HD andD2layers physisorbed on MgO studied by NMR

Abstract

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...

Direct observation of molecular hydrogen binding to magnesium oxide (100) surfaces

“…It is worth noting that no conversion from ortho-to-para hydrogen was made prior to dosing the sample. The NMR study [4] discussed above using MgO prepared in our lab showed that within 30 min the conversion was complete, probably a result of a very small amount of Mn impurities found in our MgO. While we observed no time dependence in the INS signal we discarded the first half an hour of data (out of a minimum of 12 h of measuring time).…”

“…There have been a number of relevant studies on the adsorption of molecular hydrogen on MgO [1][2][3][4]. Thermodynamic studies suggest that a number of layering transitions and other less well-identified phase changes (e.g.…”

Structure of helicoptering H2 molecules on MgO(001)

“…The entropy difference becomes ΔS = S(c(2 × 2)) − S(one-defect) = k ln(Ω) − k ln(4Ω) = −k ln(4), where k is Boltzmann's constant. The Helmholtz Free Energy difference now becomes ΔF/k = − 45 + T ln (4). At T = 32.5 K the free energy difference is zero (ΔF = 0) and the c(2 × 2) structure will be fully transformed to the defect structure and aggregation will occur.…”

“…Although one of the simplest adsorbate-substrate systems imaginable for the study of gassolid interactions, it has the additional possibility of exhibiting quantum effects, owing to the light mass of the adsorbate. Early studies using neutron scattering [2], adsorption-isotherm and heat capacity measurements [3], and nuclear magnetic resonance [4] examined the structure of multilayer and sub-monolayer films of H 2 , HD, and D 2 . The neutron scattering study found evidence of a sequence of phase transitions c(2 × 2) → c(4 × 2) → c(6 × 2) in the D 2 /MgO system.…”

Electric field gradient antishielding for HD andD2

“…Finally, molecular hydrogen is used to probe surfaces and study two-dimensional models on surfaces of various materials. 6 In particular, the electrical environment affects the deuteron nuclear spin through its quadrupole moment, and the interaction is directly proportional to the efg and the nuclear quadrupole moment. However, besides the direct effect of the efg on a nucleus, there can be an indirect effect that, in general, is much larger than the direct effect.…”