Atomic X-Ray Atomic X-Ray SpectroscopySpectroscopy

Chapter 12Chapter 12

X-ray range X-ray range 10 10-5-5Å to 100 ÅÅ to 100 ÅUsedUsed 0.1Å to 25 Å 0.1Å to 25 Å

Formation of X-Rays Formation of X-Rays (emission)(emission)

Produced by the deceleration of Produced by the deceleration of high-energy electrons. high-energy electrons.

Electronic transition of Electronic transition of electrons in the inner orbitals of electrons in the inner orbitals of atoms.atoms.

Formation of X-Rays Formation of X-Rays (fluorescence)(fluorescence)

Exposure of a substance to x-Exposure of a substance to x-ray radiationray radiation absorptionabsorption and and thenthen fluorescence fluorescence

Inner orbital electrons in K or L Inner orbital electrons in K or L shells of metal atoms are shells of metal atoms are knocked out! (big or small?)knocked out! (big or small?)

Outer shell electrons undergo Outer shell electrons undergo transitions to the lower shells transitions to the lower shells and and give off high energy X-Raysgive off high energy X-Rays

Formation of X-Rays Formation of X-Rays (decay, synchroton)(decay, synchroton)

Radioactive decay Radioactive decay X-ray X-ray emission (common in medicine)emission (common in medicine)

Synchrotron source radiation Synchrotron source radiation (accelerated particles) very few (accelerated particles) very few of these available!of these available!

X-Ray Tube X-Ray Tube (electron beam (electron beam sources)sources)

100KV!

Controlling the intensity of X-Ray

Determining the energy of the X-Ray

X-ray tube emissionX-ray tube emission

0

0 = 12,398/V

Duane-Hunt Law

•Independent of material

•Related to acceleration voltage E

Continuum Spectra: Results fromCollisions between the electrons and the atoms of target materials

Ee = E’e + hAt o, E’e = 0

h0 = hc/o = Ve

V: accelerating voltagee: charge on e-

Line spectra is possible!Line spectra is possible!

Line Spectrum of a Molybdenum target

0•Atomic number >23

•2 line series K and L

•E K> EL

•Atomic number < 23

•K only

A minimum acceleration voltage is required for

L

From electron transitions involving inner shells

A minimum acceleration voltage required for each element increases with atomic number

Line spectra Line spectra

0

Electron Transitions Electron Transitions X-RaysX-Rays

Question: which K series appear at short wavelength between W and Cr?

Which K series appear at short wavelength between W and Cr?

Which K series appear at short wavelength between metal W and W oxide (W is a heavy element)?

Radioactive sources are more Radioactive sources are more commoncommon

X-ray absorptionX-ray absorptionLn P0/P = μX

μ is the linear absorption coefficientis characteristic of the Element and # of atoms in the path of the beam.X is sample thickness

Ln P0/P = μMηX η is density of the sample

μM is mass absorption coefficient

Bragg’s Law of DiffractionBragg’s Law of Diffractionlight scattering by lattice of light scattering by lattice of

atoms!atoms!

d

d

dPCAP

PCAP

2

nsin

sin2n

sin

n

Constructive interference only at angles proportional to and d!

If is known and can be measured then you can calculate d!If d is known and can be measured then you can calculate !

X-Ray Monochromator X-Ray Monochromator (diffractometer?)(diffractometer?)

d2

nsin

X-Ray Diffraction SpectrumX-Ray Diffraction Spectrum

Debye-Scherrer Powder Debye-Scherrer Powder Diffractometer (Camera)Diffractometer (Camera)

X-Ray Spectra of Polymorph 1

X-Ray detectorsX-Ray detectors Geiger tube: formation of ions and electrons from an Geiger tube: formation of ions and electrons from an

inert gas kept at 1200-1600Vinert gas kept at 1200-1600V Phosphors (Scintillation counters): fluorescence of Phosphors (Scintillation counters): fluorescence of

ZnS when hit by a particleZnS when hit by a particle Semicoductor detectors based on a modified diodeSemicoductor detectors based on a modified diode