Breakdown diagrams

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Figure 11. Breakdown diagram of ethylene. Abscissas given for ions 28 +, 27+and 26 + indicate outer and inner appearance potentials obtained by J. E. Collin (6)...

Breakdown diagrams can be constructed on the basis of charge exchange mass spectra [812, 813]. In the ideal case, these would be identical to those obtained by photoion—photoelectron coincidence, provided the sampling intervals, and t2 in eqn. (9), were the same in the two experiments. 

Breakdown diagrams and lifetimes of metastable ions are covered here. Certain translational energy releases are discussed, but fuller coverage of energy releases is given in Sect. 8. 

Breakdown diagrams of CH4 and CD4 have been determined by a number of workers [131, 132, 797, 799, 877] and are explicable within the framework of QET. It was found [806] that there was no kinetic shift affecting the appearance energy of (CH3)+ from CH4. On the basis of translational energy releases measured with CH4 (and CD4), it has been suggested [798] that all rotational energy is available to assist the decomposition to (CH3)+ but that only 2 degrees of freedom contribute to formation of (CH2)t (from the molecular ion), cf. Sect. 2.3. 

The tetrafluoromethane ion has also been found to decay before electronic randomisation has occurred [129, 769] (see Sect. 5.3 and 5.9 for other perfluorinated molecules). The breakdown diagrams for CF3X molecules (X = a halogen atom other than F) have been reported [690]. Translational energy release distributions have also been measured for these molecules and shown to be in agreement with the predictions of statistical theory (phase space theory) [691]. Carbonyl chloride and fluoride have been studied [451] (see Sect. 8). 

Breakdown diagrams of CH3OH and CD3OH determined by PIPECO appear to be explicable by QET [130, 658, 661, 662, 877] (see Sect. 8). Breakdown diagrams have been determined for formic acid [660, 877]. 

Breakdown diagrams determined by threshold PIPECO have been reported for C2H4 and C2D4 [804] and C2F6 [446], C2H2 [270, 877],... 

Breakdown diagrams have been determined for acetone both by threshold and fixed wavelength PIPECO [154, 330, 619, 692, 797]. Perdeuteroacetone has also been studied [619]. 

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... 

Breakdown diagrams obtained using fixed wavelength PIPECO have been reported for NH3 [877] and SF6 [769] and a breakdown diagram using threshold PIPECO has been reported for BF3 [36], The results for SF6 evidenced decomposition occurring prior to randomisation of electronic energy [769]. 

The result of another type of PEPICO experiment is the breakdown diagram, which is a plot of the fractional abundance of the ions formed by the dissociation of energy-selected parent ions as a function of the photon energy. It is also represented as a plot of branching ratios to the various dissociation products as a function of the precursor ion internal energy. The breakdown diagrams are obtained directly from a series of PEPICO mass spectra at several photon energies. 

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