ADSORPTION OF HG ON CO, O, AND H ADSORBED ON W(110)

The adsorption of Hg on O/W(110), CO/W(110) and H/W(110) was studied by work function, UPS, XPS, LEED, energy loss and thermal desorption measurements. Hg adsorbs on these surfaces without causing geometric changes in the underlying chemisorbate. The monolayer coverage of Hg on these surfaces is the same as on clean W(110), and was measured to be approximately 1 x 10(15) Hg atoms/cm2 or Hg/W = 0.72. One monolayer of Hg screens the O-W and H-W dipole moments nearly completely and gives work functions almost identical to that of multilayer Hg deposits on clean W(110). In the case of CO screening is incomplete. In all cases the Hg core levels are unshifted relative to Hg/W(110), suggesting that the Hg layer is in electronic "communication" with tungsten and that a common Fermi level exists. An Hg surface plasmon at approximately 7 eV, not seen on clean W(110) for less than 2.5 Hg layers. appears slightly above Hg monolayer coverage in all cases, but is shifted upward in energy from 7 to 8.5 eV on O/W(110). There is no wetting of higher than the first Hg layer for O/W(110), and only partial wetting of higher layers in the other cases. The Hg desorption temperature from Hg1/W(110) is 500 K, and is reduced to 250 K on O/W(110) and 200 K on CO/W(110), but is 500 K on H/W(110). For Hg/H-1/W(110) the desorption spectrum of H-2 is strongly affected with 40% of H2 desorbing at very low temperature, approximately 200 K, the remainder more or less as from clean W(110). O2 and CO are not adsorbed on Hg1/W(110) but H-2 apparently can diffuse through several Hg layers to become dissociatively adsorbed on tungsten. However for preadsorbed Hg the 200 K H-2 peak is suppressed and only the fraction corresponding to the 450-500 K peak. i.e., 60% of what is seen on clean W(110) is adsorbed. These results indicate that despite its closed 6s2 shell atomic configuration Hg can behave very like bulk Hg metal in monolayer form on some, but apparently not all surfaces.