Critical Behavior of Chlorine on SILVER-100.

The quantitative study of the critical behavior of chemisorbed overlayers on single-crystal surfaces is a recent and important development in the field of surface science. We have examined the Cl/Ag(100) system with low -energy electron diffraction (LEED), Auger spectroscopy and work function change measurements with an especial interest in the critical behavior of the chlorine overlayer. We find that this system is well described as a two-dimensional hard square lattice gas system, slightly modified by the presence of weak second-nearest-neighbor (2nn) repulsions. Cl/Ag(100) behaves like a hard-square lattice gas in four important respects. First, the chemisorbed chlorine is highly mobile. Second, there is only one ordered phase, a C(2 x 2), and no observable island formation down to 90K. Third, the ordering of the C(2 x 2) overlayer as a function of coverage is independent of temperature. And fourth, the chlorine overlayer saturates at (theta) = 1/2 monolayer. These observations, and all others we have made, are consistent with a picture of this system as being one in which the adatom-adatom interactions are dominated by extremely large (effectively infinite) nearest-neighbor (nn) repulsions, the 2nn and 3nn interactions being quite weak. This is essentially a description of the hard square model. We have measured the critical exponent (beta) for this system as a function of coverage, the first such determination reported for a chemisorption system. We found (beta) to be 0.115 (+OR-) 0.025, in good agreement with the theoretical value of 1/8. We found the critical coverage (theta)(,c) to be 0.394 (+OR-) 0.007 monolayer, which is about 7% higher than the theoretical value of 0.368 monolayer. We interpret this as meaning that the Cl/Ag(100) system is not a perfect hard-square system, but is modified by the presence of slight ((TURN) 20 meV) 2nn repulsions.