Thermal X-ray radiation (PDF) - SRON

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3.6.6 Excitation-Autoionisation In the previous section we showed how a collision of a free electron with an ion can lead to ionisation. In principle, if the free electron has insufficient energy (E < I), there will be no ionisation. However, even in that case it is sometimes possible to ionise, but in a more complicated way. The process is as follows. The collision can bring one of the electrons in the outermost shells in a higher quantum level (excitation). In most cases, the ion will return to its ground level by a radiative transition. But in some cases the excited state is unstable, and a <strong>radiation</strong>less Auger transition can occur. Later on we will treat the Auger process in more detail, but shortly speaking the vacancy that is left behind by the excited electron is being filled by another electron from one of the outermost shells, while the excited electron is able to leave the ion (or a similar, process with the role of both electrons reversed). Because of energy conservation, this process only occurs if the excited electron comes from one of the inner shells (the ionisation potential barrier must be taken anyhow). The process is in particular important for Li-like and Na-like ions, and for several other individual atoms and ions. Figure 3.16: The Excitation-Autoionisation process for a Li-like ion. As an example we treat here Li-like ions (see Fig. 3.16). In that case the most important contribution comes from a 1s–2p excitation. The effective cross section Q for excitation from 1s to 2p followed by auto-ionisation can be approximated by (E H = 13.6 eV): Q = πa 2 0 2B Z2 Z 2 eff (1s) E H E Ω H (3.38) in which a 0 is the Bohr radius, and Z eff = Z − 0.43 is the effective charge that a 1s electron experiences because of the presence of the other 1s electron and the 2s electron in the Li-like ion. Just as before, E is the energy of the free electron that causes the excitation. Further, B is the so-called branching ratio. It gives the probability that after the excitation there will be an auto-ionisation (the other possibility was a radiative transition). The following approximation for B can be used: 1 B ≃ (3.39) 1 + (Z/17) 3 Finally, the scaled hydrogenic collision strength is given by: Z 2 Ω H ≃ 2.22 lnǫ + 0.67 − 0.18 ǫ 35 + 1.20 ǫ 2 (3.40)
where ǫ ≡ E/E ex and the excitation energy E ex ≡ I 1s − I 2p . Similar to the case of direct ionisation, it is straightforward to average the excitation-autoionisation cross section over a Maxwellian electron distribution. Figure 3.17: Collisional ionisation cross section for Ovi. Figure 3.18: Collisional ionisation cross section for Fexvi. In Figs. 3.17 and 3.18 we show the total collisional ionisation cross section (direct + excitation-autoionisation) for an ion in the Li electronic sequence and the Na isoelectronic sequence. Clearly, for Fexvi the excitation autoionisation process is very important. The increasing importance of excitation autoionisation relative to direct ionisation is also illustrated in Fig. 3.19. Figure 3.19: Ratio between the Excitation-Autoionisation cross section to the Direct Ionisation cross section for Na-like ions. From Arnaud & Rothenflug (1985). 36
Page 1 and 2: High Energy Astrophysics Jelle Kaas
Page 3 and 4: 3.1 Introduction Thermal X-ray radi
Page 5 and 6: Figure 3.2: The observed X-ray spec
Page 7 and 8: Figure 3.6: X-ray spectrum of the S
Page 9 and 10: series of alkaline spectra). The no
Page 11 and 12: • The first atomic shell (n = 1)
Page 13 and 14: Z El 1s 2s 2p 3s 3p 3d 4s Ground te
Page 15 and 16: m l1 s 1 m l2 s 2 M L M S +1 + +1
Page 17 and 18: 3.3 Energy levels We have seen the
Page 19 and 20: 3.4 Transitions between different l
Page 21 and 22: Figure 3.10: Transitions between a
Page 23 and 24: 3.5 Abundances of the elements With
Page 25 and 26: 3.6 Basic processes In order to be
Page 27 and 28: leading to Here the constant S 0 is
Page 29: 3.6.3 Radiative transition probabil
Page 32 and 33: Figure 3.12: Fluorescence yield as
Page 34 and 35: The formula of Younger While the Lo
Page 38 and 39: 3.6.7 Photoionisation Figure 3.20:
Page 40 and 41: 3.6.9 Radiative recombination Radia
Page 42 and 43: 3.6.10 Dielectronic recombination T
Page 44 and 45: 3.7 The ionisation balance In order
Page 46 and 47: of computing time. Therefore one us
Page 48 and 49: 3.8 Continuum emission processes Af
Page 50 and 51: decays back to the 1s orbit by a ra
Page 52 and 53: Inner-shell ionisation The third li
Page 54 and 55: 3.9.3 Line width For most X-ray spe
Page 56 and 57: Table 3.6: The strongest emission l
Page 58 and 59: Table 3.8: Some important iron line
Page 60 and 61: with τ(λ) = τ 0 ϕ(λ) (3.83) wh
Page 62 and 63: Table 3.9: The most important X-ray
Page 64 and 65: where the hydrogen column density N
Page 66 and 67: source itself. Fortunately, with hi
Page 68 and 69: Figure 3.35: Stability for the ioni
Page 70: Table 3.10: Wavelengths in Å of th

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