Synchrotron X-Ray Diffraction Studies of Phase Transitions in Physisorbed Monolayers of Rare Gases on Graphite

This study is an investigation of phase transition in monoatomic layers adsorbed on graphite. Such effects can be considered physical realizations of two-dimensional systems. The experimental technique used is synchrotron X-ray diffraction. A scattering geometry which utilizes a position-sensitive detector is developed and used for the experimental studies discussed below and carried out at the DORIS storage ring at Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany. Systems which have been investigated include the commensurate-incommensurate phase transition in krypton monolayer. By adjusting the spreading pressure in the krypton layer by means of a coadsorbent deuterium gas it has been unambiguously demonstrated that at low temperatures the phase transition is of first order. A melting study of incommmensurate argon monolayers demonstrates an experimental verification of the possibility for having a continuous melting transition in two-dimensions. Mixtures of two-components have been investigated for their phases. No (chemical) order-disorder transition is seen. A discussion is given on this lack ot a chemical order. This lack is utilized to study the commensurate-incommensurate phase transition driven by average particle size. Finally, a special low-temperature phase is identified in a xenon monolayer which is diluted with freon.