Ortho-para Conversion Research Articles

Effects of coadsorbedO2on hydrogen ortho-para conversion on Ag surfaces

Ortho-para (o-p) conversion of ${\text{H}}_{2}$ and para-ortho (p-o) conversion of ${\text{D}}_{2}$ were investigated on a Ag surface coadsorbed with ${\text{O}}_{2}$ by resonance-enhanced multiphoton ionization combined with photostimulated desorption. Compared with a bare Ag surface, both ${\text{H}}_{2}$ o-p conversion and ${\text{D}}_{2}$ p-o conversion were accelerated on ${\text{O}}_{2}$-covered surfaces, and the conversion time was found to decrease with increasing ${\text{O}}_{2}$ coverage. In order to analyze the conversion kinetics, Monte Carlo simulations taking account of hydrogen diffusion on the surfaces were performed and compared with the experimental data. The conversion time of ${\text{H}}_{2}$ and ${\text{D}}_{2}$ in the vicinity of adsorbed ${\text{O}}_{2}$ was estimated to be 8.3 and 53.4 s, respectively. The isotope dependence of the conversion is discussed.

The effect of n-H2 impurity on the heat capacity of solid Ne

The alloy Ne-5.1% n-H2 is investigated calorimetrically under equilibrium vapor pressure in an interval of 0.9–25K. The magnitude and temperature dependence of the heat capacity Csol(T) of the alloy at T=11–20K are found to be strongly dependent on the conditions of sample preparation. The temperature dependence of Csol(T) for the sample prepared in a calorimeter by direct condensation of a gas mixture at T≈15K exhibits a smeared maximum near the triple-point temperature of hydrogen and a phase transition at T=17.1K. The detected features of Csol(T) indicate that preparation of solid Ne–n-H2 from the gas phase leads to the formation of a long-lived nonequilibrium phase of Ne with high H2 concentrations and a small portion of H2 inclusions with low Ne concentrations. The phase transition is caused by decomposition of this phase. The phase does not recover on cooling of the sample after the phase transition. It is found that the rate of ortho-para conversion of the H2 molecules in the Ne–n-H2 solid solutions is higher than in solid H2.

Low temperature Raman spectra of hydrogen in simple and binary clathrate hydrates

The Raman spectrum of hydrogen clathrate hydrates has been measured, as a function of temperature, down to 20 K. Rotational bands of H(2) and HD, trapped into the small cages of simple (H(2)O-H(2)) and binary (H(2)O-THF-H(2)) hydrates, have been analyzed and the fivefold degeneracy of the molecular J=2 rotational level has been discussed in the light of the available theoretical calculations. The vibrational frequencies of H(2) molecules encapsulated in the large cages of simple hydrates turn out to be well separated from those pertaining to the small cages. Comparison with the equivalent D(2) spectra allowed us to assign the large cavity vibrational frequencies to three couples of Q(1)(1)-Q(1)(0) H(2) vibrational modes. Populations of ortho and para species have been measured as a function of time from rotational spectra and the rate of ortho-para conversion has been estimated for both simple and binary hydrates. We suggest, using the H(2) vibrational spectra, a model to analyze the cage population in simple hydrates.

Hydrogen collisions in planetary atmospheres, ionospheres, and magnetospheres

Hydrogen is the most abundant element in the universe. Molecular hydrogen is the dominant chemical species in the atmospheres of the giant planets. Because of their low masses, neutral and ionized hydrogen atoms are the dominant species in the high atmospheres of many planets. Finally, protons are the principal heavy component of the solar wind. Here we present a critical evaluation of the current state of understanding of the chemical reaction rates and collision cross sections for several important hydrogen collision processes in planetary atmospheres, ionospheres, and magnetospheres. Accurate ab initio quantum theory will play an important role. The collision processes are grouped as follows: (a) H ++H charge transfer, (b) H ++H 2( v) charge transfer and vibrational relaxation, and (c) H 2 (v, J)+H 2 vibrational, rotational, and ortho–para relaxation. In each case we provide explicit representations as tabulations or compact formulas. Particularly important conclusions are that H ++H 2( v) collisions are more likely to result in vibrational relaxation than charge transfer and H 2 ortho–para conversion is at least an order-of-magnitude faster than previously assumed.

