Mass spectrometry finds widespread application in the analysis of ion distributions in different kinds of plasmas. In DC discharges, the ions reaching the cathode are usually collected by means of a sampling orifice and focused with a set of electrostatic lenses onto the entrance of a mass filter. Mass spectra of ions coming directly from the plasma are not affected by the fragmentation problems at the detector that plague the mass spectrometry of neutrals, but the recorded mass distributions are not entirely free from ambiguity. In particular, ions with the same mass/charge (m/q) ratio, but with different chemical composition, will contribute to the same peak. This can be a problem especially for discharges in gas mixtures that can lead to a variety of different products. Additional insight can be derived from the measurement of ion-energy distributions. These energy distributions are largely determined by the acceleration of the ions in the sheath region between the plasma and the cathode, but they may also carry specific information about ion generation mechanisms within the plasma or about collisional processes in the sheath, which can in turn be of help for the identification of different ions with the same m/q ratio.

This work has been funded by the DGI (MEC) of Spain under grants ENE 2006-14577 and FIS 2007-61686.

3 pages, 2 figures.-- Printed version published Aug 2008.-- Published in section JMS Letters.

Peer reviewed