Structural Evolution of a Cyclooctatetraene Adlayer on Cu(111) during Isothermal Desorption

Here, the use of helium diffraction patterns to study desorption processes is explored as a novel extension to traditional methods based on helium specular reflection. The sample, cyclooctatetraene, adsorbed on Cu(111) provides a rich but complex structure. The modulation of cyclooctatetraene by Cu(111) is manifested as a convolution in the diffraction pattern, displaying an averaged super cell symmetry of (7$\sqrt{3}$ x 7$\sqrt{3}$)R30°. The adlayer expands during isothermal desorption, and the change in lattice constant provides a direct measure of the coverage as a function of time. We find a desorption energy of 0.96 +/- 0.01 at saturation of the first layer and an upper limit of 1.62 +/- 0.07 for isolated molecules. These values and details of the assigned structure indicate chemisorbed molecules with a planar conformation.