Mechanism of theortho-paraconversion of hydrogen on Ag surfaces

Ortho-para conversion of ${\mathrm{H}}_{2}$ and para-ortho conversion of ${\mathrm{D}}_{2}$ on an Ag surface were investigated by resonance-enhanced multiphoton ionization combined with photostimulated desorption. The conversion processes were found to be accelerated by laser irradiation at $193\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, while photo-irradiation at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ had little effect on the conversion time. The natural conversion time of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ was estimated to be 610 and $1030\phantom{\rule{0.3em}{0ex}}\mathrm{s}$, respectively. The roles of electron transfer and rotational-energy dissipation in the conversion are discussed.

Proposal of an innovative, high-efficiency, large-scale hydrogen liquefier

An innovative, efficient and large hydrogen liquefier is described. Innovations lie in the fact that (i) the feed, 10kgs−1, is refrigerated in heat exchangers catalytically promoting the ortho–para conversion (ii) down to the low temperature of 20.5K and at the high pressure of 60bar at which it is available and (iii) lastly expanded to the storage conditions of 1.5bar and 20K through a liquid-phase turbomachine; (iv) refrigeration is via four helium recuperative Joule–Brayton cycles arranged so that the refrigerant follows the cooling curve of hydrogen and the volume flow rates in compression and expansion processes are typical of axial-flow high-efficiency turbomachines; (v) compression is accomplished in 15 intercooled 8-stage devices derived from gas turbine technology. Heat exchangers require specific surfaces comparable to current state-of-the-art liquefiers. Nevertheless, the predicted work of approximately 18MJkg−1 is half as much as the requirement of those liquefiers and corresponds to a second-law efficiency of almost 48%.

Coincidence of rotational energy of H2O ortho-para molecules and translation energy near specific temperatures in water and ice

A hypothesis of the quantum nature of the specific temperatures T s of water and ice, whose values is not random, was formulated. It was found that the quantum energy hΩ mn of closely located rotational transitions in the ortho and para spin isomers of H2O molecules coincides with the translation energy kT near the well-known specific temperatures T s in ice and water. On the basis of this fact it was suggested that ortho-para conversion occurs at temperatures close to T s upon inelastic collisions and resonance energy exchange kT s ↔ hΩ mn in the rotation-translation-rotation (RTR) processes. Such conversion can induce rearrangement of the H-bond set structure and repacking of H2O molecules. The coincidence kT s ≈ hΩ mn was checked for ice and water at 12 known T s, as well as for heavy water D2O near T s = 11.2°C (maximum density) and −140°C (glassy transition). The previously observe strong deformation of the OH Raman band near T s = 4, 19, 36, and 76°C (maximum density, maximum surface tension, minimum heat capacity, and maximum speed of sound, respectively) was interpreted as a manifestation of the water structure rearrangement induced by H2O ortho-para conversion.

Molecular H2 trapped in AlH3 solid

Solid aluminum hydride, AlH3, has been proposed and studied for applications in hydrogen storage. In some samples, a comparatively narrow feature in the proton NMR spectrum is observed; we demonstrate here that this peak is due to molecular hydrogen (H2) trapped within the solid, presumably from earlier processing procedures or partial decomposition. Static and magic-angle spinning NMR show that the responsible species is highly mobile, even at 11 K. Neutron-energy-gain spectra obtained at 3.5 K yield a feature at or near the free-rotor H2 energy difference between the J = 1 and J = 0 states. Both NMR and neutron scattering demonstrate ortho–para conversion at low temperatures. Similar NMR signatures in other hydrogen-storage solids such as NaAlH4 may also be due to trapped H2.

FastOrtho-ParaConversion ofH2Adsorbed at Copper Surface Step Atoms

Ortho-para conversion of H2 adsorbed at the step atoms of a Cu(510) surface proceeds with a short conversion time constant around 1 s as observed in electron-energy-loss measurements of rotational populations. We suggest that this rapid conversion is related to the special character of the adsorption state, which involves a short H2-Cu bond length of 1.8 A. On the flat Cu(100) surface, conversion is found to occur at active sites, most likely step atoms.

Ortho-Para Conversion of InterstitialH2in Si

Ortho-para conversion of isolated interstitial H2 in single-crystalline Si is studied by Raman scattering. This process is suggested to be caused by the interaction of H2 with the nuclear magnetic moment of 29Si. At 77 K the ortho-to-para conversion rate is approximately 0.015 h(-1) for all Si samples employed in the experiments. At 300 K, the reverse para-to-ortho transition is observed. It occurs with a rate of roughly 0.18 h(-1) and results in a thermodynamically nonequilibrium ortho-para ratio.

