ortho-H2 Molecules Research Articles

Monitoring the build-up of hydrogen polarization for polarized hydrogen-deuteride (HD) targets with nuclear magnetic resonance (NMR) at 17 T.

We report on the frozen-spin polarized hydrogen-deuteride (HD) targets for photoproduction experiments at SPring-8/LEPS. Pure HD gas with a small amount of ortho-H2 (∼0.1%) and a very small amount of para-D2 (∼0.001%) was liquefied and solidified by liquid helium. The temperature of the produced solid HD was reduced to about 30 mK with a dilution refrigerator. A magnetic field (17 T) was applied to the HD to grow the polarization with the static method. After the aging of the HD at low temperatures in the presence of a high-magnetic field strength for three months, the polarization froze. Almost all ortho-H2 molecules were converted to para-H2 molecules. Most remaining para-D2 molecules were converted to ortho-D2 molecules. The para-H2 and ortho-D2 molecules exhibited weak spin interactions with the HD. If the concentrations of the ortho-H2 and para-D2 were reduced appropriately at the beginning of the aging process, the aging time can be shortened. We have developed a new nuclear magnetic resonance (NMR) system to measure the relaxation times (T1) of the 1H and 2H nuclei with two frequency sweeps at the respective frequencies of 726 MHz and 111 MHz and succeeded in the monitoring of the polarization build-up at decreasing temperatures from 600 mK to 30 mK at 17 T. Automatic NMR measurements with the frequency sweeps enabled us to omit the use of a manual tuning circuit and to remove magnetic field sweeps with eddy current heat. This technique enables us to optimize the concentration of the ortho-H2 and to efficiently polarize the HD target within a shortened aging time.

Observation of enhanced rate coefficients in the H2++H2→H3++H reaction at low collision energies

The energy dependence of the rate coefficient of the H2++H2→H3++H reaction has been measured in the range of collision energies between kB⋅10K and kB⋅300mK. A clear deviation of the rate coefficient from the value expected on the basis of the classical Langevin-capture behavior has been observed at collision energies below kB⋅1K, which is attributed to the joint effects of the ion-quadrupole and Coriolis interactions in collisions involving ortho-H2 molecules in the j = 1 rotational level, which make up 75% of the population of the neutral H2 molecules in the experiments. The experimental results are compared to very recent predictions by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)], with which they are in agreement.

Theoretical Investigation of Two H2 Molecules Inside the Cages of the Structure H Clathrate Hydrate

We report 10-dimensional quantum dynamic calculations of the translation-rotation energy levels of two hydrogen molecules inside the cages of the structure H clathrate hydrate. For the small and medium cages, positive ground state energies are reported supporting their single occupancy, while for the large cage negative energy levels are calculated and some low-lying states are characterized for two para-H2, ortho-H2, para-D2, ortho-D2, and HD molecules inside the cage. The calculations are performed using the block improved relaxation method implemented in the multiconfiguration time-dependent Hartree (MCTDH) code.

Zero-phonon emission bands of solid hydrogen at 6–12μm wavelength. An astrophysical phenomenon

Infrared emission bands in the wavelength range of 6–12μm observed in the ISO-SWS mission are assigned to rotational zero-phonon bands of solid parahydrogen by using Van Kranendonk’s approximate rigid-lattice method. This method is based on superposed electric quadrupole pair interactions and superposed quadrupole induced dipole moments of pairs in the hcp crystal. Accordingly, the approximate formalism uses zero-order H2 pair wave functions. Symmetry effects of the hcp crystal require preference of rotational pair transitions. The interaction potential of the pairs is confined to the electric quadrupole-quadrupole interaction. Zero-phonon emission bands of H2 pair transitions fitted to the spectrum contain at least one delocalized j=2 state initially and/or finally because of their significantly enhanced emission rates. They also yield the characteristic bandwidths which fit nicely to the widths of the observed features. The frequency positions of the seven pure parahydrogen pair transitions used, obtained from experimentally determined rotational solid hydrogen energy levels, are in perfect agreement with the observed features, whereas the three mixed ortho-parapair transitions need a presently unknown frequency correction, caused by the migration of the ortho-H2 molecules into the parahydrogen crystal prior to emission, the so-called initial excess binding energies. The astrophysical setup of the observed source is discussed in the end of the paper.

