[Ag,]-ions emerging from a silver target under bombardment with energetic krypton ions have been measured for n ranging up to 60. The relative intensities of ions with odd numbers are strikingly favoured. The numbers n = 8 and 20 mark minima in the abundance of ions with even numbers. The observations are briefly discussed.The range of a magnetic mass spectrometer used to measure the yield of ions from a secondary ion emission apparatus previously described 1 was extended to higher mass ranges. It became possible to observe heavy clusters of silver with negative charge which contained as many as 60 atoms. These clusters emerged from a cesium-coated silver target bombarded with energetic krypton ions. Clusters tentatively identified as [Ags]-have been reported a few years ago 2. In this paper we present data for 4 < n < 30, where n is the number of atoms forming a cluster. The massspectra were not sufficiently resolved above this range to enable us to distinguish completely between pure silver clusters and contaminated ions.The measured intensities of heavy clusters decrease with a high power of n. It is, however, well known that the energy necessary to separate an electron from a negative single atomic ion does not remarkably differ from the work function of solid silver. It can therefore be concluded that the decrease in intensity is not due to the electron configurations but must depend on the probability for the emission of the duster. In order to get a rough estimate of this probability, the measured intensity function I(n) is split into two terms, the reduced intensity R(n), and a term describing arbitrarily the gross trend of the probability for the emission of the large body, as
The production of negative ions specifically depends on the chemical properties of the substance under consideration. This dependence influences the stability and existence of the particular heavy ions as well as the choice of the best procedure of their production. For substances which have high temperatures of evaporation it is difficult to produce the ions from a gaseous atmosphere, and it should bear some advantages to produce these ions directly from solid surfaces. We therefore tried to apply the secondary ion emission for this purpose. In this paper the practical realisation of an ion source of this type is described. A number of measurements using this source has been made to look for the optimum operation conditions and to search the periodic table for these ion species for which this source is particularly suited.Under ion bombardment a great variety of different ions is emitted from the target surface.The yield of perticular ion species, however, depends sensitively on the composition of the surface. It is known from measurements by KROHN [ I that the yield of negative ions is increased by evaporation of cesium onto the emitting surface. Also other surface contaminations show the same tendency, and it was observed by LELAND and OLSON [ 2 1 that the ion yield reduces in time shortly after beginning of the ion bombardment.These two effects -cesium activation and time dependence -led to the construction of the ion source presented here. It was concluded that the activating surface layer is removed under the ion bombardment by sputtering, and one could expect to achieve high, stationary ion currents if the activating surface layer was regenerated continously. The regeneration of this layer can be made by continuous evaporation of cesium onto this surface or by exposure of the surface to a suitable gaseous atmosphere. In both cases the vacuum pressure near the target is high, and therefore the negatinve ions leaving the surface are destroyed. In order to achieve proper activation without destruction of the negative ions, it was found advantageous to separate the place of regeneration of the activating surface layer from the place of negative ion emission. A rotating disc was used to transport the activated region to the place where the primary ion beam induces secondary ion emission. By this provision the vacuum at the emission place can easily be maintained [ 3 ].
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