cm}}^{\ensuremath{-}1}$ IR line assigned to ortho-${\text{H}}_{2}$ is reduced in intensity with a single exponential time constant of 229 h as the system relaxes to the para-${\text{H}}_{2}$ state. The previous Raman data had been analyzed by a model in which the ortho-para transition is caused by an interaction with the nuclear magnetic moment of $^{29}\text{S}\text{i}$ (4.7% abundant) that is present in natural Si. However, in order for every ${\text{H}}_{2}$ molecule to interact with a $^{29}\text{S}\text{i}$, at least six shells of Si neighbors must be considered. Because of the strong dependence of the transition rate on the $^{29}\text{S}{\text{i-H}}_{2}$ distance, the kinetics of such a transition should be highly nonexponential. Thus, our IR data are not consistent with this mechanism. We have also investigated the para-ortho transition of interstitial ${\text{D}}_{2}$ and have found decay rates similar to those for the ortho-para transition of ${\text{H}}_{2}$. We have considered two possible mechanisms for this transition: ${\text{H}}_{2}$ or ${\text{D}}_{2}$ tunneling, which would allow the molecule to sample tetrahedral sites adjacent to $^{29}\text{S}\text{i}$, and spin-rotation coupling that can act while the molecule is dynamically off-center. Consideration of a possible tunneling rate by theory and the similarity of the ${\text{H}}_{2}$ and ${\text{D}}_{2}$ results seem to rule out the tunneling mechanism but not the spin-rotation mechanism. These results clearly show that the mechanism of the ortho-para transition for ${\text{H}}_{2}$ and ${\text{D}}_{2}$ in Si remains an experimental and a theoretical challenge.

Ortho-para transition in molecular hydrogen

The radiative ortho-para transition in molecular hydrogen is studied. This highly forbidden transition is very sensitive to relativistic and subtle nonadiabatic effects. Our result for the transition rate in the ground vibrational level $\ensuremath{\Gamma}(J=1\ensuremath{\rightarrow}J=0)=6.20(62)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{yr}}^{\ensuremath{-}1}$ is significantly lower in comparison to all the previous approximate calculations. Experimental detection of such a weak line by observation of, for example, cold interstellar molecular hydrogen is at present unlikely.

Direct observation and modelling of ordered hydrogen adsorption and catalyzed ortho–para conversion on ETS-10 titanosilicate material

Hydrogen physisorption on porous high surface materials is investigated for the purpose of hydrogen storage and hydrogen separation, because of its simplicity and intrinsic reversibility. For these purposes, the understanding of the binding of dihydrogen to materials, of the structure of the adsorbed phase and of the ortho-para conversion during thermal and pressure cycles are crucial for the development of new hydrogen adsorbents. We report the direct observation by IR spectroscopic methods of structured hydrogen adsorption on a porous titanosilicate (ETS-10), with resolution of the kinetics of the ortho-para transition, and an interpretation of the structure of the adsorbed phase based on classical atomistic simulations. Distinct infrared signals of o- and p-H2 in different adsorbed states are measured, and the conversion of o- to p-H2 is monitored over a timescale of hours, indicating the presence of a catalyzed reaction. Hydrogen adsorption occurs in three different regimes characterized by well separated IR manifestations: at low pressures ordered 1:1 adducts with Na and K ions exposed in the channels of the material are formed, which gradually convert into ordered 2:1 adducts. Further addition of H2 occurs only through the formation of a disordered condensed phase. The binding enthalpy of the Na+-H2 1:1 adduct is of -8.7+/-0.1 kJ mol(-1), as measured spectroscopically. Modeling of the weak interaction of H2 with the materials requires an accurate force field with a precise description of both dispersion and electrostatics. A novel three body force field for molecular hydrogen is presented, based on the fitting of an accurate PES for the H2-H2 interaction to the experimental dipole polarizability and quadrupole moment. Molecular mechanics simulations of hydrogen adsorption at different coverages confirm the three regimes of adsorption and the structure of the adsorbed phase.

Ortho-Para Transition Rate inμ-Molecular Hydrogen and the Proton’s Induced Pseudoscalar Couplinggp

We report a measurement of the ortho-para transition rate in the p mu p molecule. The experiment was conducted at TRIUMF via the measurement of the time dependence of the 5.2 MeV neutrons from muon capture in liquid hydrogen. The measurement yielded an ortho-para rate Lambda op = (11.1 +/- 1.7 +/-(0.9)(0.6)) x 10(4) s(-1), which is substantially larger than the earlier result of Bardin et al. The result has striking implications for the proton's induced pseudoscalar coupling g(p), changing the value of g(p) obtained from the most precise ordinary muon capture measurement from 10.6 +/- 2.7 to 0.8 +/- 2.8, and from the sole radiative muon capture measurement from 12.2 +/- 1.1 to 10.6 +/- 1.2, bringing the latter result closer to theoretical predictions.

Mathematical modeling of the process of isothermalortho-para conversion

Production of liquid hydrogen includes a stage of ortho-para conversion (i.e., obtaining the para form equilibrated to the saturation temperature), without which prolonged storage of liquid hydrogen is not possible. Obviously, it is economically expedient to withdraw some of the heat of the ortho-para transition at the highest temperature level using the liquid nitrogen cooling loop traditionally used in hydrogen equipment. According to current terminology, equipment for carrying out ortho-para conversion with cooling by a boiling cryogenic agent are called isothermal converters. This does not reflect the essence of the process since it actually occurs at a variable temperature, despite the fact that the initial and final temperatures of the process are equal in some cases. Characteristics of the process of isothermal ortho-para conversion have been studied for the following assumptions: the reaction occurs in the kinetic region, the rate constant for the chemical reaction does not depend on temperature, the catalyst is a homogeneous irregulate structure, the apparatus is an ideal displacement reactor, the thermal resistance of the walls of the reactor tubes is negligibly small, and there is no economizer zone.