Rotational spectroscopic study of carbonyl sulfide solvated with hydrogen molecules

Rotational spectra of small-sized (H(2))(N)-OCS clusters with N = 2-7 were measured using a pulsed-jet Fourier transform microwave spectrometer. These include spectra of pure (para-H(2))(N)-OCS clusters, pure (ortho-H(2))(N)-OCS clusters, and mixed ortho-H(2) and para-H(2) containing clusters. The rotational lines of ortho-H(2) molecules containing clusters show proton spin-proton spin hyperfine structure, and the pattern evolves as the number of ortho-H(2) molecules in the cluster increases. Various isotopologues of the clusters were investigated, including those with O(13)CS, OC(33)S, OC(34)S, and O(13)C(34)S. Nuclear quadrupole hyperfine structures of rotational transitions were observed for (33)S (nuclear spin quantum number I = 3/2) containing isotopologues. The (33)S nuclear quadrupole coupling constants are compared to the corresponding constant of the OCS monomer and those of the He(N)-OCS clusters. The assignment of the number of solvating hydrogen molecules N is supported by the analyses of the proton spin-proton spin hyperfine structures of the mixed clusters, the dependence of line intensities on sample conditions (pressure and concentrations), and the agreement of the (para-H(2))(N)-OCS and (ortho-H(2))(N)-OCS rotational constants with those from a previous infrared study [J. Tang and A. R. W. McKellar, J. Chem. Phys. 121, 3087 (2004)].

Rotational State Effects in the Dissociative Recombination ofH2+

We have studied the dissociative recombination (DR) of molecular hydrogen ions with slow electrons over a range of collision energies from 0 to 400 meV. By employing a pulsed expansion source for rotational cooling and by exploiting super elastic collisions with near-0-eV electrons in a heavy ion storage ring for vibrational cooling, we observe a highly structured DR cross section, comparable to that reported for HD+. Using para-hydrogen-enriched ion beams, we identify for the first time features in the DR cross sections attributed to nu=0, J=even molecules (para-H2) and nu=0, J=odd (ortho-H2) molecules, separately. Indications are given that para levels have different DR rate coefficients from ortho levels for the first four vibrational levels at near-0-eV collisions.

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.

Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies

Diffusion and recombination of H atoms were studied in solid hydrogen containing ortho-H2 molecules at relative concentration Xo=0.001→0.75 using electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and electron spin echo (ESE) methods at around 4 K. When the rate-determining step for recombination is assumed to be the diffusion of H atoms, the rate constant for recombination at Xo⩾0.1 is consistent with the diffusion coefficient estimated from the analysis of ENDOR spectra and longitudinal spin relaxation behaviors. The recombination rate constant at Xo<0.1, however, is too slow to be explained using the diffusion coefficient estimated from longitudinal spin relaxation and forbidden spin-flip satellite transition studies. This result suggests that, even if one H atom finds another H in its immediate neighborhood, these H atoms do not react to form a H2 molecule at Xo≪0.1. The absence of recombination of H atoms is due to lack of the energy dispersion path required for the recombination of diatomic molecules. Since the absence of recombination becomes less significant at higher Xo, ortho-H2 molecules are found to play an important role in the energy dispersion which accompanies the recombination reaction.

Resonance Effect on the Tunneling Reaction: H+H2→H2+H in Solid Hydrogen

The tunneling abstraction reaction: H+H2→H2+H in γ-irradiated solid hydrogen has been studied using electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) spectroscopy and gas-chromatography. The rate constant for the tunneling reaction in solid hydrogen was found to decrease with the increase in the concentration of ortho-H2 molecules in solid hydrogen. We concluded that the decrease in the rate constant is due to the energy level mismatching between reactant species of H+H2 trapped and product species of H2+H trapped in the reverse induced by inhomogeneous intermolecular interactions between ortho-H2 molecules in solid hydrogen. This result indicates that resonance effects play an important role in tunneling reactions in solids.

Decay Dynamics of H Atoms in Solid Hydrogen at 4.2 K. Controlling Factor of Tunneling Reaction H + para-H2 → para-H2 + H

H atoms produced by the γ-ray irradiation of solid para-H2 (p-H2) and solid normal-H2 (n-H2) decay gradually when the irradiated sample is stored at 4.2 K. ortho-H2 (o-H2) molecules at the sites nearest an H atom in solid n-H2 convert to p-H2 molecules by interacting with the H atom. H atoms in solid H2 decay by the process of tunneling diffusion H + p-H2 → p-H2 + H and finally recombine with other H atoms. The rate constants for the tunneling reaction were obtained from the decay rates of H atoms in solid H2 at various concentrations of o-H2. When solid n-H2 is stored for 10 h at 4.2 K after γ-ray irradiation, a number of o-H2 molecules change into p-H2 molecules by a catalytic chain reaction induced by mobile H atoms. The G value of the o-H2 yields from H atoms amounts to 1−2.6 × 104. The rate constant for the tunneling reaction H + p-H2 → p-H2 + H decreases drastically with increasing numbers of o-H2 molecules, which exist as orientational defects in solid p-H2. It was concluded that a very small amoun...

