Chapter 12 X-Ray Spectrometry X-Rays are short wavelength electromagnetic radiation produced by the deceleration of high energy electrons or by electronic transitions of electrons in the inner orbital of atoms. The wavelength range of X-rays is from about 10 -5 Å to 100 Å; conventional X-ray spectroscopy is largely confined to the region of about 0.1 Å to 25 Å. X-ray spectroscopy is a form of optical spectroscopy that utilizes emission, absorption, scattering, fluorescence, and diffraction of X-ray radiation The basics... X-rays are short-wavelength (hence, high frequency, and hence, relatively high energy) electromagnetic radiation. Two ways to produce X-rays: 1) Deceleration of high-energy electrons 2) Electronic transitions involving inner-orbital (e.g. - d or f) electrons Approximate wavelength range: 10 -6 nm - 10 nm Wavelength range used in conventional applications: 0.01 nm - 2.5 nm X-rays are the shortest wavelength, i.e., highest energy, electromagnetic radiation associated with electronic
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Chapter 12
X-Ray Spectrometry
X-Rays are short wavelength electromagnetic radiation produced by the deceleration of
high energy electrons or by electronic transitions of electrons in the inner orbital of atoms.
The wavelength range of X-rays is from about 10-5Å to 100 Å; conventional X-ray
spectroscopy is largely confined to the region of about 0.1 Å to 25 Å.
X-ray spectroscopy is a form of optical spectroscopy that utilizes emission, absorption,
scattering, fluorescence, and diffraction of X-ray radiation
The basics...
X-rays are short-wavelength (hence, high frequency, and hence, relatively high
energy) electromagnetic radiation. Two ways to produce X-rays:
1) Deceleration of high-energy electrons
2) Electronic transitions involving inner-orbital (e.g. - d or f) electrons
Approximate wavelength range: 10-6 nm - 10 nm
Wavelength range used in conventional applications: 0.01 nm - 2.5 nm
X-rays are the shortest wavelength, i.e., highest energy, electromagnetic radiation
associated with electronic transitions in atoms. Calculation of the energy states of an atom
is in general, very difficult, except of course in the particular case of the hydrogen atom,
where the problem is readily soluble and the results, shown schematically below, are very
well known.
The orbital picture of the H-atom associated with this energy level diagram is readily
extended to multi-electron atoms and the again familiar result is qualitatively represented
below.
Energy levels of many electron atoms in the periodic table. The insert shows a magnified view of the order near Z = 20.
An important feature of both the above diagrams is that the differences in orbital energies
decrease as they themselves increase. This means that the energy required for excitation, or
given out on relaxation of an electron from a higher orbital to a lower orbital is greater
when "inner" orbitals are involved and least when "outer" orbitals are involved. Except for
light elements (say, those preceding Na) the innermost orbitals are not significantly
influenced by bonding interactions involving the atom and, hence, their energies may be
regarded as characteristic of that atom regardless of its state of combination. Inner orbital
transitions involve X-rays, and it is for this reason that X-ray spectrometry can be a form
of atom detection and, hence, of non-destructive chemical analysis.
Of course, for a multi-electron atom, a conventional orbital diagram as given above cannot
simply be converted into an energy level diagram. The energy level diagram for any atom
is considerably more complex and depends in detail upon the particular atom. However,
for X-ray emissions of importance in elemental analysis, a simplified treatment is
sufficient and the diagram below is useful.
Partial energy level diagram showing common transitions leading to X–radiation.
The most intense lines are indicated by the widest arrows.
The orbital shells for which the principal quantum number n = 1,2,3, etc. are labelled the
K, L, M, etc. and, hence, emissions due to a higher energy electron entering these shells
are said to form the K, L, M, etc. series of lines. Generally, only the K and L series of X-
rays are of analytical utility and the wavelengths of these lines for a selection of elements
spanning the Periodic Table are shown below.
Wavelengths/Å for Intense X–ray Emission Lines
Element K Series L Series
a1 b1 a1 b1
Na 11.909 11.617 – –
K 3.742 3.454 – –
Cr 2.290 2.085 21.714 21.323
Rb 0.926 0.829 7.318 7.075
Cs 0.401 0.355 2.892 1.282
W 0.209 0.184 1.476 1.282
U 0.126 0.111 0.911 0.720
(Note that all possible electronic transitions are not of equal probability, i.e., the nature of a spectrum depends on specific selection rules, so that the complexity of a spectrum is not as
great as might be expected from first consideration of an energy level diagram.)
