Dissociation of MgO films under heat and electron bombardment and its effects on secondary emission

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Abstract
The dissociation of MgO films under heat and bombardment by 1000‐eV electrons has been measured in line of sight with a quadrupole mass analyzer. The samples were prepared by oxidizing Mg films in O18. In the range 230<TYO≫YMg. Electron bombardment strongly increases YMg but has no effect on YO; YMg is quadratic in the current density j and saturates at a value close to YO. Both atoms were detected as neutral species; Mg is evolved solely as the monomer, and oxygen is evolved mainly as the monomer, with YO18YO36?10. A model in which the evaporation rates of Mg and O are independently limited by charge transfer at the surface of the insulator is proposed. It accounts for our results as well as the dependence YO∝j2 reported by other investigators, and correctly predicts the dependence on primary energy Y (Ep) ∝j2Ep2.7. The secondary emission δ of cleaned films of MgO is insensitive to prolonged electron bombardment at 0.1 A/cm2. The application of heat alone in vacuo causes changes in δ which have been correlated with changes in surface stoichiometry.