Ion clusters in He–Co and Ar–Co glow discharges

Positive ion spectra from the positive column of a discharge in a mixture of He–Co with different amounts of O2 are presented. Without O2 the positive ions are CO+ and clusters of the type CO+⋅Co⋅Cn (n=0,1,...,11). The proportion of these clusters is about 75% of the positive charge carriers. Upon addition of small quantities of O2 (few parts per thousand of the total pressure) the ion clusters disappear gradually, starting with the higher mass clusters, while O2+ becomes the dominant positive ion. It has been found that in Ar–Co discharges the addition of Ar causes an increase of cluster density, indicating the importance of three-body processes for cluster formation. A strong dependence of cluster concentration upon gas temperature has been found.