Ellipsometric study of the two dimensional phase of physisorbed rare gases.

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Quasi-two dimensional behavior is observed in the system of rare gases physisorbed on the atomically flat surface. Statistical thermodynamics of this system has been investigated experimentally by various methods: (1) absorption isotherms (2) LEED, X-ray and neutron diffraction, (3) calorimetry, (4) NMR, etc. But it has been impossible to compare, for example, the results on the single crystal surface measured by LEED with the well-extablished phase diagram on the exfoliated graphite by the adsorption isotherm measurement. This is mainly because of experimental restriction.The purpose of this project is to establish a new experimental method in this field, that is, to adapt ellipsometric techniques to the study of physical adsorption. Ellipsometry is believed to give a new aspect in this field because of the following merit: (1) a small amount of adsorbed species, as thin as 1/100 atomic layer, on a single crystal surface can be detected, (2) non-destructive to weak bonds of a physisorption system,(3) no limitation on the operating gas pressure (from 1 atm to ultrahigh vacuum).The experimental apparatus which was constructed in the project has the following features.(1) The vacuum chamber was made of an aluminum alloy and its final pressure is lower than 10^7 Pa. (2) Sample temperature can be controlled between 20 K and 150 K with the stability of 0.01 K. (3) Sample surface is cleaned by ion bombardment in Xe glow discharge followed by annealing. We chose Xe/Ag(111) system as the first sample. Preliminary experiment on multilayer adsorption of Xe showed step-like dependence of <delta> on Xe pressure in gas phase. Each step corresponds to the formation of 1st, 2nd, and 3rd atomic layer. The parameter <delta> measured by null-ellipsometry is reproducible within <plus-minus> 0.2 ゜ , which value corresponds to the sensitivity of 1/10 atomic layer.