Adsorption of ortho and para H2 on NaCl(001)

Fourier transform infrared spectroscopy (FTIR) experiments together with interaction potential calculations are performed to interpret the adsorption–desorption properties of para and ortho H2 molecules physisorbed on the (001) faces of NaCl crystallites. Isotherms of the adsorption process show preferential adsorption of para-H2 from gaseous normal-H2 at surface temperature around 10 K. Isobars of the desorption process show preferential desorption of para-H2 from a mixed ortho-H2 and para-H2 adsorbate. A quantum description of the molecular motions corroborates preferential adsorption of p- and (M=±1) o-H2 on the cation sites, as proposed in previous classical studies. It is shown that the adsorption energy is smaller for the para than for the (M=±1) ortho species in agreement with experimental measurement of desorption energy, while (M=0) o-H2 is only slightly bound to the substrate. This indicates that the ionic surface can discriminate all the H2 species. The determination of the bar spectrum for a set of independently adsorbed H2 species can interpret most of the infrared signals assigned to adsorption on the terrace sites. More specially the experimental behavior, with temperature and coverage, of the frequencies and relative intensities of the pure vibrational peaks and of the translation–vibration combination bands is very well explained in terms of hindered external motions of the admolecule.

Decay Behaviors of H2- Anions in Solid Parahydrogen: Effect of Nuclear Spins on Chemical Reactions

Decay processes of H2- anions in γ-ray-irradiated solid parahydrogen were studied by using an ESR spectrometer. The following interesting results were obtained. First, the initial yields of ortho-H2- anions in the γ-irradiated solid parahydrogen were 3 times as large as those of para-H2- anions. Second, the yields of para-H2- anions decrease faster than those of ortho-H2- anions upon storage of the irradiated samples at 4.2 K. Third, the decay rate of H2- anions is accelerated by the addition of HD and D2 molecules. Fourth, H2- anions at 2.2 K decay faster than at 4.2 K. According to the parity conservation rule in a homonuclear diatomic molecule, the energy of ortho-H2- anions at the ground state is lower than that of para-H2- anions, whereas that of ortho-H2 molecules is higher than that of para-H2 molecules at low temperatures. The first and second results are ascribed to para → ortho conversion of H2- anions. The third result indicates that nearby HD and D2 accelerate the decay of H2- anion or broaden...

Specific heat of quench-condensed hydrogen films

The specific heatC(X, T) of quench-condensed films of H2 has been measured as a function of ortho concentration X with 0.28<X<0.75 for coverages between 24.3 and 92.3 A−2 at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperaturesT. cond between 1.0 and 3.5 K. The observed specific heat is attributed to orientational ordering of the ortho-H2 molecules. For the films withX = 0.74 condensed atT cond>2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution toC, which is not present in bulk H2, and which we attribute to short-range ordering size effects. AsT cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence ofC onT cond, the entropy per molecule at 3 K is only weakly dependent onT cond and comparable to that for bulk H2. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H2. As in bulk H2, the transition temperature inferred from the location of the specific heat peak or anomaly decreases withX. Unlike in bulk H2, there is no temperature hysteresis inC for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.

Orientational ordering in thick films of molecular hydrogen on BN

We report NMR studies of the orientational ordering of ortho-H2 molecules in 12-layer films physisorbed on hexagonal boron nitride. The phase diagram for the ordering has been explored for ortho-H2 concentration 0.23<X<0.72 and for temperatures 0.05<T<10K. For high ortho concentrations we observe a two-component NMR line shape with a broad central gaussian line (∼60 kHz) and a broad outer wing structure (∼240 kHz), corresponding to an incompletely orientationally ordered component. The broad component closely resembles the shapes expected for a quadrupolar glass for this fraction of the molecules. The most unusal feature occurs at low concentrations,X<0.40, for which we observe a double wing structure, an inner pair with a separation ∼240 kHz, and an outer pair at ∼400 kHz. The latter are attributed to fully ordered molecules in the first two layers near substrate.

Local ordering in dilute ortho-para-hydrogen mixtures at low temperatures

We report experimental observations of the local orientational ordering of ortho-H2 molecules in dilute solid ortho-para mixtures for low temperatures. The results indicate a smooth, continuous freezing of the rotational degrees of freedom without any evidence for a distinct discontinuity in the temperature dependence of the local order parameters for the 37–600 mK temperature range.

