FUNDAMENTALS OF X-RAY FLUORESCENCE - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 19

X-ray spectra generally show the intensity of radiation or rather the number
of its photons in relation to the wavelength of radiation or the energy of these
photons. Normally, X-ray spectra consist of two different parts, the line
spectrum and the continuous spectrum.

1.3.2.1TheLineSpectrum

A line spectrum will be produced if a target or sample is irradiated with X-ray
photons, as just mentioned, or is bombarded with electrons (or ions). In both
cases a sufficient energy of photons or electrons is needed. The energy must
exceed the binding energy of a bound inner electron of the target atoms, which
therefore is called the criticalexcitationenergy . The ensuing effects can be
described best by Niels Bohr's atomic model, which supposes Z electrons
revolving around a nucleus in different orbitals or shells and subshells, where Z
is the atomic number of the respective element.
Owing to the high-energy impact, an inner electron can be ejected from the
atom so that a vacancy is created within the respective inner electron shell. The
atom with the vacancy is in an instable state of higher energy and tries to regain
its stable ground state by two different processes. In both processes an outer
bound electron fills the vacancy and the atom instantly emits either an X-ray
photon, which is the basic process of XRF, or what is called an Auger electron.
The energy of the X-ray photon must be equal to the difference of the previous
and the subsequent energy state of the atom:

E photon E previous E subsequent (1.3)

The newly created vacancy in the outer shell can be filled in turn by an electron
still farther out, and another X-ray photon can be emitted. These processes will
follow each other successively and a series of photons will be emitted until a
free electron ultimately replaces an outermost valance electron so that the
atom has finally returned to the ground state.
Since the energy states of atomic electrons are quantized and characteristic
of all atoms of an element, the X-ray photons emitted in this way have
individual energies that are equal for all atoms of the same element but
different for atoms of different elements. Consequently, these photons cause
discrete sharp lines or peaks as intensity maxima in an X-ray spectrum that are
characteristic for any single element of the sample target. Conversely, any
element of the sample can be identified by its characteristic lines or peaks,
comparable to a fingerprint or barcode. For this reason, the line spectra are also
called characteristic spectra. Of course, line spectra of the same sample either
produced by X-ray photons or by electrons are similar.
Although not every outer electron is permitted to fill an inner vacancy,
there are a lot of allowed transitions according to the selection rules of
quantum theory. The most important transitions are indicated in Figure 1.9