Threshold energy, fragmentation

The experimental approaches described above are examples of relative methods, wherein a thermochemical property is measured with respect to that of a standard, or an anchor. The quality of these measurements ultimately depends on the quality of the anchor. Alternatively, there are methods of determining thermochemical properties, in which the energy for a chemical process is measured on an absolute basis. Among the more common of these are the appearance energy measurements, in which the threshold energy for formation of an ionic fragment from an activated precursor is measured. There are mauy differeut methods of activation that can be used. Some of these are discussed here. 

Notes "Threshold energies, E , in kcal/mol. Data from Klassen et al Fragment ion notation a, y, b2 as used in peptide sequencing work. For details see Harrison et al.l00c Threshold energies include correction for the kinetic shift. 

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... 

The low threshold energies for the production of D( S), 0( P), and 0( D2) show the importance of valence excited states in the BSD of neutral fragments [47]. The pathway for D( S) desorption probably involves D O D -I- OD. Ffowever, the thresholds for producing 0( P2) and 0( D2), which are the same within experimental error, are lower than the 9.5-and 11.5-eV thermodynamic energies required to produce 0( P2) + 2D( S) and 0( D2) + 2D( S), respectively. The low threshold values therefore indicate that the formation of 0( P2) and 0( D2) must occur by a pathway which involves simultaneous formation of D2. Kimmel et al. have in fact reported [46] a threshold for the production of D2 from D2O ice at — 6 to 7 eV, which supports this conclusion. Above the ionization threshold of amorphous ice, these excited states can be formed directly or via electron-ion recombination. 

Note here the relatively long wavelength of the threshold energy, and note also that the O2 fragment is itself formed excited, although that is not the immediate point of interest. The noteworthy feature is that excited atoms are able to participate in processes such as... 

The threshold energy for the dissociative electron attachment process e" + SPg(g) = SF "(g) + F(g) has been measured directly by means of mass spectrometry ( .- ). In addition, mass spectrometric studies (5) have been reported on various electron transfer reactions which provide Independent estimates of the enthalpy for the above process. Presented below is a summary of the results derived from these measurements. Also included in the summary are results obtained from an electron-impact study ( ) of the process e + SFgCl(g) = SFg (g) + Cl(g). We assume that the fragment-ions are formed in each process with no excess or kinetic energies. As a result, the derived electron affinities represent lower limits to the true value. Other reported values of EA(SFg) include >1.465 eV (9), 3.2 eV (5) and 3.66 0.04 eV (10). The first two results are based on charge-transfer studies (5-9) while the latter value represents a direct measurement of EA using the magnetron technique (10). 

Properties of afomic and molecular states, the latter along the potential energy surfaces (PES) that are known up to their fragmentation thresholds, rather than at just the region of equilibrium geometries. These include rates of fransitions befween states. 

The threshold energy for a particular fragmentation channel satisfies the condition T = 0, and a particular fragmentation channel can be associated with a particular electronic relaxation route only if (elec) for that route is less than a critical value, E, which is obtained when (int) = 0, that is,... 

Irradiation of methyl bromide, again on a silver(lll) surface, follows the same pattern. The principal photochemical event is the fission of the C—Br bond with the bromide being retained on the surface and with expulsion of the methyl fragment. The study reports that the threshold energy for the C—Br bond fission was higher for multilayer systems... 

An ionization-efficiency curve is a plot of the decrease in intensity of a certain peak (molecular ion or fragment) as the ionizing energy is lowered. There is usually a particular threshold energy below which a negligible number of ions appear this is the ionization potential. 

One common conceptual denominator of the various publications on resonance (nonstationary) states is the association of the appearance of a resonance with a quasi-localized single- or multi-particle wavepacket whose energy is above the fragmentation threshold. This means that, on resonance, there is an "inner" and an "outer" (asymptotic) component of the wavefunc-tion, each contributing in a different way to the intrinsic characteristics of the state and to the quantity which is measured. 

---