Direct Evidence of Abrupt Changes in Two-Dimensional Fluidity in Melting of Monolayers

Abstract

Debye-Waller factors $\exp \mathrm{}(-2W)\mathrm{}$ of monolayer films of butadiene iron tricarbonyl molecules adsorbed on basal-plane graphite surfaces have been measured in the surface-normal and surface-parallel directions. For each coverage $x=0.3, 0.7, \mathrm{and} 1.0$, the normal exponent $2W_{\perp }$ indicates strong substrate binding at all temperatures, while the parallel one $2W_{\parallel }$ increases abruptly at a definite temperature, indicating a first-order melting transition in the plane of the film.