PHASE-TRANSITIONS, SURFACE-STRUCTURES, AND ADSORBATE BONDING IN THE H/MO(100) CHEMISORPTION SYSTEM

Low energy electron diffraction (LEED) and surface infrared spectroscopy (SIRS) have been used to investigate the phase diagram of the H/Mo(100) chemisorption system, for hydrogen coverages up to saturation in the temperature interval 100 K < T < 350 K. A strikingly large number of surface phases are observed as the temperature and coverage are varied. We find that the hydrogen atoms bond in bridge sites under all conditions while each phase is due to a distinct reconstructed geometry of the Mo(100) substrate. Results obtained from the combination of LEED and SIRS show that most of the phase transitions observed for this system are first order, involving regions of coexisting phases. In particular, for low coverages, it is found that formation of dilute hydrogen islands occurs; the effective H-H interactions which lead to this can be explained by means of a substrate-phonon-mediated interaction mechanism. Possible substrate surface structures for each of the observed phases are considered. Finally, the results obtained in this work are compared to those from previous studies of the closely related H/W (100) system.