Photoelectric effect

The photoelectric effect occurs when a photon ejects an electron from an atom; the photon ceases to exist. The most abundant atom in interstellar space, hydrogen, can be ionized by photons with energies above 13.6eV(A = 91.2nm),i.e.,byultraviolet or x-ray photons. For pure hydrogen, the cross section for the photoelectric effect is effectively zero below 13.6 eV; at 13.6 eV, it jumps abruptly up to a high value. This transition is called the K edge (Fig. 6a). This alludes to the inner orbital K shell of electrons (n = 1) from which the electron is ejected.

At photon energies greater than 13.6 eV, the neutral hydrogen atoms can still absorb the photons. The excess photon energy appears as kinetic energy of the ejected electron. As the photon energy increases, beyond 13.6 eV, the cross section decreases roughly as the inverse 8/3 power of the photon energy or frequency, a a v-8/3 (Cross section for photoelectric absorption) (10.38)

This takes the form of a straight line of slope — 8/3onalog a — log v plot (Fig. 6a).

Heavier elements, e.g., oxygen, have electrons in higher energy levels, e.g., n = 2 (L shell) and higher, as well as in the K shell. The cross sections for these elements will show not only a K edge but also "L" edges at the energies where the n = 2 electrons can be ejected, an energy lower than that of the K edge (Figs. 1 and 6b). The L edges are actually a triplet because the n = 2 state has coupling between the electron spins and orbits. The L edges are somewhat less pronounced than the K edge. There are also M edges, even less pronounced, from the n = 3 level, etc.

The cross sections for heavier elements such as carbon and oxygen are very large at and just above their K edges which are in the x-ray range. Therefore they play an important role in x-ray absorption. The global view of cross section variation with photon energy for the several processes mentioned herein is shown in Fig. 6c.

(a) Hydrogen (b) Heavy element

Log <r

^^ K edge

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