Chem 454

X-rays defined 0.1-100 A

non-optical (no lenses, mirrors)

source: inner electron transitions

• typically to vacant n=1 (1s)
• or to vacant n=2 (2s, 2p)
  • becomes vacant by high energy electron impact
  • perhaps vacant by radioactive decay
  • perhaps vacant due to exposure to radioactive element
  • (no possibility of thermal or chemical reaction as source)

• E = -13.6 eV Z² / n²
• really want Z-0.3 for 1s (screening)
• want about Z- 3 for 2s, 2p
• but really those changes won't affect our estimates

• E(1s) = 400 x 13.6 = 5440 eV
• E(2p) = (400) x 13.6 /2 = 1000 eV
• 2p-->1s is about 4400 eV
  • wavelength = 12398 A / (V or eV)
  • wavelength is about 3 A
• this is a K transition (into n=1 shell)
  • actually a Ka
• 3p-->1s is a bit more energetic
  • E(3p) = "400"X13.6 / 9 = 400 eV
  • transition is 5000 eV
  • wavelength is with shorter
  • Kb

• for Z=40 can expect about 4x energy, 1/4 wavelength
• generally wavelength, energy unique to atomic species

• from 3p or 4 p
• into 2s
  • E(2s) = 1/4 E (1s)
  • E(3s) = 1/9 E (1s)
  • 3p-->2s is (1/4-1-/9) Es =0.15 E(1s)
  • 2p -->1s was 0.75 E(1s)
    • so La is about 1/5 energy or 5* wavelength

Uses of X-rays

• diffraction-- important, later in text
  • molecule or crystal is diffraction grating
  • pattern gives spacing (mol dimension, geometry)
• absorption
  • some, but very specialized
• imaging
  • based on relatively low absorption, but some
• ionization sources
• monitoring emission by samples
  • must be able to produce
  • must be able to detect
  • must be able to measure wavelength or energy

Detection-- through ionization

• directly: gas, high voltage, measure current
• directly: gas, high voltage, one current pulse/particle
• directly: solid state, measure pulse
• indirectly: photographic film, phosphors (scintillation)
• generally E(X-ray) >> Ionization potential
  • get many electrons
  • solids, could absorb completely
  • energy is proportional to pulse height
  • energy discriminating detection
• Typical detector Si or Ge, cooled with liq N2, 2-6" in size

Dispersive methods

• grating (based on a crystal)
• wavelength selection, then detection

Cleaning Up X-Rays

• Absorption spectra typically show sharp edges
• Represent onset of a transition
• Filter made of lower MW metal
  • can remove lower energy radiation
  • can isolate a K lines (remove L and background)
  • simple, helpful, but crude
• Grating to select wavelength
  • better, bigger, costly
• Live with the background (corrections)

Producing X-Rays

• Some radioactive elements
  • produce their own X-rays
  • or beat up target atoms
• X-Ray tubes
  • accelerate electrons beam (4-20 KV)
  • electrons slam into target
  • produce complex radiation
    • Bremstrahlen (continuum)
    • Atomic Emission lines
    • characteristic of the target
• X-Ray Fluorescence
  • short wavelength X-ray source; aim at sample
  • monitor longer wavelengths from sample
• Scanning Electron Microscopes
  • again, electron beam
  • carefully focused
• Particle Accelerators
  • important research sites
  • high energy, intense, focused, more monochromatic

• Generally use an Energy Dispersive Detector
• Portable units often use radioactive source
  • Mars Sojourner
• Lab Units often favor an X-Ray source

SEM Scanning Electron Microscope

• Tightly focused electron beam
  • maneuvered by magnetic coils (lenses)
  • scanned across sample
  • raster scan (image pixel by pixel)
• Mode 1-- Backscattered electrons
  • electrons collected, current measured
  • made brightness at that pixel
  • plotted as 2D image
  • shows morphology
• Mode 2 -- Energy dispersive Detector
  • measures X-rays produced
  • decides which signals to display
    • all, all above xxx A
    • only that for one element
  • produces a more discriminating image
  • Intensity is measure of composition of sample
  • Analysis in area of 10-50 A is possible
• Analysis limited to near surface
  • X-rays penetrate
  • Electrons stopped in first few atoms
  • So X-rays originate in first few atomic layers
  • Sometimes, etch surface to see successive layers