Ionization onset

Figure 8. Photoionization efficiency curve for FeO near the ionization onset, corresponding to production of FeO X, E.

Another consequence of the stronger interactions upon ionization is that the equilibrium geometry of the ionized complex may differ signihcantly from that of the neutral states. Broadened ionization onsets are frequently attributed to the spectral superposition of ionization into several vibrational levels for which Franck-Condon factors are more favorable. As a result, the adiabatic ionization potential may be considerably lower than the vertical potential, and the observed ionization onsets may occur above the adiabatic potential. Another factor to be considered is the conformation-dependent efifect, due to the different conformations of the solvent molecules. The most populated form of a complex may involve a less stable form of the solvent. After photoionisation, the lowest-energy dissociation channel in the complex ion leads to the most stable form of isolated solvent, which has to be taken into account for the estimate of the binding energy. 

A study of the photoionization of tetramethyl-/ -phenylenediamine (TMPD) in solution [49] showed the dependence of the wavelength of ionization onset on the energy Fq of the sol-... 

According to precise photoconductivity measurements, the ionization onset, which is usually taken as the ionization potential, is ca. 1.5 eV lower in liquid alkanes than in the gas phase [38]. The ionization potentials in liquid and gas phases (7/ and Ig, respectively) are related by the equation ... 

Af//°nj(4) has been obtained experimentally from the dissociative ionization onset for the 2-propyl chloride, bromide and iodide PFI-PEPICO TOF. The experimental value of the PA of propene leads to a very close result. These data are summarized in Table 1. The reader is referred to Ref. 102 for an extremely careful discussion on the experimental and calculated values of Af//°ni(4). High-level theoretical calculations using various methods," are also in good... 

Figure 5.7 The dissociative ionization onsets for CH4 loss from energy selected iso-butane ions obtained by PEPICO. The shift in the crossover energy is the median thermal energy removed when the room-temperature sample (effusive beam) is replaced by a seeded beam (20% C4H,o in 400 Torr Ar). The cw nozzle had a diameter of 180 pm. Taken with permission from Weitzel et al. (1991).

FIGURE 1 Ionization cross section o-/(e) for N2 and SFe close to the ionization onset /. Electron-scattering cross section as a function of e for N2 and electron-attachment cross section cra(s) for SF0. Normalized electron energy distribution function E/N) as a function of s for N2 at two values of E/N. [From Christophorou, L. G., et al. 

Most other studies on multiphoton ionization were carried out on solutions. Early work is due to Pilloff and Albrecht (1968), who studied the biphotonic ionization of TMPD (N,N,N N" tetramethyl-p-phenylene diamine) in 3-methylpentane. They found a quadratic dependence on the light intensity indicating that only virtual states were involved. Siomos et al. (Siomos and Christophorou, 1980 1982 Siomos et al., 1981) investigated the two-photon-induced ionization threshold of several aromatic molecules in n-pentane. The values obtained were lower than the corresponding thresholds measured in single photon ionization experiments. In a later paper Faidas and Christophorou (1987) attributed these lower values to three-photonic processes near the threshold which were not identified in the earlier work. For fluoranthene in tetramethylsilane they report a threshold of 5.7 eV. In another more complete study two-photon ionization onsets were reported for several aromatic molecules in various hydrocarbons (Faidas and Christophorou, 1988). Some of their results are compiled in Table 7. Within the variation of the relative dielectric constant and the limits of error, the polarization P+ energy is constant. 

The energy shift in and the ionization onset I has generally been treated within the free-electron model of Fermi (1934). In this simple... 

In contrast to the situation on electron impact ionization, there have been some studies on the photoionization processes and energetics of molecules in dense gases [e.g., see Reininger et al. (1984) and references quoted therein] and in liquids [e.g., see Lipsky (1981), Schmidt (1984), Christophorou and Siomos (1984), and references quoted therein]. The ionization studies in dense gases have shown that the ionization onset (photoconductivity threshold) I(N) at a number density N can be represented by (Raz and Jortner, 1969)... 

In Eq. (20) Iq is the low-gas-density adiabatic ionization onset, (a negative quantity) is the polarization energy of the positive ion produced in the photoionization process, (a negative or positive quantity) is the ground state energy of the photoionization electron, and A E is a small quantity associated with the broadening of the valence levels of the isolated atom (molecule) in the dense gas or the liquid. 

Apart from the higher resolution relative to similar electron ionization experiments, photoionization has an additional advantage in that there is a finite cross section at threshold, making it easier to detect the actual ionization onset. This also applies to unimolecu-lar fragmentation reactions and is a result of the different threshold laws for the two ionization processes. An ionization efficiency curve produced by photoionization is comparable to a first derivative of its electron-ionization counterpart. 

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