| Abstract
| - The adsorption of Ca atoms on pristine and electron-irradiated poly(methyl methacrylate) (PMMA)surfaces at 300 K has been studied by adsorption microcalorimetry, atomic beam/surface scattering, andlow-energy He+ ion scattering spectroscopy (ISS). On pristine PMMA, the initial sticking probability of Cais 0.5, increasing quickly with Ca coverage. Below 0.5 ML, the heat of adsorption is 730−780 kJ/mol,much higher than Ca's sublimation energy (178 kJ/mol). The Ca here is invisible to ISS, which is attributedto Ca binding to ester groups below the CH3/CH2-terminated PMMA surface. The adsorption energyincreases with coverage, suggesting attractions between neighboring Ca−ester complexes. Above 0.5ML, Ca starts to grow as three-dimensional (3D) Ca clusters on top of the surface, which dominate growthafter 2 ML. It is proposed that each Ca reacts with two esters to form the Ca carboxylate of PMMA, becausethis reaction's heat would be close to that observed. The total amount of Ca that binds to subsurface sitesis estimated from the integral heat of adsorption to involve 4−6 layers of ester groups. Exposing the PMMAsurface to electrons increases Ca's initial sticking probability but lowers its adsorption energy. This isattributed to electron-induced defects acting as nucleation sites for 3D Ca islands, whose growth nowcompetes kinetically with Ca diffusing to subsurface esters. Consequently, only two layers of subsurfaceesters get populated at saturation. The heat eventually reaches Ca's bulk heat of sublimation on all PMMAsurfaces, where pure, bulk-like Ca thin films form.
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