Repulsive interactions of potassium on Re(001)

Repulsive interactions of potassium on Re(001) were investigated using temperature programed desorption (TPD), work-function measurements, and optical second-harmonic generation (SHG). The TPD and work-function results were used for the first time to evaluate critically the validity of available electrostatic models for explaining simultaneously both sets of data. The activation energy for desorption in the limit of zero coverage was determined to be 68.4 +/- 0.1 kcal/mol, decreasing monotonically with increasing potassium coverage to 22.1 +/- 0.1 kcal/mol at a full monolayer (ML) coverage. The TPD data were fit well by a depolarization model which includes the adsorbate-surface distance. The work function decreases monotonically upon potassium adsorption until reaching a minimum value of 4.35 eV below the work function of the bare rhenium surface at a coverage of approximately 0.5 ML, after which the work function increased to an ultimate value of 3.00 eV below the work function of Re(001). The work function of the bare Re(001) surface was determined to be 5.26 +/- 0.05 eV. The work-function data were fit using a model which considers the change in work function assuming local contributions by the substrate and by the adsorbate, using the same parameters obtained from fitting the TPD data. SHG measurements were consistent with previous alkali-transition metal systems, including a characteristic resonant like signal enhancement at a potassium coverage of 0.3-0.4 ML. The SHG data taken during potassium adsorption and desorption complement information obtained by TPD and work-function measurements. (C) 1997 American Institute of Physics.