The ionization of caesium vapour by the method of space charge amplification

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Abstract
The production of positive ions following the irradiation of caesium vapour with photons whose energy is less than the atomic ionization threshold was investigated with the aid of a space charge limited diode. An amplification factor of 105-106 was observed for the diode over the range of temperatures from 400-500 K and was ascribed to the trapping of ions within the space charge potential well. The space charge released is related to the time spent by the ion in the trap. Data on the ions are compared with total absorption cross section measurements and the ionization potential of Cs2 is found to lie within the range 3.60 to 3.71 eV. Ions formed by the associative ionization processes Cs+hv to Cs*, Cs*+Cs to Cs2* to Cs+s+e, Cs*2 to Cs++Cs- are analysed in terms of potential energy curves for the ionic and excited molecular states. The dissociation energy of Cs+2 is estimated to be 0.66+or-0.07 eV.

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