Photoionization, cross sections thresholds

The adiabatic ionization potential (1A) of a molecule, as shown in Figure 4.1, equals the energy difference between the lowest vibrational level of the ground electronic state of the positive ion and that of the molecule. In practice, few cases would correspond to adiabatic ionization except those determined spectroscopically or obtained in a threshold process. Near threshold, there is a real difference between the photoabsorption and photoionization cross sections, meaning that much of the photoabsorption does not lead to ionization, but instead results in dissociation into neutral fragments. 

The distinction between photoabsorption and photoionization is important, particularly near threshold, where the probability that ionization will not occur upon photoabsorption is significant. Thus, the ionization efficiency is defined by TJ. = a. /photoionization cross section and photoabsorption cross section, is related to the absorption coefficient a by a = n(7, n being the absorber density. 

The presented data illustrate the noticeable sensitivity of threshold values of the photoionization cross sections of confined atoms to the principal quantum number n. Indeed, as seen from Figure 5, the photoionization cross section anoticeably increases, whereas er 5 decreases, at threshold, compared to the free Ne atom. As a result, for confined Ne, the threshold... 

Mb. Note, also, that, away from the threshold, the confinement resonances in er1 <5 and a s emerge at different energies and have different resonance amplitudes. Caution since the data displayed in Figure 5 were obtained in the 5-potential model, the predicted amplitudes of confinement resonances in these photoionization cross sections are, most likely, overestimated, as in the case of Xe C60-... 

For outer subshells of the encaged atom, the ionization thresholds of which vary from a few eV to a few tens eV, the dynamical-cage model is required. The photoionization cross section of the encaged atom in the dynamical-cage approximation will be marked with a tilde sign 5 s and... 

The quintessence of the reversed electron correlation effect is illustrated in Figure 20 by nonrelativistic HF and RPAE calculated data of the 4s photoionization cross section of free Ca and encaged Ca, Ca C60 near threshold, both at the frozen-cage approximation level, afs A, [20] and dynamical-cage approximation level, afsA [64]. 

Some partial photoionization cross sections, derived in this way for neon, are shown in Fig. 2.11 as a function of photon energy. The uppermost curve is the total absorption cross section. At the onset of the ionization thresholds for the ejection of Is, 2s and 2p electrons this quantity shows the corresponding absorption edges (see the discussion related to equ. (2.11)). The partition of the total cross section into partial contributions cr(i) clearly demonstrates that the dominant features are due to main photoionization processes described by the partial cross sections satellite transitions from multiple photoionization processes are also present. If these are related to a K-shell ionization process, they are called in Fig. 2.11 multiple KL where the symbol KLX indicates that one electron from the K-shell and X electrons from the L-shell have been released by the photon interaction. Similarly, multiple I/ stands for processes where X electrons from the L-shell are ejected. Furthermore, these two groups of multiple processes are classified with respect to ionization accompanied by excitation, (e, n), or double ionization, ( ,e). If one compares in Fig. 2.11 the magnitude of the partial cross sections for 2p, 2s and Is photoionization at 1253.6 eV photon energy (Mg Ka radiation) and takes into account the different... 

Above the ionization threshold (see Chapter 8), the absolute photodissociation cross section can be obtained as the difference between the absolute total absorption cross section and the absolute photoionization cross section (see for example, for N2, Fig. 6 of Shaw, et ai, 1992). Another experimental quantity is the ionization efficiency defined as the total photoionization cross section divided by the total absorption cross section. 

Figure 8.12 Relative photoionization cross section of para-H.2, at 78 K, in the region o t e ionization threshold, recorded at a wavelength resolution of 0.016 A. On the right-han si e of the figure, the rotationally autoionized lines of the np2 series appear as emission win °ws-The large peaks on the left are the result of vibrational autoionization (see Section 8.6) [ rom Dehmer and Chupka (1976)].

The direct knockout mechanism had earlier been evoked by Samson [36, 37], who pointed out that the variation of some double photoionization cross sections as a function of energy closely resembles the form of electron impact ionization cross sections of ions. This is, of course, the expected behavior if direct knockout dominates, which is expected in the energy range relatively near threshold. Further consideration of these model mechanisms is left for a later section where the role of initial state electron correlation is discussed. 

C.A. Nicolaides, C. Haritos, Th. Mercouris, Theory and computation of electron correlation in the continuous spectrum Double photoionization cross- section of H and He near and far from threshold, Phys. Rev. A 55 (1997) 2830. 

Y. Komninos, C.A. Nicolaides, Many-electron approach to atomic photoionization Rydberg series of resonances and partial photoionization cross sections in Helium, around the n=2 threshold, Phys. Rev. A 34 (1986) 1995. 

---