The role of O and Cl adsorbates on the secondary emission properties of tungsten

Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na+ incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na+ impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum.