The Isotope Effect in the Evaporation of Lithium, Potassium and Rubidium Ions

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Abstract
Abundance ratio measurements are necessarily complicated by the possibility of an isotope effect at the source. In the emission of positive thermions of lithium, potassium and rubidium the extent to which the ion ratio represents the atom ratio within the source can be estimated by observing the change in the abundance ratio as a function of the percent of alkali evaporated. In the case of lithium an initial ratio of Li7/Li6=11.60 was observed; the abundance ratio followed the computed ratio for ideal evaporation until the lithium was 40 percent exhausted, beyond which it fell below the ideal value. The abundance ratios of K39/K41=14.22 and Rb85/Rb87=2.59 remained almost unchanged during the process of exhaustion. It is apparent therefore that a correction factor needs to be applied to the ion abundance ratio for lithium, but is unnecessary for potassium and for rubidium. The atomic weights computed from the abundance ratios are Li=6.939, K=39.094 and Rb=85.46.

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