Inverse Photoelectron Spectroscopy
It has already been pointed out in Chap. 1 that photoemission experiments can also be performed in a reversed mode, namely by sending electrons of varying energy onto a sample and detecting the photons that are thereby produced by them (Bremsstrahlung) [9
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It has already been pointed out in Chap. 1 that photoemission experiments can also be performed in a reversed mode, namely by sending electrons of varying energy onto a sample and detecting the photons that are thereby produced by them (Bremsstrahlung) [9.1]. If only photons of one particular energy are detected, which is a common measuring mode, the technique is called Bremsstrahlung Isochromate Spectroscopy (BIS) and, a little misleadingly, this term is sometimes used to characterize the IPES technique in general. For the sake of accuracy, the technique as such - sending electrons onto a sample and detecting the photons produced in the Bremsstrahlung process - will henceforth be called Inverse PhotoElectron Spectroscopy (IPES) and only the mode in which the photon detection energy is kept constant while varying the energy of the incoming electrons will be called BIS. The techniques of PES and IPES are complementary to one another (Fig. 9.1) in that PES connects the energy levels below the Fermi energy and above the vacuum level and IPES connects the energy levels above the vacuum level and between the vacuum level and the Fermi level. For each of the techniques there is thus an energy range which is inaccessible. Figure 9.1 can be used to demonstrate the complementary nature of PES and IPES. If one uses the convention that energies are referenced to the Fermi energy E F , one has
PES: - Ei
(9.1 )
Ef - Ei ,
fLw =
=
Ef =
fLw - Ekin Ekin
and since - Ei
=
¢ ,
+ ¢,
(9.2) (9.3)
EB ,
(9.4) IPES: fLw =
+ Ekin + ¢ ,
(9.5) (9.6)
+ ¢,
(9.7)
Ei - E f ,
Ef
= -
Ei
=
fLw
Ekin
S. Hüfner, Photoelectron Spectroscopy © Springer-Verlag Berlin Heidelberg 2003
552
9. Inverse Photoelectron Spectroscopy
I PES (BIS)
PES
nw Fig. 9.1. Schematic diagram of PhotoEmission Spectroscopy (PES) and Inverse PhotoEmission Spectroscopy (IPES). In PES the energy range between the Fermi energy and the vacuum level is not accessible, while in IPES the unaccessible range is that below E F . Thus the two techniques complement each other
and since -Ej
-EB
=
= EB
-IU.;
+ Ekin + ¢ .
(9.8)
The comparison of (9.1-9.4) with (9.5-9.8) gives a nice picture of the complementary nature of the two processes. The equations also illustrate why it is sometimes claimed that PES measures an initial state energy while IPES measures a final state energy. This statement, however, is somewhat misleading because in both spectroscopies one measures a final state energy. The problem of screening in the final states for PES has been discussed at length in this volume and that of screening in IPES has not yet been explored sufficiently for us to make definite statements. On a formal basis, IPES measures the electron addition spectrum or the spectral function (above the Fermi energy) multiplied by a transition matrix element. Given their similarity, it might seem desirable to describe the results of PES and IPES in the same context. This separate chapter on IPES might thus seem unnecessary at first sight. However, IPES does have certain dis
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