X-Ray Spectroscopy and The Development of the Periodic Table


Spectroscopy Information

Immediately upon the discovery of x-rays in 1895, Rontgen also discovered the most important use for x-rays.  With his hand in front of the machine he saw his carpals and metacarpals displayed on the screen.  While fascinated by this striking discovery, one can’t help but ask how it works.

Most types of spectroscopy rely on vibrational, rotational and translational movement of bonds in a molecule or atoms.  However x-ray spectroscopy is different in that electron transitions are involved.  Usually energy is absorbed by an electron and it moves to the excited state and upon return to the ground state, a photon of certain energy and wavelength is emitted as shown below.

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This concept still holds true x-rays are unique in that the electrons which are excited are not valence electrons.  The collisions with energy particles are so powerful that the inner shell core electrons are excited.  This causes an inner orbital to be vacant therefore an outer electron drops to a lower orbital.  This drop in energy of an electron causes the x-ray photons to be released.  The figure below illustrates this.

   
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Figure 2 (above) shows a free electron colliding with a tungsten atom knocking one of the inner shell electrons out of a lower orbital.  A higher orbital electron fills the empty position releasing excess energy.

This is an example of primary excitation.  The high energy particles that excite the inner shell electrons are electrons in this case. Secondary excitation or Fluorescence excitation is caused when a photon hits an inner shell electron allowing it to reach the excited state and another electron drops down to fill the opening releasing less energy than was absorbed.  This is the premise of fluorescence and why x-ray emission can be seen.  As an example, a 1s electron is excited because it is bombarded with a photoelectron during primary excitation.  This electron is excited from the K level below which is the notation for 1s. 

 X-ray diffraction

In 1913 Bragg and Bragg used Barklas and Sadler’s data to do many x-ray experiments using different crystals as a three-dimensional diffraction grating.  They came up with a law that allows one to calculate details about the crystal structure, moreover, if the crystal structure is known, to determine the wavelength of the x-rays incident upon the crystal.

For the diagram below:
Bragg’s Law identifies the angles of the incident radiation relative to the lattice planes from which diffraction peaks occur.  Bragg derived the condition for constructive interference of the x-rays scattered from a set of parallel lattice planes (represented by the blue dots shown in the diagram below).

Bragg considered crystals to be composed of parallel planes of atoms.  Incident waves reflect only a small fraction of radiation. The diffracted beams occur when the reflections from different planes of atoms interfer  constructive. 

The equation below quantatively represents Bragg's Law. For the equation n represents the number of wavelengths, d represents the distance between the planes of atoms and Θ represents the incident angle of reflection.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/bragg.html

Moseley used this information to show that wavelengths were not only a characteristic of the element the target was made of, but also they had the same sequence as the atomic numbers. This allowed atomic numbers to be determined with certainty for the first time.

Soon after it was also established that secondary fluorescent x-rays were excited in any material irradiated with beams of primary x-rays. This started investigation into the possibilities of fluorescent x-ray spectroscopy as a means of qualitative and quantitative elemental analysis

Since there is now a vacancy on the K level shown in the 2nd picture, an electron from the L or M level will replace it creating a new vacancy and yield a photon in the x-ray range. Since it is lower in energy than the initial x-ray absorbed it will fluoresce.  New vacancies will be created on the L or M level and new electrons will occupy the vacancies releasing lower energy creating the L lines and this pattern will continue.  The criterion that must be met in order for this to occur is that an absorption limit must be met.  This means that the initial x-ray must have the minimum energy required to excite an electron from a given one-electron state, which also corresponds to a minimum wavelength requirement.

Each atom because it has a different electronic structure will emit a characteristic x-ray line emission spectrum.  This is one way in which x-ray spectroscopy can be used to quantitatively measure elements in a sample. The two types of spectrometers that accomplish this are wavelength dispersive and energy dispersive.  The former system uses a diffraction crystal to focus specific wavelengths onto a detector, the latter focuses all emitting x-rays onto an energy analyzing detector.  Wavelength dispersive spectrometers are the preferred instrument due to their heightened sensitivity.  The diffraction crystal, standard being lithium fluoride crystal, is rotated and the x-ray emission is focused on the detector.