Photoemission in Solids I General Principles
- PDF / 5,413,318 Bytes
- 104 Pages / 454 x 684 pts Page_size
- 18 Downloads / 241 Views
With 26 Figures
Antonio Machado Electrons and photons are the m o s t easily available particles with which to p r o b e matter. Hence, m a n y spectroscopic techniques involve the use of these two types of particles. In a typical spectroscopic experiment (see Fig. 1.1), an electron or a p h o t o n in a m o r e or less well-defined state (energy, direction of propagation, polarization) impinges on a sample. As a result of the impact, electrons a n d / o r photons escape from that sample. In any given spectroscopic technique the state of one type of escaping particles is at least partially analyzed with a spectrometer (analyzer, filter, m o n o c h r o m a t o r ) . In photoelectron spectroscopy, p h o t o n s (visible, uv, x-rays, ~,-rays) are the incoming and electrons, the outgoing particles to be analyzed (see Fig. 1.2). In such an experiment the photon ~ 0 photon-,-photon
~
°
absorption :Britlouin,
Raman. ~ Compton k.~d:~fl~.;~' photon ~ electron :Photoelectron ~ o ~ Spectroscopy
0
~
photon ~(elect rord .electron :Auger
ISAMPLE[ . ~~t~.?//1 l'~eh. -
eleetron~.(electmn) ~
emmslOrl. appeonance potential lAPS)
electron-..eleetron :criamderistic energy tosses
Fig. 1.1. Schematic diagram of spectroscopic methods involving photons and electrons po'lj ,~oO
noS
SAMPLE l J ~ r , . o ~
to analyzer
Fig. 1.2. Schematic diagram of a photoelectron spectroscopy process. The variables involved are: for the photons their energy he), their polarization p and the azimuthal and polar angles of ilacidence~op,0p; for the electrons their energy E, polarization a, and polar and azimuthal angles 0=, q%
2
M, Cardona
and L. Ley
sample is left in an "ionized" state after an electron is emitted. Sample and photoemitted electron must be rigorously viewed as a joint excited state. The difference between the energy of this excited state and that of the ground state the sample was in before being hit by the photon must equal the energy of the annihilated photon. While this view is rigorous, it is not easily amenable to treatment. Very often, however, the photoemission process can be treated in the one-electron picture: The emitted electron comes from a one-electron orbital within the sample without suffering losses in the escape process. In this case the energy of the emitted electron equals the photon energy minus the binding energy of the corresponding bound electronic state : an analysis of the energy distribution of the photoexcited electrons yields information about the energies of occupied one-electron states. The photoelectron current measured by an ideal spectrometer, i.e., one capable of resolving the energy E, angles 0o (polar) and q~e (azimuthal), and electron spin ~r, is a function F of the parameters of the impinging photon and the settings of the electron spectrometer (see Fig. 1.2)
I=F(E, Oe,qg¢,a; he),po, Op, ~pp),
(1.1)
where 0p and q~p give the direction of the incoming photon, pp its polarization, and e) its frequency. Equation (1.1) is a function of 10 variables which, as such, is impractical to me
Data Loading...
