THERMAL-DESORPTION SPECTROSCOPY OF MAGNESIUM FROM A CHEMICAL-VAPOR-DEPOSITED ALUMINUM-OXIDE SURFACE

The binding of vapor deposited magnesium atoms to amorphous aluminum o xide was investigated by thermal desorption spectroscopy (TDS) combine d with mass spectrometric detection of the desorbed species and Auger electron spectrometric examination of the oxide surface, as well as by sticking probability measurements. The TDS studies of magnesium desor ption from the aluminum oxide film showed that at coverages less-than- or-equal-to 7 x 10(13) CM-2, one first-order TDS peak was observed wit h T(m) = 489 K. An activation energy for desorption (E(d)) of 163 kJ m ol-1 was determined for this process at low coverages. A crystal equiv alent bond enthalpy model was used to calculate H(L), the bond enthalp y value for an adsorbed metal-surface oxygen bond. For Mg on alumina(a m) the calculated value 2H(L) = 166.2 kJ mol-1 agrees with the experim ental value and indicates that two magnesium-oxygen bonds are broken i n the desorption process (E(d) = 2H(L)). In a separate experiment Mg w as deposited on an alumina film and then exposed to 02(g). TDS indicat ed that oxygen exposure results in increased thermal stability of adso rbed Mg.