Dissociation of Solid SrO by Impact of Slow Electrons

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Abstract
Dissociation of solid SrO by impact of electrons having energy below 400 v was measured by a mass spectrometer method. Positively and negatively charged dissociation products were observed with equal sensitivity; no uncharged products were observed with an arrangement having poorer sensitivity. The fundamental dissociation products are Sr+, Sr++, and O+ with essentially no O−. Surface impurities such as CO, Cl, F, and H were also abundant. The only source of chlorine found was an impurity in the original SrCO3, of about 1 part in 105. However, its abundance in the ion current is about equal to that of oxygen, and the yields of Cl+ and Cl− are about equal. All particles must come essentially only from the surface layers. Yields of Sr+ and Sr++ increase with the square of electron density; yields of O+, Cl+, and Cl− increase, respectively, with the 1.5, 1.0, and 1.0 power of electron current density. The process is quite inefficient, the impact of 107 electrons giving about 1 fundamental dissociation product for 20 to 30 ma per cm2 of bombarding current. Yields are independent of electron energy from slightly above threshold to beyond 200 v, above which they decline slowly. Yields increase slightly with temperature, corresponding to activation energies of a few tenths of a volt. At least some of the chlorine on the surface probably exists in the uncharged form; threshold energies are too low to convert a Cl− on the surface to a Cl+ in the vacuum. Removal of O+, Sr+, and Sr++ ions is discussed in terms of a crystal composed of singly charged Sr+ and O− ions; a process involving two incoming electrons in rapid succession on a minute area is apparently required for removal of an O+, Sr+, or Sr++ ion.