Orientational ordering of hydrogen molecules adsorbed on graphite

Kubik, P.R.; Hardy, W. N.; Glättli, H.

doi:10.1139/p85-094

Cited by 60 publications

(28 citation statements)

References 49 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…The fit shown by the solid line in Fig. 8 is given by V c i ϭϪ2.45 K. This is to be compared with the value ͉V c ͉ϭ0.65 K for H 2 on graphite determined by Kubik et al 5 and the value of ͉V c ͉Ӎ30 K estimated by Novaco and Wroblewski. 68 The value of ͉V c ͉ for bulk H 2 is Ӎ0.02 K. 69 The above analysis gives a good quantitative description of the outer peak structure AA ͑region III of the phase dia-gram͒ in terms of the orientational ordering driven by the substrate crystal field.…”

Section: B Analysissupporting

confidence: 56%

Orientational ordering in films of molecular hydrogen on boron nitride: Thick (12-layer) films and bilayer films

Kim

Sullivan

1998

Phys. Rev. B

View full text Add to dashboard Cite

We report NMR studies of the orientational ordering of molecular hydrogen in films physisorbed on hexagonal boron nitride ͑BN͒. The local orientational ordering of the molecular axes is determined for both thick films ͑12 layers͒ and thin films ͑bilayers͒. The ordering observed is in both cases very different from that observed for bulk solid hydrogen. Quadrupolar glass ordering occurs for intermediate layers in thick films at low ortho concentrations 0.25ϽXϽ0.40 and Tр0.7 K. The ortho molecules remain rotationally disordered in the outer layers of the thick films and invariant to small changes in thickness. In the bilayers one observes a long range ordering for Tр1.95 K and Xр0.56, and a quadrupolar glass ordering for intermediate concentrations (0.37ϽXϽ0.55) and intermediate temperatures (0.

show abstract

Section: B Analysissupporting

confidence: 56%

Orientational ordering in films of molecular hydrogen on boron nitride: Thick (12-layer) films and bilayer films

Kim

Sullivan

1998

Phys. Rev. B

View full text Add to dashboard Cite

show abstract

“…[1][2][3][4][5][6] The phase diagram and the structure of the monolayer have been investigated by a number of experimental techniques, including heat capacity, 1-6 neutron scattering, [7][8][9][10][11][12][13][14][15][16][17][18] nuclear magnetic resonance, 19 and low-energy electron diffraction. 20,21 Similarly to helium physisorbed on graphite, submonolayer of molecular hydrogen adsorbed on graphite forms a ͱ3ϫͱ3 commensurate structure due to the graphite surface potential corrugation.…”

Section: Introductionmentioning

confidence: 99%

Submonolayer molecular hydrogen on graphite: A path-integral Monte Carlo study

Nho

Manousakis

2002

Phys. Rev. B

View full text Add to dashboard Cite

We have used path-integral Monte Carlo ͑PIMC͒ to simulate molecular hydrogen on graphite at submonolayer coverage. First we use a flat substrate and we study the first layer for various values of the coverage up to layer completion. We found that the first layer has a solid-gas coexistence phase at low densities and a triangular solid phase at and above the equilibrium density 0 ϭ0.0705 Å Ϫ2 . We also determine that the first layer promotion coverage is at 0.094 Å Ϫ2 in agreement with experiment. Second we introduce the full H 2 -graphite interaction, i.e., we include the effects of substrate corrugations. In this case we carry our PIMC simulations on a variety of systems at and below the 1/3 coverage. We calculate the energy as a function of coverage, contour plots of the molecule probability distribution, the pair distribution function, the static structure function and the specific heat. When the substrate corrugation part of the interaction is included we find that at 1/3 coverage the system is in a ͱ3ϫͱ3 commensurate solid phase. At coverages below that and at low enough temperature the system exists in solid clusters surrounded by vapor. At coverages below a critical density, defining a tricrical point, as the system is heated up these clusters melt into a uniform fluid phase. We find that below the commensurate density and above the tricritical point, as the clusters are heated up, first they undergo a transition into a phase where the vapor phase disappears and a commensurate phase with vacancies arises. This commensurate solid melts at higher temperature into a uniform fluid phase.

