Competing Photoemission Processes

The photoemission processes M0 ^ M+ are not the only processes that lead to the appearance of electrons with well-defined kinetic energies. Depending on the energy of the exciting radiation, some other processes also contribute to the PE spectrum. These processes are indicated by the circled numbers in Fig. 2, which shows a schematic diagram for the electronic states of the initial system (M) and the systems that have lost one (M+) or two (M2+) electrons.

Process 1 is the process discussed in the previous section. It corresponds to photoionization by emission of either a valence (1a) or a core (1b) electron.

Process 2 represents an autoionization. If the energy hv of the exciting radiation coincides with an electronic transition of the neutral system (which in the valence re

FIGURE 2 Schematic representation of different processes that lead to the appearance of electrons with well-defined kinetic energy.

gion is very likely for larger molecules), a photon can be absorbed by the neutral system. In case the final state Mk of this process has a higher energy than the ground state of M+, Mk can decompose into an electron and a low-lying state of M+. Since both the initial and the final state of the autoionization (AI) process are well-defined electronic states, the electron created in this process has the well-defined kinetic energy eAI(e-) = E(Mk) - E(M+)

In Eq. (3) we again neglected the rebound energy of the heavy particle. Unlike the kinetic energy of an electron produced in process 1, eAI(e-) does not depend on hv. Therefore, the binding energy scale has no meaning for autoionization peaks. These peaks appear primarily at low kinetic energies since autoionization is usually efficient only when the initial and final states of the process are close in energy.

Process 3 in Fig. 2, is a direct transition from M0 to states of M2+ under simultaneous emission of two electrons. As discussed in Section II, such a process has a much lower probability than process 1 and leads to a continuous energy distribution of the produced photoelectrons.

Process 4 represents an Auger transition. As discussed above, photoionization can lead to the creation of a core hole provided the energy of the exciting radiation is high enough. The resulting state of M+ is highly excited. In about 10-16 sec, it relaxes to a lower excited state of M+ by emission of an X-ray photon or to a lower lying state of M2+ by emission of another electron. The latter transition, known as an Auger process, has a higher probability for light atoms, up to about Z = 40. Since the initial and final states of an Auger transition are well-defined electronic states of M + and M2+, respectively, the emitted Auger electron has the well-defined kinetic energy

FIGURE 2 Schematic representation of different processes that lead to the appearance of electrons with well-defined kinetic energy.

Auger transitions contribute strongly to high-energy PE spectra. For example, in Fig. 1c all peaks indicated with an asterisk result from Auger transitions. As in autoion-ization, the kinetic energy of the Auger electron does not depend on hv. Autoionization and Auger processes, therefore, can be separated from photoemission processes by variation of the excitation energy.

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