Effects of rotational-symmetry breaking on physisorption of ortho- and para-H2 on Ag(111)

Significance Statement

Hydrogen molecules are physisorbed on any materials surfaces, which is a key to hydrogen storage technology. While materials with a larger physisorption energy for H2 is being intensively searched for efficient H2 storage, fast ortho-para conversion and ortho-para separation of H2 is of considerable importance for H2 liquefaction. When H2 is physisorbed on a surface, the physisorption potential can be anisotropic depending on the molecular-axis orientation, which leads to symmetry breaking of the H2 rotational motion. Whereas the originally degenerate rotational levels are split under the anisotropic potential, the rotational wavefunction is almost unaltered and H2 still behaves as a nearly-free rotator within a first-order perturbation. Since the nuclear-spin triplet (ortho-H2) and singlet (para-H2) states take only odd and even rotational quantum numbers (J), respectively, a crucial consequence due to the rotational-symmetry breaking appears in the dependence of physisorption properties on the nuclear-spin state including the physisorption energy and rotational partition function, which has been applied for nuclear-spin isomer separation for many years.

With temperature-programmed desorption (TPD) in combination with the quantum-state-resolved detection (please see, for example, Figure 2(b) of [T. Sugimoto and K. Fukutani, Nature Physics. 7, 307 (2011)]), we successfully demonstrated that not only the adsorption enthalpy but also the adsorption entropy is significantly different between ortho-H2 and para-H2 physisorbed on Ag(111). This is a clear manifestation of the symmetry breaking due to the surface anisotropic potential on the quantum rotator of the H2 molecule. Our careful analysis of the experimental data unambiguously showed that H2 prefers perpendicular orientation on Ag(111) with an anisotropy of -5 meV, which is in agreement with recent ab initio calculations. In this anisotropic potential, almost all of the ortho-H2 (J=1) exist in the M=0 state where its molecular axis distribution is out-of plane, whereas the molecular axis of para-H2 (J=0) remains isotropically distributed due to the rotational quantum effect. Clearly, the present results are at variance with the conventional van der Waals theories and suggest essential importance of partial hybridization interaction with the H2 unoccupied orbital in the weakly physisorbed system. Considering that the weak hybridization would not be limited to H2, the term ‘physisorption’ so far used for weakly adsorbed system may well be rephrased as ‘chemicalphysisorption’. It is noted that the rotational-state-selective TPD is a powerful and non-invasive method to clarify the physisorption properties of adsorbed H2, which can be easily and versatilly applicable to other surfaces.

 

Effects of Rotational-Symmetry Breaking on Physisorption of Ortho- and Para-H2 on Ag(111) - Advances in Engineering

 

 

 

 

 

 

 

 

Phys Rev Lett. 2014 ;112(14):146101. Sugimoto T, Fukutani K.

Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan.

Abstract

 Quantum-state-selective thermal desorption of H2 weakly physisorbed on Ag(111) demonstrates significantly different desorption features between the nuclear-spin modifications. An energy shift due to the rotational-symmetry breaking induced by an anisotropic interaction affects not only the enthalpy but also the entropy of adsorption. The preexponential factor for desorption of the ortho-H2 is about three times as large as that of the para-H2. The entropy difference indicates a perpendicular orientation preference of anisotropic physisorption potential, which also suggests the importance of partial hybridization interaction for weak physisorption.

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