Electron-stimulated desorption of lithium atoms from oxygen-covered molybdenum surfaces

We have measured electron-stimulated desorption (ESD) yields and energy distributions for lithium (Li) atoms from Li layers adsorbed at 300 K on an oxygen-monolayer-covered molybdenum surface as a function of electron energy and lithium coverage. The measurements have been carried out using a time-of-flight method and surface ionization detector. The appearance threshold for Li atoms is close to the O 2s level ionization energy of 25 eV and is independent of the lithium coverage. Above the 25 eV threshold the lithium atom ESD yield increases linearly with increasing lithium coverage. An additional threshold of 55 eV is observed in the lithium coverage range from 0 to 0.45; this appears to be associated with the Li 1s level ionization energy. For energies above the 55 eV threshold the incremental change of lithium atom ESD yield exhibits a maximum as the lithium coverage increases, and decreases to zero at lithium coverages greater than 0.45. Additional features for lithium atoms appear at about 40 and 70 eV, correlating with the molybdenum 4p and 4s ionization energies at Li coverages of 0.6 and 0.75, respectively. There are broad peaks at the latter energies indicating that desorption proceeds via a resonance process, in which the initial excitation may lead to a spectator electron in a quasi-bound state. The average kinetic energy of lithium atoms is several tenths of eV. It depends on the lithium coverage and electron energy. The ESD energy distribution for lithium atoms consists of a single peak with a maximum of around 0.18 eV for electron energies less than 55 eV. A second peak with a maximum of about 0.25 eV appears at electron energies greater than 55 eV. The data can be interpreted in terms of the Auger-stimulated desorption model, in which neutralization of adsorbed lithium ions occurs after filling of holes created by incident electrons in inner shells of substrate and lithium atoms.