METHODS FOR MEASURING MEAN LIFETIMES OF LONG-LIVED MOLECULAR-IONS FORMED IN FAST COLLISIONS

The direct measurement of the mean lifetime of molecular ions in a fast-beam (0.1-1 MeV) which were produced in collisions with a thin target requires methods which are different from the ones used for metastable atomic cases. This is due to the nature of the molecular ions breakup. The kinetic energy released in the dissociation is sufficient to spread the fragments on a relatively large area on the detector plane, thus special care and appropriate detectors have to be used to ensure the detection of all fragments. Several experimental techniques to determine the mean lifetime of long lived molecular ions are described. They are based on evaluating the ratio of dissociated molecular ions to the non-dissociated ones as a function of the distance they traveled. Measurements of the mean lifetimes of the recently discovered long lived rare gas dimers, NeAr2+ and HeNe2+ are used to describe these methods.