Appearance potential electron impact

In the electron impact mass spectrum of C02, 0+ ions are formed with an appearance potential (AP) 19.1 e.v. according to the process ... 

A potential source for generating monomeric 23 is found in the l,2Xs-oxa-phosphetanes 21 and 22 18-20>, Their mass spectra contain peaks at M+ — 140, corresponding to [2 + 2]-cycloreversion to olefin and 23 however, the latter fragment (m/e = 140) was not found in the mass spectra. Although it cannot be explicitly stated whether this fragmentation is induced by electron-impact or thermally, a thermal reaction in the mass spectrometer certainly appears plausible. Such a reaction can indeed be accomplished on a preparative scale under milder conditions, as previously reported for 21 (R = C6HS) (Sect. 2.1). 

Electron impact studies10 of the appearance potentials and reaction cross-sections for the decomposition of W(CO)6 have shown that this decomposition occurs by a cascade mechanism... 

The appearance energy (formerly known as appearance potential) is a widely used concept in threshold mass spectrometry experiments, which involve measuring the minimum energy required to cause a certain process. However, there are a number of theoretical and practical problems associated with the determination of reliable values of H o(A+/AB). In the following paragraphs we summarize the discussion of this subject made by the groups of Traeger for photoionization [64,65] and Holmes for electron impact [66]. 

This experiment may be regarded as the forerunner of mass spectro-metric appearance-potential determination in that both are threshold techniques, that is they depend on slow variation in the energy supplied by the impacting electron until a change in the electron-molecule interaction is observed. Thus, just as the Hertz experiment did not distinguish between excitation and ionization potentials, mass spectrometric appearance potential measurements are subject to similar ambiguities in interpretation as between ionization and autoionization. 

Fio. 12. Fhotoelectron spectrum of methanol vapour using the helium resonance line (21-21 e.v.). Ionization energy increases from left to right. The adiabatic ionization potentials measured (Al-Jobomy and Turner, 1964) are indicated by vertical arrows, and can be compared with (probably) vertical I.P. values derived from electron impact appearance potentials by Collin (1961) (dotted arrows). 

A number of excellent reviews and books have included consideration of the fundamental electron impact ionization process, and the attention afforded the experimental measurement of ionization potentials and Augment ion appearance energies over the years is reflected in the comprehensive database of ionization potentials and gas phase ion enthalpies of formation published through the National Bureau of Standards in printed and electronic forms. In contrast, few absolute ionization cross sections have been measured. The most comprehensive compilation of molecular ionization cross sections are relative values measmed with a modified commercial electron impact mass spectrometer ion source using the cross section for Ar as a reference. ... 

An electron impact study of 1,3-dichlorotetrafluoroacetone has provided a value of 11.95 e.v. for the appearance potential of the CF2C1+ ion. There is at present no value for the ionization potential of the CF2C1 radical available but it would be safe to assume that it would lie between the limits of 8.78 e.v. for CC13 and 10.0 e.v. for -CF3, so that the C-C bond strength will not differ greatly from that of acetone itself. 

Herzberg63 has recently determined an upper limit for D(CH2—H) of 113 kcal. by combining the spectroscopically established ionization potential of CH2 (240 kcal.), with the appearance potential of CH24 from CH3i which was independently determined by Langer et al.83 and Waldron136 to be 353 kcal. The value determined in this manner is taken as an upper limit because of the possibility that CH2+ is formed in an excited state in the electron impact process. 

Three more estimates based on electron-impact studies have appeared recently. Hobrock and Kiser89 measured appearance potentials in CF2C1H and deduced AH298 CF2 = — 20 kcal/mole based on the assumption that the reaction was... 

Excess energy terms such as the value E in equation (8) must be either negligible or cancel for thermochemical arguments to lead to accurate bond dissociation energies. There is little evidence that excess energy terms and other errors of interpretation are not more important in more complex cases than the reaction (8). Further, the empirical assumptions used to determine ionization and appearance potentials by electron-impact methods work best for very simple molecules. 

The flash vacuum pyrolysis and the mass spectrometric fragmentation of a series of five methyl substituted phenanthrenes were correlated with each other in a quaUtatively predictable way.i ) The major thermochemical products for these compounds generally corresponded to one of the five most intense fragment ions in the compound s electron impact mass spectrum. The importance of products which corresponded to ions with high appearance potentials increased with increasing temperature in the thermolysis system. 

The identification of m e for particles In the mass spectrometer is a standard technique discussed in the text-books, but in view of the fact that there is at present much interest in dissociation energies measured by electron impact and relatively little general discussion of the measurements, we shall devote the two follo ving short sections to the discussion of measurement of appearance potential and of kinetic energy, and follow them by examples of the use of the method in Section 5.2.4. 

D N2) was determined as 9 79, 7 90, 7 42, 6 23, or 5-76 eV according to the assumed states of excitation of the nitrogen ion and the nitrogen atom produced. Spectroscopically obtained values for Z)(N2) are 9 76 or 7 38 eV, depending on the assumptions made. The retarding potential and appearance potential measurement alone is satisfactory for the interpretation of electron impact processes in homonuclear diatomic molecules, where there can be no doubt about the mass number of the ions. Possible confusion for heteronuclear diatomic molecules is not likely to be very great, but the method by itself is clearly inapplicable to dissociative ionization processes in polyatomic molecules, where the number of possible products is large. 

The electron impact results for carbon monoxide will now be discussed in some detail, as an illustration of the method applied to a diatomic molecule. The ion current against electron energy curves for C " from CO and for O from GO are given in Figure 5.2A.L It will be seen that the initial appearance potential of G + is well defined at 20-9 0 2 eV, and that the appearance potential of 0 is at almost exactly the same voltage, given by Hagstrum... 

The original electron impact work is due to Stevenson 71, He determined the appearance potential of various ions from 1-butene and found in particular that. 4(G2H5+) was 11 97 01 eV. This was combined with other appearance potential and thermochemical data as follows ... 

Electron impact results give 94 2 kcal (4 07 eV) for Z)(Me2GH H) Stevenson found the appearance potentials, A(G3H7+), in the mass spectra of uobutane, wopentane and 2 3-dimethyl butane, to be ll 0i 0 l, 10 84 0 1, and 10 79 0 1 eV respectively. These values were combined with the following data ... 

Electron impact measurements of the appearance potentials of various ions in the mass spectrum of methylamine and other amines by Collin 2 lead to upper limits to Z)(GN) in these compounds which appear to be very much larger than the most probable values. These probable values are not directly determined, but can be obtained from the heats of formation of the amines and the radicals concerned. Collin, for example, gives 140 kcal as an upper limit to D(CH -NH2), whereas the most probable value is about 80 kcal. 

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