Novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) by the use of temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), and spot-profile-analysis low energy electron diffraction (SPA-LEED). The desorption energy of deuterated cyclohexane (C
6D
12) is lower than that of C
6H
12. In addition, the work function change by adsorbed C
6D
12 is smaller than that by adsorbed C
6H
12, indicating that C
6D
12 molecules are slightly more distant from the surface than C
6H
12 molecules (vertical geometric isotope effect). These isotope effects originate from a shallower adsorption potential of C
6D
12 than that of C
6H
12. The lateral geometric isotope effect was also observed in the two-dimensional cyclohexane superstructures as a result of the different repulsive interaction between interfacial dipoles. The observed isotope effects should be ascribed to the quantum nature of hydrogen involved in the C−H⋅⋅⋅metal interaction.
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