Chemi-ionization uses the stored potential of an excited atom rather than kinetic energy to transfer a valence electron in low energy collisions between two neutral atoms. One result of chemi-ionization can be an ion pair, with the excited atom losing an electron to the collision partner. This process was recently adopted to explain the anion production of the cesium sputter ion source, particularly as applied in accelerator mass spectrometry (AMS). Neutral sample and Cs atoms are both sputtered from a sample surface to form an infrared-dense (1012 - 1014 cm-3) plasma which contains Cs atoms excited by secondary sputtered electrons if the sample is recessed within the sample holder. Alternatively, the Cs is excited to specific states using lasers. The theory immediately explained the low anion production of elements such as Ca, Be, and Al which have low electron affinities (EA) as opposed to the high outputs of halogen anions. A blue glow is seen from recessed or pitted carbon samples producing high anion outputs (&gt;100 µA), indicating that Cs atoms are excited to the 7p state, where the Cs ionization potential nearly matches carbon’s EA. Elements in the samples or holders with EA similar to or lower than a desired ion reduce anion production by competition for or blocking excitation of the optimal Cs state. Oxygen and Al2 (both EA = 1.46 eV) are particularly a problem in ion sources fed by CO2 as well as for Al2O3 samples. Methods for reducing these problems are shown, along with the relevant excitations of Cs for maximal anion output from other AMS sample elements.

Chemi-ionization in the Cs sputter ion source: Selective ionization for AMS

253rd National Meeting (2017)

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