show abstract

“…From perturbation theory, such a large value of P2 implies that the anisotropic potential for HD on MgO is not much smaller than the energy splittings between J states (e.g., 128 K). This is quite different from that for ortho H2 (odd /) on graphite, where small crystal fields (only -1 K) were found [10,11], as confirmed by calculation [12]. Presumably the large electric field gradients from the ionic substrate [6] interacting with the H2 molecular quadrupole moment are important.…”

mentioning

confidence: 64%

HD andD2layers physisorbed on MgO studied by NMR

et al. 1992

View full text Add to dashboard Cite

The D NMR spectra of HD and ortho D2 are sensitive to the crystal fields from the MgO surface sites. At fractional coverages 0.6 < x < 0.9, beyond the limit of island growth, rapid motion among inequivalent sites occurs, even at 4.2 K. For 1 < x < 2, broad and narrow lines arise from the first and second layers, respectively. Isotopic bilayer mixtures show nearly complete separation, with the more massive specie in the first layer.PACS numbers: 76.60.-k, 68.6a-p, 64.75.+gWe report a deuteron NMR study of HD and D2 layers physisorbed onto MgO powder. The D line shapes provide unambiguous evidence of molecular motion in the compressed monolayer and reveal isotope separation in the bilayer. This information is essentially inaccessible by the more frequently employed thermodynamic and diffraction measurements. This particular system of hydrogen on MgO is an intriguing one, because of recent neutron scattering evidence [1] that multilayers retain a liquid component down to 7 K. Suppression of freezing to lower temperatures may result in a new superfluid [2].The MgO substrate avoids many problems that the popular graphite substrate presents for NMR. The conductivity of graphite results in skin depth limitation of the rf fields, dictating use of low NMR frequencies and/or small samples, resulting in low sensitivity. Further, magnetism of the graphite, both natural and from impurities, makes interpretation of NMR results difficult [3]. The MgO for most of this work had specific area 30 m Vg and uniformity somewhat poorer than exfoliated graphite. A few experiments were performed on MgO prepared according to Coulomb and Vilches [4]. This material is much more uniform but has only 6 m Vg. In both cases, a 1.25-cm^ vessel held about 0.55 g of MgO powder.The origin of spectral width is as follows: Free HD molecules have a 7=0, nondegenerate rotational ground state. The spherical symmetry results in the vanishing of P2^(3cos^O-l>/2, with 6 the angle between the molecular axis and the external field. The interaction between the quadrupole moment of the D nucleus and_the molecular electric field gradient is proportional to P2, as is the intramolecular proton-deuteron dipole coupling. Thus, in the J=0 state the D NMR spectrum of HD should be relatively sharp, broadened only by intermolecular dipole interactions of a few kHz. Thermal excitation of higher-J states (> 128 K higher) is unimportant at the low temperatures of this work, ;S 10 K [5].Deuteron spectra at 4.2 K and 8.3 T (54.4 MHz) for HD on MgO are presented in Fig. 1. The coverage x = 1 corresponds to the start of second-layer formation. The first and most obvious feature of the data is the large linewidth ( -30 kHz) compared to that expected for y =0 molecules. Evidently the crystal field from the ionic MgO substrate [6] admixes higher-/ states into the rotational ground state [7]. The result ^s a rotational ground state that is not spherical. Thus P2 no longer vanishes and the quadrupole and intramolecular dipole interactions appear in the spectrum. The above explanat...

show abstract

Orientational ordering of hydrogen molecules adsorbed on graphite

Cited by 60 publications

References 49 publications

Orientational ordering in films of molecular hydrogen on boron nitride: Thick (12-layer) films and bilayer films

Orientational ordering in films of molecular hydrogen on boron nitride: Thick (12-layer) films and bilayer films

Submonolayer molecular hydrogen on graphite: A path-integral Monte Carlo study

HD andD2layers physisorbed on MgO studied by NMR

Contact Info

Product

Resources

About