HIGHLY EXCITED-STATES OF CESIUM FROM A DIFFUSION SOURCE - GAS-PHASE IONIZATION REACTIONS

The Cs flux from a diffusion source of the open type contains excited states which can give ions by field ionization [E. Wallin, T. Hansson and L. Holmlid, Int. J. Mass Spectrom. Ion Processes, 114 (1992) 31]. The flux further contains small clusters which decompose by the action of an electric field. In the present experiments we study the processes caused by collisions of these excited states of Cs with gas molecules at pressures less than 3 x 10(-5) mbar by time-of-flight mass spectrometry. Cs2+ ions can be observed which indicates that doubly excited states are present. Reactions with N2, O2 and CO2 give ions such as N2+, N+, O2+ and O+ with large cross sections which require energies in the range 12-25 eV. Also He+ can be observed. The time position of the gas ion peaks shows that the corresponding ions are formed in the gas phase around the pulsed grid. The ionization reactions may be due to three different kinds of excited species: electronically excited clusters or doubly excited states, which may be interrelated through field induced cluster dissociation processes, or fast excited states which have been observed recently in other experiments.