Mark Edward Lagus

Electron impact excitation out of the metastable levels of argon into the level

We have measured the direct cross section for electron impact excitation out of the metastable level ( in Paschens notation) into the level of argon from threshold to 800 eV. The direct cross section is at 10 eV.

APPARATUS FOR MEASURING ELECTRON-IMPACT EXCITATION CROSS SECTIONS USING FAST METASTABLE ATOMS PRODUCED VIA CHARGE EXCHANGE

An apparatus for measuring absolute cross sections due to electron‐impact excitation out of the metastable levels of rare‐gas atoms via the optical method is described with the focus specifically on excitation out of the 2 3S metastable helium level. The metastable helium target (He*) is prepared by charge exchange between 1.6 keV He+ ions and cesium vapor. An electron beam crosses the fast metastable beam target at a right angle and the fluorescence is collected at right angles to both beams. The charge transfer reaction produces He atoms mainly in the n=2 He levels. Because the target contains a negligible ground state He fraction, we can measure excitation cross sections from excitation threshold up to an arbitrarily high energy (keV regime) which represents a major improvement over previous metastable excitation cross sections measurements. The He* target density is extremely small (∼106 atoms/cm3) yielding minuscule signal rates. We describe steps taken to maximize the signal‐to‐noise ratio. We discu...