Dependence of Rare-Gas Adsorbate Dipole Moment on Substrate Work Function
The distinctly different behaviors of rare gas adsorded on various metals has been a puzzle for more than two decades. On refractory metals, the deduction of the metal work functions is unexpectedly large, while on simple metals, the effect is nearly undetectably small. The "switching" theory proposed by Flynn and Chen explains the general trend of the substrate dependent work function change reasonably well and also produces the observed linear dependence of the induced dipole moment on the heat of adsorption. The present research is to provide further experimental evidence for the theory and mainly focuses on the simple metals to supplement the paucity of data in this area. The Kelvin method is adopted for the relative work function measurements on freshly evaporated samples: Al, Mg, and alkali metals. Samples were held at about 15(DEGREES)K inside a liquid He cooled shielding can. Belljar pressures rose typically into the 10('-9) Torr range during metal evaporation and fell back into the 10('-10) Torr range afterwards. The effective pressure around the samples was certainly much lower. Rare gas dopant, Xe, Kr or Ar, was successively added to one face of the Kelvin electrodes from a calibrated molecular beam, with intervening measurements. The other face was left intact as reference which was driven by a bimorph at a frequency near 145 Hz. The electronic sensitivity for the measurements is better than 0.2 meV at a response time of a few seconds. The coverage dependence of work function changes was measured and the Topping model was employed to deduce the effective polarizabilities and the initial dipole moments. The data obtained in this work as well as those reported recently in the literature provides further support to the proposed "switching" theory in explaining the work function change due to the rare gas absorption.
- Pub Date:
- Physics: Condensed Matter