Microcanonical statistical study of ortho-para conversion in the reaction H3++H2→(H5+)*→H3++H2 at very low energies

The ortho-para conversion of H(3) (+) and H(2) in the reaction H(3) (+)+H(2)-->(H(5) (+))(*)-->H(3) (+)+H(2) in interstellar space is possible by scrambling the five protons via (H(5) (+))(*) complex formation. The product distribution of the ortho-para conversion reaction can be given by ratios of cumulative reaction probabilities (CRP) calculated by microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin, and parity. A statistical method to calculate the state-to-state reaction probabilities for given initial nuclear spin species, rotational states, and collision energies is developed using a simple semiclassical approximation of tunneling and above-barrier reflection. A new calculation method of branching ratios for given total nuclear spins and scrambling mechanisms is also developed. The anisotropic long-range electrostatic interaction potential of H(2) in the Coulomb field of H(3) (+) is taken into account using the first-order perturbation theory in forming the complex. The CRPs and the product distribution of the ortho-para conversion reaction at very low energies with reactants in their ground vibronic and lowest rotational states for given initial nuclear spin species are presented as a function of collision energy assuming complete proton scrambling or incomplete proton scrambling. The authors show that the product distribution at very low energies (or very low temperatures) differs substantially from the high energy (or high temperature) limit branching ratios.

Time-differential observation of the ortho-para conversion of liquidD2under irradiation

We measured an absolute ortho-${\mathrm{D}}_{2}$ concentration of gas samples taken from irradiated liquid ${\mathrm{D}}_{2}$ samples as a function of the integrated radiation dose and the irradiation time by means of a frequency-resolved Raman spectroscopy to study the ortho-${\mathrm{D}}_{2}$ conversion mechanism under irradiation. The measurement was carried out by irradiating liquid ortho-${\mathrm{D}}_{2}$ at a temperature of $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with Bremsstrahlung photons, which were produced by bombarding tantalum with $33\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electrons, and whose absolute fluxes were determined experimentally by employing a foil activation method. The obtained ortho-${\mathrm{D}}_{2}$ concentration was found to decrease from 98% to 82% monotonically with increasing the radiation dose, irrespective of the electron-beam power. This fact clearly indicates the important role of a radiation-induced breakup effect of the ortho ${\mathrm{D}}_{2}$ under the present experimental conditions of radiation dose and irradiation time. The obtained conversion rate is ten times faster than the evaluated value based on an existing model, requiring an alternate mechanism to enhance the conversion.

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.

Adsorption properties of HKUST-1 toward hydrogen and other small molecules monitored by IR

Among microporous systems metal organic frameworks are considered promising materials for molecular adsorption. In this contribution infrared spectroscopy is successfully applied to highlight the positive role played by coordinatively unsaturated Cu2+ ions in HKUST-1, acting as specific interaction sites. A properly activated material, obtained after solvent removal, is characterized by a high fraction of coordinatively unsaturated Cu2+ ions acting as preferential adsorption sites that show specific activities towards some of the most common gaseous species (NO, CO2, CO, N2 and H2). From a temperature dependent IR study, it has been estimated that the H2 adsorption energy is as high as 10 kJ mol(-1). A very complex spectral evolution has been observed upon lowering the temperature. A further peculiarity of this material is the fact that it promotes ortho-para conversion of the adsorbed H2 species.

Nuclear spin conversion of formaldehyde in protostar environments induced by non reactive collisions

Context. Formaldehyde is an important diagnostic of the physical conditions in star forming regions. The temperature of formation is determined by measuring the relative abundance of para-H2CO and ortho-H2CO. The key hypothesis for this determination is that the ortho-para interconversion is strictly forbidden once the molecule is formed. However H2CO nuclear spin conversion mechanisms do exist in a gas phase either involving reactive or non-reactive collisions. This last process is governed by internal properties of the molecule, such as the mixing of energetically closed ortho-para level pairs, which are coupled through magnetic intramolecular interactions. This mixing is interrupted by a collision that makes the molecule leave the mixed state and puts it in a pure state, ortho or para, with a non zero probability for both isomeric forms. Aims. This model allows us to estimate the spin conversion induced by non reactive collisions in different conditions encountered in the interstellar medium. Methods. We calculated the ortho-para conversion rate in H2CO for different temperatures and H2 abundances. Results. It is shown that the characteristic conversion time is always much longer than the H2CO lifetime. Conclusions. Consequently, the conversion probability is zero in gas-phase protostar environments for these non reactive collisions.

