Instrumental Chemistry

Chapter 12

Atomic X-Ray Spectroscopic

Brief Summary

X-ray spectroscopy is a form of optical spectroscopy that utilizes emission, absorption, scattering, fluorescence, and diffraction of X-ray radiation

About X-Rays

• 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

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

Schematic of an X-ray tube

Energy-level diagram showing common transitions producing X-rays

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.

The Duane-Hunt law

The maximum photon energy corresponds to total stopping of the electron and is given by:

hvo = (hc)/o = Ve

vo is the maximum frequency

V = accelerating voltage

e = electron charge

X-ray Fluorescence

Since X-rays are rather energetic, excitation of sample electrons will give rise to fluorescence as the sample electrons are excited and return to their ground states in a series of electronic transitions.

Bragg equation

sin = (n)/2d

= angle of incidence

= wavelength

d = interplane distance of crystal

Diffraction of X-rays by a crystal

X-ray monochromator and detector

References

http://www.anachem.umu.se/jumpstation.htm http://userwww.service.emory.edu/~kmurray/mslist.html

http://www.chemcenter/org

http://www.sciencemag.org