PIPECO experiments

In these rather special PIPECO experiments, ions are being produced with selected internal energy and their lifetimes are measured. FIK achieves measurement of time, and PIPECO in its usual form and charge exchange fix energy. To both preselect energy and measure time is the desired, yet rare, level of experiment in mass spectrometry. 

There are disagreements between certain results obtained by PIPECO on metastable ions and the results for the same decompositions obtained in PIPECO experiments, which vary the source residence time [717, 718] (see Sect. 5). The origin of these conflicting results has, however, yet to be agreed upon. 

Rate coefficients k(E) have been obtained in this way for decompositions of benzene, thiophene and benzonitrile over an energy range of some electron volts. These were the first direct experimental determinations of rate coefficients, k(E), for ionic decompositions. Moreover, a range of energies was accessible, since the times at which rates were measured extended over the 3 orders of magnitude, which contrasts with the later PIPECO experiments in which rates have been measured only in the microsecond time-frame. 

Pyridine has also recently been studied in the special type of threshold PIPECO experiment (see Sect. 4.1.1), in which the residence time of the ion in the source is varied [719]. In terms of eqn. (9), the limit t2 on the sampling interval or observation window was variable. Breakdown diagrams for formation of (C4H4)t were determined at two residence times (t2 = 0.805 and 5.925 ps). Detailed analysis of the results suggested that the k(E) vs. E curve for this decomposition was somewhat steeper than indicated by the earlier PIPECO study [274], It was concluded that the transition state for... 

The decompositions of bromobenzene [717] and chlorobenzene ions [716] have been studied by the special PIPECO experiment using variable source residence times. In the case of chlorobenzene, increasing the residence time from 0.7 to 8.9 ps resulted in a shift (kinetic shift) in the breakdown curves by 0.4 eV. Detailed analysis of the effects of varying residence time provided information on the k(E) vs. E curve in the vicinity of 104—106 s-1. The k(E) vs. E curve obtained differed significantly [by almost an order of magnitude in k(E) at some energies] from the curve reported in the earlier PIPECO study of metastable ions [22], The initial analysis [716] placed the critical energy for chlorine loss at 3.40 0.05 eV, but this has subsequently been revised to 3.19 0.02 eV [717]. The transition state was found to be loose . 

If metastable ions are observed in charge exchange experiments, both the lifetime and the internal energy of the reactant are defined as in the analogous PIPECO experiment. The energy at which the intensities of the reactant and fragment ion in the breakdown curve become comparable may be used as a guide as to where to look for metastable ions,... 

PIPECO experiments have provided direct estimates of kinetic shifts. The limit of the observation window has been varied and the changes in the measured appearance energies observed. The limit t2 is the source residence time and was known in these experiments, being determined by a delay prior to applying the drawout pulse. Changing from 0.7 to... 

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