Dissociative ionization process

The cross-section for dissociative ionization process (19.56) is experimentally known for v = 0 [83] (see also [14]). The cross-sections for v > 1 can be roughly estimated from the cross-section for v = 0 by using the I 1 scaling of ionization cross-section, where Iv is the transition energy from H (X v) energy level to the (H+ + H+) interaction potential at the internuclear distance equal to the equilibrium distance of (X v). 

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 indirect method was put forward by Stevenson in it the appearance potentials of the same ion produced from two different but related molecules are measured, and their difference is combined with thermochemical data to give the required dissociation energ) It is thus possible to calculate bond dissociation energies from appearance potentials and thermochemical data without a knowledge of ionization potentials. The essence of the method is to produce as the non-ionized partner in a dissociative ionization process of the type... 

There is an interpretation of the results which would allow us to obtain the high value of the heat of atomization of carbon, although it is rather a strained one. If the dissociative ionization process yields 0+, H2, H, and X, instead of 0+, 3H, and X, as originally assumed, the derived upper limit for the heat of atomization of carbon is increased by i)(H2) or about 103 kcal, thus becoming A/i (0 gas) <240 kcal. This allows any of the presently canvassed values for this quantity, if the appropriate amount of kinetic or excitational energy is assumed. This is 3 eV for Lq == 170 kcal, and correspondingly more for other estimates. It seems unlikely that this explanation is correct, particularly in view of McDowell s work, but the possibility exists. 

Rather different, and higher, values of the boron-boron bond energies were obtained by Dibeler and his co-workers from photoionization mass spectrometric studies of B2F4 and B2CI4 and the corresponding trihalides. Essentially, this work combines known heats of formation of the subhalides (106) with threshold energies for dissociative ionization processes such as... 

Blais and Cottin (1 ) have measured the appearance potential (AP) of SCl (g) from SClgCE) as 12.5 0.2 eV using the electron impact technique. Subsequent work by Hartmann et al. (2) using the same method led to AP(SC1 /SClg) = 12.2+0.1 eV which confirms the earlier results. We assume that the fragment ions produced in the dissociative ionization process e" + SClgCg) = SCI (g) +... 

A summary of the information available on the appearance potential of PFg from PF is presented below. We analyze these electron-impact results using the expression A H (0 K) = AP-E by assuming that the dissociative ionization process is PFg(g) + e = PFg Cg) + F(g) + 2 e . Values of A H and the ionization potential are derived by setting E in the above expression equal to zero. As a result, the values are listed in the table as inequalities to emphasize that they are limiting (upper) values. 

One can see from Table VI that the dissociative ionization process which is of greatest importance in isopropyl benzene and of overwhelming importance in t-amylbenzene is displacement of the alkyl group from the aromatic nucleus to form the alkyl ion. The large alkyl ion intensity is accompanied by a diminution in the intensities of (M + H)" " and (M -I- CjHj), and it is postulated that the alkyl ions are formed by alkyl ion displacement reactions ... 

Several dissociative ionization processes have been observed, as is presented below, with the various onsets in parentheses ... 

Metastable atom bombardment (MAB) is another suitable ionization option for gas-phase organic compounds [11,12]. In this approach, ionization occurs through Penning processes of the type shown in reactions (2.21) and (2.22) by bombarding the sample with a beam of metastable species. The process depicted in reaction (2.21) involves an electrophilic reaction of the metastable atom (A ) to produce an odd-electron molecular ion of the analyte. The excitation energy of A must exceed the IE of the analyte molecule. With much higher excitation energy, dissociative ionization process also takes place [reaction (2.22)]. 

Some reported AfH values of cations were derived from appearance energy (AE) measurements in dissociative ionization processes (10.1), either by photon impact or by electron impact. In their determination it is usual to follow the approximation [61], that two heat capacity terms in the full energy balance equation [62] cancel each other so that the 298 K AfH values of fragment ions mi can be obtained from measured AE s using (10.2) ... 

One of the most useful applications of peak widths in the TOF distributions is in distinguishing an ionization from a dissociative ionization process. This is particularly useful in the case of photoionization of clusters. An example of such a study is the case of ArCO", which can arise from a variety of process in a molecular beam expansion of Ar and CO gases, including those shown in Equations [18]-[20]. 

However, the first reaction can be distinguished from the dissociative ionization processes because its TOP peak is expected to be narrow, whereas the latter two reactions will impart kinetic energy to their fragments and thus result in broadened TOP peaks. This is illustrated in Pigure 4. The narrow peak with a width of 34 ns is due to reaction [18], while the broad peak with a width of 160 ns is due to the dissociative processes. By this approach, an onset of 13.03 eV for the ArCO" ion from ArCO could be measured. In addition, by measuring the width of the broad peak as a function of the cluster ion energy, it was possible to extrapolate the kinetic release in the second reaction down to the threshold (as shown in Pigure 5). 

Finally, it should be pointed out that dissociative ionization yields fragment ions with small to moderate kinetic energies. To describe the dissociative ionization process completely, these kinematic properties must be known. 

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