Impact Ionization and Photoionization

In Eqs. (4.8a, b), fef is the rate of fluorescence, k is the intersystem crossing rate (Sj—-Tj), k is the radiationless decay rate to the ground state (S1— S0), 0( is the quantum yield for producing the triplet, and T is the probability of phosphorescence from that state. Note that in many hydrocarbons, 0 =1 — 0f, and for a group of carbonyl compounds, 0t = 1. 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

Photoionization refers to the ionization of a neutral entity. Its inverse is the radiative capture of a free electron. By virtue of detailed balancing, the cross section of radiative capture,cr, and that of photoionization, 7., involving the same initial and final states, i and f, are related by 

TABLE 4.3 Luminescence Lifetimes and Quantum Yields (qy) of Some Selected Compounds 

V is the frequency of radiation and a . and (Ox are the statistical weights of the initial and final states. It should be remembered that Eq. (4.9) refers to the photoionization cross section, not the total photoabsorption cross section (see Sect. 4.2). 

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Figure 9 Idealized threshold laws for electron impact ionization and photoionization.

Fig. 26. Product branching ratio, (frYCR 4>yc3H4> from collisions of Y + propene as a function of con obtained using 157 nm photoionization (squares) and electron impact ionization (triangle).

It is apparent that in both cases energy E is deposited in [AB+ +eej] and that, as in the case of excitation, the photon energy is analogous to the electron energy loss. However, since there are now two electrons sharing the excess energy in electron-impact ionization, it is necessary to use time correlation (coincidence techniques) for the simulation of photoionization... 

Silylenium ions are common in gas-phase organosilicon chemistry, where they may be generated by various techniques including electron impact (29-33), photoionization (34-36), chemical ionization (37-45), collision-induced dissociation (25,46), and chemical-nuclear methods (15). Although this article is concerned with reactions in solution, a short account of gas-phase studies cannot be omitted, since they provide important information about chemical and physical properties of silylenium ions, which are so elusive in condensed phases. 

On the other hand, Hansen et al. [28] measured A -x-ray intensity ratios for various elements following A -capture decay of radioactive nuclides and pointed out that the KP/Ka ratios by electron capture (EC) decay are considerably different from those by photon and electron impact ionization. Paic and Pecar [29] found that the Kp/Ka ratios for Ti, V, Cr, and Fe by EC are smaller by almost 10% than those by photoionization (PI), but no appreciable difference was observed for Cu and Zn. A similar excitation mode dependence was measured for Mn by Arndt et al. [30]. They stated that the reason for the difference is due to the excess 3d electron in EC and the large shakeoff probability in PI. Rao et al. [31] also observed smaller KP/Ka intensity ratios by EC for Mn and Fe. Since no appreciable difference was found for high-Z elements, they concluded that the difference observed for 3d elements can be ascribed to the chemical effect. It is usual that the chemical forms of the samples for EC measurements are different from those for PI. In order to elucidate the excitation mode dependence on the Kp/Ka ratios in 3d elements, it is necessary to perform theoretical calculations which takes into account the chemical effect as well as the difference in the electron configurations. 

Binding energies and momentum distributions for electrons in the valence orbitals of CH4 have been presented 104 these are the first data obtained by the (e, 2e) reaction (electron-impact ionization with complete determination of the kinematics of the incident and emitted electrons) for a polyatomic molecule. The vertical I.P. s obtained for the lt2 and 2ai electrons of CH4 are compared with values from u.p.s. and photoionization studies in Table 6 the data derived from the three techniques are in satisfactory agreement. 

Two experimental parameters are critical to properly interpret the experiments. First, the molecular formula of the cluster producing a spectrum must be known. Second, the laser intensity along the molecular beam must be determined. Determination of the cluster formula is aided by mass selective detection. For polyatomic van der Waals molecules, however, fragmentation of the cluster upon ionization is often extensive. For electron impact (2,) or photoionization (14) of the dimer of ethylene, for instance, the parent ion is a minor product. Instead (loss of CH and (loss of H) dominate the... 

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

Cation radicals are formed by the removal of one electron from a neutral molecule. The result is the formation of a species which is at the same time a cation (the positive charge caused by the loss of an electron) and a radical (the remaining unpaired electron). One-electron oxidations may be achieved with a variety of chemical oxidants, e.g. concentrated sulfuric acid, by physical means, e.g. photoionization, pulse radiolysis, and electron impact (mass spectrometry), and by anodic oxidation. In this section we shall confine ourselves mainly to chemical oxidations, to oxidations on acidic surfaces, and to photoionizations. Anodic oxidation is dealt with more fully in section 2 and in Volume 12 of this series (Eberson and Nyberg, 1976). Formation by electron impact ionization will not be discussed here. 

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