Studies of Metastable Ion Transitions with a 180° Mass Spectrometer

Abstract

The decay of metastable ions through transitions giving an ionized fragment and a neutral radical has been investigated with a 180° mass spectrometer. Four aspects of the main problem have been studied. These are: (a) the decay of metastable ions within the ion source to produce a continuous distribution of ``metastable ions''; (b) an examination of the mathematical conditions relating dissociation beyond the ion-source exit slit to the formation of a ``metastable'' peak, to the shape of such a peak, to the cutoff imposed by the analyzer walls and to the length of travel during which detectable dissociation may occur; (c) an examination of the conditions for determining lifetimes with a 180° instrument; and (d) a consideration of the elements leading to the broadness of metastable peaks. The distribution function resulting from dissociation within the slit drops very rapidly and is ordinarily hidden within the peak due to the daughter ion. The formation of a metastable peak in a 180° instrument results from a low-order dependence of where the ion fragment resulting from dissociation hits the focal plane on the position of dissociation beyond the exit slit. The cutoff mass due to ions striking the walls of the analyzer tube may be readily calculated. In a CEC model 21–103 instrument there is adequate distance of free travel beyond the exit slit to allow the instrument to be used for lifetime measurements. Using repeller voltages up to 120 V, shorter lifetimes than those previously reported have been observed. For the m*=31.9 metastable peak from n-butane, for example, there is evidence that the metastable ions are created in at least three classes, each with its own lifetime. The shortest lifetime observed was of the order of 9×10—8 sec. Evidence has been obtained to indicate that the broadness of the observed metastable peaks is probably the result of the perturbation of focusing conditions by the increased angular spread in the ion beam.