Cross sections for scattering of deuterium mesic atoms on deuterium nuclei

Cross sections have been calculated for elastic scattering of deuterium mesic atoms in ortho and para states on deuterium nuclei in the 0–50 eV energy range; cross sections for spin-flip have also been calculated. The calculations have been performed in the adiabatic representation where the initial problem of slow collisions in a three-body system is reduced to the multichannel scattering problem. The temperature dependence of the ortho-para transition rate λ hf( T) for dμ atoms is obtained for a deuterium target. The calculated rate 37.2×10 6 s -1 (at 30 K) is in good agreement with the recent experimental value, λ exp(30 K) = (37.0±0.74)×10 6 s -1 (Vienna-SIN group, 1983).

Inelastic neutron scattering of H2O isolated in solid argon

Abstract Samples of H2O suspended in Ar were prepared which permitted us to record inelastic incoherent neutron scattering spectra of matrix-isolated water. In the energy loss spectrum, five peaks were found below 45 cm−1, including a strong ortho-para transition. Earlier assignments of free monomer rotations observed by far-IR spectroscopy could be confirmed. In the range 45 to 90 cm−1, only very weak intensity is found. The respective band positions do not agree with earlier far-IR data.

Measurement of the ortho-para transition rate in the pμp molecule and deduction of the pseudoscalar coupling constant gpμ

The ortho-para transition rate in the pμp molecule has been found experimentally to be λ OP = (4.1 ± 1.4) × 10 4 s −1. Our recent result for the muon capture rate in liquid hydrogen can now be interpreted to extract the ortho-molecular capture rate: λ OM = (531 ± 33) s −1. A deduction of the pseudoscalar coupling constant g p μ is presented: we find g p μ = 8.7 ± 1.9.

Hydrogen Storage and Transmission

Three basic methods for the storage and the transmission of hydro-gen are discussed in this review. These forms are gaseous hydrogen, liquid hydrogen and metal hydrides. Gaseous storage and transmission have not received much attention because of safety problems due to the high-pressure containers and the container weight penalty. Liquid storage and transmission have also the various penalties, that is to say, liquefication energy, boiling off, ortho-para transition etc. But this lipuid hydrogen storage method shall be most applicable on a large scale in future. The methods to recover some of the liquefication energy are discussed. The use of metal hydrides as hydrogen storage and transmission media has been received increasing attention over the past several years. The recent studies for the hydrides of Mg (Ni), AB5 type alloys and TiFe and the other promising hydrides are reviewed. The storage abilities of the various hydrogen absorbing alloys are sensitive to the simplicity of theis phases and affected by the impurities and activation procedures. The application and demonstration studies are described concerning with off-peak energy storage, metal hydride containers and vehicles.

Self-induced emission of electrons from the surface of solid hydrogen due to the ortho—para transition

Electrons bound on the surface of solid hydrogen catalyze the ortho—para transition. Under certain conditions the energy of the o—p transition is greater than that binding the electron to the surface. As a result, the electron may be ejected from the surface by the transition that it has initiated. We have calculated the transition rate for the self-induced electron emission and determined the conditions under which it could be observed.

Ortho-Para Transition in Molecular Hydrogen.

view Abstract Citations (26) References (2) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ortho-Para Transition in Molecular Hydrogen. Raich, J. C. ; Good, R. H., Jr. Abstract An estimate of the probability of spontaneous transitions between rotational levels j H J - 1 the ground electronic and vibrational states of the hydrogen molecule has been made. The probabili per unit time was found to be about Jo t (2I+1)(2J+1½ ) year where I is the initial nuclear spin. Publication: The Astrophysical Journal Pub Date: April 1964 DOI: 10.1086/147835 Bibcode: 1964ApJ...139.1004R full text sources ADS |

On the theory of the catalysis of the ortho-para transformation by paramagnetic gases

According to quantum statistics, there exists for a diatomic molecule containing similar atoms an important difference between states with even and uneven rotational quantum numbers j . These two kinds of states have different statistical weights in the ratio of (S + 1)/S or S/(S + 1) where S is the nuclear spin. Whether the states of larger statistical weight possess even or uneven values of j depends on the symmetry properties of the electronic configuration and on the nuclear statistics. The states with the larger weight are called the ortho-states, those with the smaller weight the para-states. Under normal conditions, the transition probability between ortho-and para-states is zero. There are, however, two ways of inducing an ortho-para transformation. The one consists in dissociating the molecules, after which the atoms will recombine at random. The other possibility is to introduce a perturbation which depends both on the spins and on the positions of the nuclei. Under such conditions a transition may take place for the following reason: the unperturbed eigenfunction can be written as a product of two factors Ψ = ψ ( q 1 , q 2 ) ψ (s 1 , s 2 ) depending respectively on the space-co-ordinates q and the spin-vectors s of the two nuclei. Now, in order that an ortho-para transition may take place, it is necessary that the symmetry properties with respect to a permutation of the nuclei shall be changed in both factors. A simultaneous transition in ψ( q 1 , q 2 ) and ψ(s 1 , s 2 ) can, however, only occur if the perturbation contains a term depending both on q and s. It is due to a perturbation of this kind that the ortho-para hydrogen transformation is catalysed by paramagnetic gases as was found by Farkas and Sachsse. The interaction between the magnetic field of the paramagnetic molecules and the magnetic moment of the H-nuclei depends on the nuclear spin and, as the field is inhomogeneous, it also involves the nuclear positions. The theory of this catalysis was given by Wigner.