CHEM3440Lec13F06 - X-Ray Energies very short wavelength...

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CHEM*3440 Chemical Instrumentation Topic 13 X-Ray Spectroscopy X-Ray Energies very short wavelength radiation 0.1Å to 10 nm (100 Å) Visible - Ultraviolet (UV) - Vacuum UV (VUV) - Extreme UV (XUV) - Soft X-ray - Hard X-ray - Gamma Ray often report photon energies, rather than wavelength. (above range is from 125 keV down to 125 eV) usually working in the range of a few keV. Chemical bonds are in the range of 2 - 10 eV. An x-ray photon has LOTS of energy to break bonds; that ! s why they can be biologically damaging. Bremsstrahlung Radiation this word is German for “braking” whenever a moving charged particle slows down rapidly, it emits photons commensurate with that kinetic energy loss. When high energy electrons crash into a solid target, they lose energy and emit continuum radiation in the x-ray region. Note how the Bremsstrahlung envelope reaches to the energy of the electron beam accelerating voltage. Each electron is decelerated in a series of collisions, emitting an x-ray photon each time. X-ray Line Spectra Atoms in excited states will lose energy by giving off photons. If that excitation is in the inner core levels, the energies of the emitted photons are in the x-ray regions. K Shell L Shell M Shell K " K ! L ! L "
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Typical K and L Transitions Element Atomic # K " K # L " L # Na 11 11.909 Å 11.617 Å 1041 eV 1067 eV Rb 37 0.926 Å 0.829 Å 7.318 Å 7.075 Å 13380 eV 14946 eV 1693 eV 1751 eV W 74 0.209 Å 0.184 Å 1.476 Å 1.282 Å 59282 eV 67337 eV 8394 eV 9664 eV Sources - Tubes Most common x-ray source. Bombard a metal target with high energy electron beam. X-rays emitted are target atom ! s line spectrum imposed on top of Bremsstrahlung spectrum. <1% power goes into x-ray emission; balance heats target and it must be water-cooled. Acceleration voltage and beam current are critical parameters. Sources - Fluorescence If x-rays are used to irradiate another atomic target, the target re-emits its own x-ray spectrum. There is no Bremsstrahlung radiation in this case. Intensity is lower, but spectrum is “cleaner”. Ni X-ray Fluorescence Spectrum Sources - Radioactive Various nuclear decay processes emits photons in the x-ray region. Can be sharp line emission (from electron capture decay processes) or continuum (from beta decay processes). Useful as long lasting, low level sources of monochromatic x-rays. 3 H ! 3 He + e continuum radiation from 3 - 10 keV 55 Fe + e ! 54 Mn Mn K x-rays at 5.9 keV 57 Co + e ! 56 Fe Fe K x-rays at 6.4 keV 109 Cd + e ! 108 Ag Ag K x-rays at 22 keV 210 Pb + e ! 209 Bi Bi L x-rays at 11 keV
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Sources - Synchrotron The Canadian Light Source (CLS) in Saskatoon. Very important x-ray source; intense, continuously tunable, pulsed. X-Ray Absorption
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This note was uploaded on 05/15/2010 for the course CHEM 3440 taught by Professor Danthomas during the Fall '06 term at University of Guelph.

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CHEM3440Lec13F06 - X-Ray Energies very short wavelength...

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