Breakdown Graphs

Example For the case of 4-methyl-1-pentene, the breakdown graph, the internal energy distribution from the photoelectron spectrum, and the 70 eV El mass spectrum are compared (Fig. 2.22). [99] From the fragmentation threshold to about... 

Fig. 2.22. Relationship of breakdown graph (a), internal energy distribution from PES (b), and mass spectrum of 4-methyl-1-pentene (c). Reproduced from Ref. [99] by permission. John Wiley Sons, 1982.

From the breakdown graph of methanol (Fig. 6), it follows that when a ion in its ground state hits a methanol molecule, recombination is energetically possible, as RE is 11.26 eV. The molecular ion CH30H is then formed in a stable state. A metastable ion gives, on the other hand, fragmentation of the methanol ion due to the RF s 12.40 and 16.58 eV. The relative abundances, 60% and 40%, of the states seem therefore to follow immediately from the mass spectrum. 

Figure 8 shows the breakdown graph of ethane studied by von Koch/ Several appearance potentials seem to be very well defined in this graph. For this molecule, the agreement with the electron-impact appearance potentials " " is good. A remarkable feature, which cannot be deduced from electron-impact studies, is that the intensity of a fragment ion often becomes very small above a certain energy. 

The correspondence between the photoelectron spectrum and the ethane breakdown graph in Fig. 8 is obvious. The appearance of C2H4 and C2H5 corresponds to the first minimum in the photoelectron spectrum. Formation of C2H3 and C2H2 corresponds to ionization of a (tc -h n) electron. 

Methanol and other lower aliphatic alcohols have been studied in detail by use of the quasiequilibrium theory of mass spectra by Friedman et This was the reason that alcohols were the first larger molecules to be studied in the tandem machine at Stockholm. It is therefore interesting to find that the comparison of the breakdown graph and photoelectron spectrum of methanol in Fig. 10 indicates that the mass-spectrometric dissociation of at least this alcohol does not seem to be ruled by any statistical laws. 

Breakdown graphs can be used to compare the energetic demands of those different fragmentation pathways, hi addition, breakdown graphs help to correlate ion internal energy distributions derived from other methods such as photoelectron spectroscopy [107] with mass spectral data. 

Figure 2 Top photoelectron spectrum of ethylene. Bottom breakdown graph of ethylene determined by photoion-photoelectron coincidence measurements. Triangles and dots yields of C2H2" and C2H3 ions, respectively, obtained by collision experiments. The ground and excited electronic states of the C2H4 ion are denoted by X, A, B and C.

Malinovich Y, Arakawa R, Haase G and Lifshitz C (1985) Time-dependent mass spectra and breakdown graphs. 6. Slow unimolecular dissociatioon of bromobenzene ions at near threshold energies. Journal of Physical Chemistry 89 2253-2260. 

Figure 6 Breakdown graph of propane. The relative abundances of the ions indicated are plotted as functions of the sum of the IE of propane and the internal energy of the parent propane ion.

---