Two-Dimensional Quantum Rotation of AdsorbedH2

Abstract

We report experimental and theoretical evidence of H 2 adsorbed in a confined quantum rotor state on a stepped copper surface. Rotational transitions of step-adsorbed para-H 2 and ortho-H 2 observed in electron-energy-loss measurements occur close to the energies expected for an ideal 2D rotor. Dramatic enrichment of adsorbed ortho-H 2 is observed at elevated surface temperatures and hydrogen background pressures. Calculations reveal a molecule residing above a step atom and confined to rotate in a twodimensional manner. PACS numbers: 68.45.Kg, 67.70. + n, 82.65.Pa Recently, spirited attention has been directed to rotorlike states of single molecules adsorbed on metal surfaces, stimulated by ideas regarding molecular-based mechanical and memory devices. Experimental visualization using scanning tunneling microscopy has been reported, concerning, for example, supramolecular bearings formed by large organic molecules on a copper surface [1] and tunneling-current-controlled rotation of small molecules adsorbed on platinum [2] and copper surfaces [3]. Confined metal dimer rotation on a closepacked metal surface has been theoretically predicted [4]. The rotary motion, driven either by thermal energy or inelastic electron tunneling is in these cases restricted by energy barriers and is diffusive.Free rotary motion confined to two dimensions (2D rotor) is also conceivable, at least for weakly adsorbed light molecules such as hydrogen. For example, the mechanism of preferential adsorption of metastable orthohydrogen species was long ago [5] suggested to be associated with a 2D rotor adsorption state [6]. Enrichment occurs readily on activated alumina. Silvera and Nielsen [7] observed, using inelastic neutron scattering, that the rotational spectrum of hydrogen adsorbed on alumina powder at low temperatures was greatly distorted from that of the free molecule. They found that their observations were more consistent with an axially constrained rotor state. In the extreme limit the confining potential will in this case lead to a loss of two rotational degrees of freedom and the molecule can only vibrate against the surface. Whereas hydrogen molecules physisorbed on flat crystal surfaces [8-10] experience only weak rotational hindering, for local adsorption at low-coordinated surface sites such as adatoms or step atoms, a much stronger hindering may be conceived.In this Letter we present electron-energy-loss measurements and density functional calculations, which provide evidence of hydrogen adsorbed as a confined rotor on a stepped copper surface. Our measurements reveal, for step-adsorbed molecules, rotational transitions of parahydrogen ( p-H 2 ) and orthohydrogen (o-H 2 ) at energies close to the values expected for an ideal 2D rotor. This adsorption state is readily identified in total energy calculations and corresponds to a molecule residing above a single atom at the step edge in an axially symmetric orientational potential barrier, which confines the molecule to rotate in a two-dimensional manner. Our measurement...

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