Section E – Spectrometric techniques
E6 X-RAY EMISSION
SPECTROMETRY
Principles Each element has electrons occupying specific energy levels, characterized by
quantum numbers. A simple description refers to the lowest energy level as the
K shell, the next as the L shell, M shelland so on. Although with elements of
low atomic number these electrons may be involved with bonding, for high
atomic number elements, such as nickel and copper, they are inner electrons,
and largely unaffected by valency changes and bonding.
A simple example is shown in Figure 1. A target atom is bombarded with
high-energy radiation, for example accelerated electrons or radioactive particles.
This causes the excitation of an inner (K shell) electron completely out of the
atom, leaving a vacancy in the K shell (Fig. 1(a)). An electron from a higher
energy shell (L) can relax into the lower level, emitting primary X-rays whose
wavelength corresponds to the difference between the energies of the L and K
shells (Fig. 1(b)). Similar behavior will occur if an electron is excited out of the L
shell or higher levels. Since the L shell has two slightly different energy levels,
corresponding to 2s and 2p orbitals, the emission is actually a doublet, the Ka 1
and Ka 2 lines. If the electron relaxes from the M shell, Kblines are produced,
and if from M to L, Laand so on. High-energy X-ray photons may then interact
with the sample and are absorbed, causing the ejection of inner electrons as
shown in Figure 1(c). This produces an X-ray fluorescence emission spectrum. A
competitive process involves the Auger effectwhere the photon is internally
converted and an electron emitted.
Moseley’s law states that the reciprocal of the wavelength of each character-
istic series of X-rays (for example, the Ka 1 series) is related to the atomic number
Z of the element by the formula:
Key Notes
Excitation of the inner electrons of atoms promotes some to higher
energies. In falling back to lower levels, they emit radiation in the X-ray
region, characteristic of the element concerned.Excitation by high-energy electrons, radioactive particles or X-rays may
be used. Analysis of the emitted X-rays using crystal analyzers is
followed by detection using gas ionization detectors or scintillation
counters. Nondispersive semiconductor detectors and multichannel pulse
height analyzers are often used in conjunction with scanning electron
microscopes.Elemental analysis of metal and mineral samples as well as surface
studies and the determination of heavy metals in petroleum are typical
uses.Related topics Other topics in Section E.PrinciplesInstrumentationApplications