Reconstructive phase transitions and effective adsorbate-adsorbate interactions: H/Mo(100) and H/W(100)

Low-energy electron-diffraction and infrared-absorption measurements of H/Mo(100) and H/W(100) surfaces show that, at low hydrogen coverages, the effective H--H interactions are very different for the two substrates, leading to island formation on Mo(100) and to a uniform H layer on W(100). The results can be explained by a phonon-mediated interaction mechanism for the H adatoms, according to which the difference in behavior is due to the difference in the coupling between substrate distortions induced by the individual adatoms. It is concluded that a realistic treatment of the adatom interactions should go beyond the rigid-substrate model and include the crucial role of soft phonons. 21 ref.--AA