Phys. Rev. Lett. 83, 124 (1999)
K. Svensson, L. Bengtsson, J. Bellman, M. Hassel, M. Persson, and S. Andersson,
Department of Applied Physics, Chalmers/Goöteborg University, S-412 96
Goöteborg, Sweden
We report experimental and theoretical evidence of H2 adsorbed in a confined quantum rotor state on a stepped copper surface. Rotational transitions of step-adsorbed para-H2 and ortho-H2 observed in electron-energy-loss measurements, occur close to the energies expected for an ideal 2D rotor. Dramatic enrichment of adsorbed ortho-H2 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 two-dimensional manner.