Low temperature ordering of sodium overlayers on RU(001)

The adsorption of alkali metals on transition metals can produce several technologically important effects, but only limited results have been reported on the geometrical structure of such adlayers, especially for adsorption temperatures below 300 K. We have examined the adsorption of Na on Ru(001) as a function of coverage and temperature using LEED to determine the adlayer structure and thermal desorption spectroscopy to characterize binding kinetics and relative Na coverages. The only Na LEED pattern observed following adsorption at 300 K was that of $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ structure which occurred near saturation of the first layer. However, Na adsorbed at 80 K produces a progression of distinct, ordered LEED patterns with increasing coverage which does not include the $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ pattern. These patterns result from increasingly compressed, hexagonal arrangements of adsorbate atoms which are uniformly spaced due to mutually repulsive interactions. The order-disorder transition temperature for each structure was also determined by LEED and used to develop a 2D phase diagram for Na on Ru(001). Ordered structures were observed only when Na thermally induced motion was sufficiently limited and the repulsive Na-Na interaction could force the uniform spacing of Na atoms. Thus, low coverage structures only developed where Na mobility was limited by low temperature. High coverage structures were stable to much higher temperatures since motion was inhibited by the high Na density.