Relaxation of stress-split orthoexcitons inCu2O

By applying Hertzian stress fields up to $1.1\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ along a (100) direction we break the cubic crystal symmetry of a cuprite crystal causing the triply degenerate orthoexciton level to split into a singlet and doublet. The interconversion rate between the singlet and lower-lying doublet is measured as a function of stress at $T=2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by using subnanosecond time-resolved luminescence. Based on the experimentally observed stress dependence, we propose that this transition occurs via exciton and TA-phonon scattering associated with the off-diagonal shear tensor field (abbreviated ``shear scattering''), in contrast to the axial vector field scattering (abbreviated ``rotational scattering'') mechanism for the ortho-para conversion.

Quantum Effects on Hydrogen Adsorbed on Metal Surfaces

Hydrogen is adsorbed on metal surfaces either molecularly or atomically. Due to the light mass, hydrogen has a large zero-point vibrational energy and an extended feature of its nuclear wavefunction. The zero-point motion affects the hydrogen adsorption site and the wavefunction is possibly delocalized over the surface. Because of the nuclear spin, on the other hand, molecular hydrogen is classified into ortho and para species, that have different rotational quantum numbers. The ortho and para species have different adsorption energy and undergo ortho-para conversion through magnetic interaction with metal surfaces.

Rates of complex formation in collisions of rotationally excited homonuclear diatoms with ions at very low temperatures: Application to hydrogen isotopes and hydrogen-containing ions

State-selected rate coefficients for the capture of ground and rotationally excited homonuclear molecules by ions are calculated, for low temperatures, within the adiabatic channel classical (ACCl) approximation, and, for zero temperature, via an approximate calculation of the Bethe limit. In the intermediate temperature range, the accurate quantal rate coefficients are calculated for j = 0 and j = 1 states of hydrogen isotopes (H2, HD, and D2) colliding with hydrogen-containing ions, and simple analytical expressions are suggested to approximate the rate coefficients. For the ground rotational state of diatoms, the accurate quantal rate coefficients are higher compared to their ACCl counterparts, while for the first excited rotational state the reverse is true. The physical significance of quantum effects for low-temperature capture and the applicability of the statistical description of capture are considered. Particular emphasis is given to the role of Coriolis interaction. The relevance of the present capture calculations for rates of ortho-para conversion of H2 in collisions with hydrogen-containing ions at low temperatures is discussed.

H6+ in irradiated solid para-hydrogen and its decay dynamics: Reinvestigation of quartet electron paramagnetic resonance lines assigned to H2?

The quartet electron paramagnetic resonance (EPR) lines observed in gamma- and X-ray irradiated solid para-H2, which have previously been assigned to H2-, are reinvestigated. We have reassigned the quartet lines to H6 rather than H2- mainly due to comparison of experimentally obtained EPR parameters to theoretical results. Based on the new assignment, trapping site, rotation, ortho-para conversion, quantum diffusion and isotope effect of H+ have been reinterpreted by the precise reanalysis as follows. The H6+ ion is composed of the collinearly aligned H2+ core at the center and two H2 rotors at both ends, occupies a single substitutional site, and has a precession motion around a crystalline axis with the angle of approximately 57 degrees. The ortho-para conversion of H2+ core of H6+ is completed within the time-scale of hours, whereas ortho-H2 molecules near H6+ convert much faster. H6+ diffuses quantum mechanically by the repetition of H6+ + H2 --> H2 + H6+ reaction. The diffusion terminates by the reaction, H6(+) + HD --> H5D(+) + H2, with a HD impurity contained in the para-H2 sample at natural abundance. Finally, we will propose a possible reason why H6+ is produced instead of H3+ in the irradiated solid H2.

Efforts towards a dynamically polarised HD-target

The molecular hydrogen isotopes contain no unpolarisable background. From this point of view they appear to be the material of choice for a polarised bulk target in scattering experiments. The fast nuclear relaxation of H 2 and D 2 is one reason that these substances are not highly polarisable. This fact brings hydrogendeuteride (HD) into focus. In Bochum a device to freeze out gases into the consisting 4 He -cryostat has been built up. The principle of the Dynamic Nuclear Polarisation requires a sufficient amount of paramagnetic electrons. These have been produced by cracking HD molecules at 1 K using a 90 Sr β -source with an activity of 3.7 GBq . Six days of effective irradiation resulted in a density of paramagnetic centres in the order of 10 18 spins/cm 3 . This could be estimated from bolometric electron paramagnetic resonance (EPR) measurements. Dynamic polarisation could not be achieved. This is accounted to isotopic impurities in the used HD, which accelerate the nuclear relaxation. The constant of ortho–para conversion in H 2 could be confirmed to be 1.9%/h.