Interpretation of1H NMR results of HD impurities in solid H2. I. Spin-spin relaxation

Previous1H NMR results on HD impurities in solid H2 samples of low ortho concentration (X≲0.01) are reexamined, resulting in a different interpretation of the spin-spin relaxation timeT 2. The relaxation time is found to be measured by using short rf pulses rather than the long (100–120 µsec) pulses used previously for the spin-echo pulse sequences. This is due to the fact that the nuclear spin dipolar flip-flop interactions between the HD and ortho-H2 molecules are effectively quenched because of the large intramolecular dipolar splittings of the H2 molecules. The measured spin-spin relaxation times for the HD impurities in a sample ofX≲0.01 are found to be consistent with those from statistical theory, revealing no appreciable motional narrowing, which was invoked previously.

Orientational ordering of hydrogen molecules adsorbed on graphite

The orientational behaviour of ortho-H2 and para-D2 molecules adsorbed on graphite has been studied by NMR from 0.3 to 12 K at coverages where the molecules form a [Formula: see text] registered lattice. We have observed that for 90% ortho-H2, the splitting of the high-temperature NMR doublet increases rapidly near 0.6 K and additional structure appears. We have identified this as a transition to the orientationally ordered pinwheel phase: one of the phases predicted by the mean field theory of Harris and Berlinsky. In contrast, the spectrum of 90% para-D2 evolves smoothly to a broad, weak structure as the temperature decreases, showing no evidence of ordering down to 0.3 K. This very different behaviour is unexpected, given that in the bulk solid both isotopes order at comparable temperatures. From the splitting of the high-temperature NMR doublets, we have deduced the crystal fields Vc and the effective quadrupole coupling constants Γ. For D2, Γ is somewhat less than the rigid lattice value, as expected in the presence of translational zero-point motion. However, for H2, Γ is anomalously small. The ortho–para conversion rates of H2 and D2 on our Grafoil substrate have also been measured.

Orientational ordering in solid ortho–para H2 mixtures and analogy with a spin glass

A review is presented of our understanding of the orientational ordering in solid hydrogen, where ortho-H2 molecules are subject to anisotropic electric quadrupole–quadrupole interactions. This review pertains to solid H2 at low pressures and is concerned mostly with NMR experiments. After a discussion of the phase transition from the disordered hexagonal close-packed (hep) to the long-range ordered cubic phase, a description is given of experimental investigations of the ordering in the hep phase for ortho concentrations X &lt; 0.53. This ordering appears to take place continuously as the temperature decreases. The question as to whether or not this phase is to be called a quadrupolar glass phase is addressed, and comparison is made with the orientational freezing in other compounds. A theoretical analysis is given to show that the ortho–para mixture is the analog of a spin glass in a random field for which no phase transition is expected.

The S(0) line in solid H2

The far infrared absorption spectrum of solid H2 has been measured in the frequency region of the S(0) line. We have performed measurements at different ortho H2 concentrations in order to investigate the influence of ortho H2 molecules. By increasing the ortho H2 concentration Co from Co = 10% up to 75%, we found that the excitonic absorption is the dominant feature of the S(0) line at low Co values, but it still exists when Co is relevant.

Pulsed NMR studies in solid H2. II. Stimulated echoes and cross relaxation times

A study of the stimulated echo following a three-pulse sequence is presented for solid H2 samples with ortho concentrations X between 15% and 65%, and over the temperature range between T=0.1 and 2 K. The stimulated echo decay time τE is found to increase sharply upon cooling through the transition into the long-range ordered phase for X>0.55. However, the increase is only gradual during the progressive orientational ordering in the hcp phase for X ≲ 0.55. The predictions of a density matrix method for the stimulated echo amplitude are used for comparison with the experiment. A phenomenological approach shows that the results for τE can be interpreted in terms of a combination of longitudinal relaxation times T1 and cross relaxation times T12, the latter expressing the flip-flop transitions between the spin states of neighboring ortho-H2 molecules. These cross relaxation times are calculated from τE and T1 data and found to increase with linewidth as the orientational ordering progresses. The results are compared with the predictions from a simple model and qualitative agreement is obtained. Our observations of the stimulated echo shape and its decay lead to the conclusion that in the investigated range there is no sudden freezing of the orientational fluctuations over a narrow temperature range for X<0.5. Rather, combination of T1 and stimulated echo experiments indicates that the orientational fluctuation times increase progressively with decreasing temperature, but are likely to stay shorter than 10−4 sec above about 0.15 K. Therefore, in this temperature range, solid H2 cannot be regarded as a glass. This conclusion differs from that by Sullivan and co-workers.