OPEN SOURCE FOR EXCITED SPECIES OF ALKALI ATOMS AND IONS USING DIFFUSION THROUGH A THIN METAL FOIL

A novel type of source for excited species of cesium is described. It employs an effusive flux to the back of a metal foil, with the excited -state emission from the other side of the covered foil. The foil is k ept at a temperature of 1000-1900 K by direct heating with an electric current. From the foil, various excited species of ions and atoms, so me of which are rather short lived, are emitted at large densities. On ly the species observed at a distance of at least 40 mm from the sourc e emitter are of interest here. Excited species are observed by both s teady-state and pulsed field ionization, at current densities up to 10 (-4) A cm-2, which is several percent of the total beam flux. Positive and negative excited ions can also be observed in the flux from the s ource, by their ability to emit electrons with energy-independent effi ciency at impact on surfaces. The corresponding flux densities are up to 10(-5) and 10(-4) A cm-2, respectively. Doubly excited atoms can be observed as Cs2+ after field ionization in a mass spectrometer. They also produce ions after energy transfer to small molecules.