The fact that the wavelength of a line of given type decreases as the atomic number of the
element increases is rather important in that it means that an X-ray from a given element
must be able to cause inner shell ionization and, hence, emission of radiation of lower
energy from any lighter element.
For analytical purposes, X-rays are generated in three ways:
1) bombardment of metal target with high-energy electron beam
2) exposure of target material to primary X-ray beam to create a secondary
beam of X-ray fluorescence
3) use of radioactive materials whose decay patterns include X-ray
emission
Synchrotron radiation is an indispensible tool for x - ray absorption studies of biological materials. The XAFS technique yield information on the number, type and radial distribution of ligands coordinated to a metal are revealed at subatomic resolution.
The oxidation state of a metal and the effects of substrate binding or catalysis on the metal centre can be probed. Three dimensional structural information is obtained when x - ray absorption data and protein crystal data are combined in the 3D - XAFS method.
Identification and measurement of concentration of elements based on the fact that
primary-emission x-rays emitted by an element excited by an electron beam have a
wavelength characteristic of that element and an intensity related to its concentration. It
may be performed by an electron probe microanalyzer, an electron microscope
microanalyzer, or by an electron microscope, or scanning electron microscope, fitted with
an x-ray spectrometer.
Below is a schematic of an X-ray tube.
X-ray sources can emit two forms of X-rays:
1) continuous (white radiation or Bremsstrahlung - “Bremsstrah-
lung” refers to radiation arising from the deceleration of particles)
2) discontinuous (line)
Below is the partial energy-level diagram showing common transitions producing X-rays.
The most intense lines are indicated by wider arrows.
Electron beam sources...
In electron beam sources, X-rays are produced by heating a cathode to produce
high-energy electrons; these electrons are energetic enough to ionize off the cathode and
race towards a metal anode (the target) where, upon collision, X-rays are given off from
the target material in response to the colliding electrons. By varying the conditions, one
can obtain either a continuous spectrum or a discontinuous spectrum.
The reaction between the electron beam and the target material involves
deceleration of the electron and ejection of a target photon and emission of X-rays. The
energy lost by the electron as it smashes into the target material is equal to the energy of
the ejected photon. Since any given electron can be retarded differently by the same target
material, a range of photon energies are possible. The maximum photon energy
corresponds to total stopping of the electron and is given by:
hvo = (hc)/o = Ve,
where, vo is the maximum frequency,
V = accelerating voltage,
e = electron charge
This is the Duane-Hunt law.
Electron beam source line spectra characteristic...
1. Elements with Z > 23 exhibit two spectral series: a K line (corresponding to
shorter wavelengths) and an L line (corresponding to relatively longer
wavelengths). Elements with Z < 23 exhibit only the K series
2) As Z increases, so too does the minimum amount of energy required for
excitation
For all but the lightest elements, the X-ray line spectra are independent of
either physical or chemical states. This is because the electrons involved in
the transition are not participating in any chemical bonds.
Continuum Spectra from Electron Beam Sources…
In an X-ray tube, electrons produced at a heated cathode are accelerated toward a metal
anode by a potential as great as 100kV; upon collision, part of the energy of the electron
beam is converted into X-Rays. Under some conditions only a continuum spectrum is
results. The continuum X-Ray spectrum is characterized by a well-defined, short
wavelength limit, which is dependent upon the accelerating voltage but independent of the
target material. The continuum radiation from an electron beam source results from
collisions between the electrons of the beam and the atoms of the target material.
Figure 1.1: Schematic representation of an AGN continuum spectrum including a possible source for each emission component.
Line Spectra from Electron Beam Sources…
Bombardment of a molybdenum target produces intense emission lines. The emission
behavior of molybdenum is typical of all elements having atomic numbers greater than 23,
that is, the X-Ray line spectra are similar when compared with ultraviolet emission and
consist of two series of lines.
Line spectra are composed of distinct lines of color, or in the case of our graphs, sharp
peaks of large intensity at a particular wavelength. Line spectra are characteristic of
elements and compounds when excited (energized) under certain conditions. These spectra
helped develop the current atomic theories. Line spectra thus provide a “fingerprint”
unique to each element, and as with continuous spectra, the combination of the prominent
lines in the spectrum produce the observe light color.
The fluorescent lamp’s spectrum is a mixture of line and continuous spectra. Because of
the exact correlation with the principal mercury vapor lines and the fluorescent lamp’s
lines, we can conclude that a major component of the fluorescent lamp is mercury vapor.
But what produced the continuous portion of the fluorescent spectrum?
A phosphor is a substance that can accept energy in one form and emit the energy in the
form of visible light. Fluorescent lights are produced by coating the inside surface of the