Two-dimensional quantum rotation of adsorbed H-2

We report experimental and theoretical evidence of H-2 adsorbed in a confin ed 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 m easurements occur close to the energies expected for an ideal 2D rotor. Dra matic enrichment of adsorbed ortho-H-2 is observed at elevated surface temp eratures and hydrogen background pressures. Calculations reveal a molecule residing above a step atom and confined to rotate in a two-dimensional mann er.