X射线特征谱线

Microscopy 1986 Ernst Ruska in Physics for his fundamental work in electron optics, and for the design of the first electron microscope (a TEM, in Berlin, 1933) and G. Binnig and H. Rohrer in Physics for their design of the scanning tunneling microscope (at IBM, 1981)
Crystallography: * The study of crystals. * Rene-Just Hauy is considered the founder of the modern science of crystallography, c. 1800. * Auguste Bravais correctly described the 14 fundamental lattices in 1848. Diffraction: 1) Latin: diffractus - to break into pieces 2) The breaking up of a wave caused by interference of one part of the wavefront with another at the edge of an opaque object or slit. Instructor will sketch the latter on the board.
• Instead of red light, λ = 6500 Å, use x-rays, λ = 1.5 Å for CuKα. • As the wavelength is reduced by a factor of 4000, we can examine objects smaller by a factor of 4000. • Instead of measuring spacing between rows of pits on CD, measure spacing between planes of atoms in a crystalline solid!
Rows of bumps on the CD can act as a diffraction grating.
Using a red laser …
1st order diffraction visible to the left and right of the primary spot (transmitted beam).
(ASM, Metals Handbook, vol. 8 p. 230)
Terminology
Crystal: 1) Greek: kryos - cold, frost 2) Greek: krystallos - ice or something frozen 3) heavy clear glass, as in "crystal ball" - NOT our interest in this class 4) A solid composed of atoms or molecules in a fixed repeating pattern and having an external shape bounded by plane faces in a symmetrical arrangement. 5) Any material having a discrete diffraction pattern.
Electrons and Neutrons 1905 Philipp Lenard in Physics for important work on cathode rays. (discovered around 1869 by Hittorf) 1906 J. J. Thomson in Physics for researches into the discharge of electricity in gases (discovery of the electron, in 1897) 1937 C. Davisson and G. Thomson in Physics for their experimental discovery of the diffraction of electrons by crystals. 1981 K. M. Siegbahn in Physics for high resolution electron spectroscopy. 1982 Aaron Klug in Chemistry for development of crystallographic electron microscopy and structural elucidation of biologically important nuclei acidprotein complexes. 1994 B. Brockhouse and C. Shull in Physics for neutron diffraction and spectroscopy for studies of condensed matter.
Scanning tunneling microscope photo of CD stamper. Height of bump is ¼ the wavelength of light used to read them, 7800 Ang. Track pitch is 1.6 μm.
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(units are nm)
http://news.bbc.co.uk/onthisday/hi/dates/stories/april/25/newsid_2932000/2932793.stm
1964 Dorothy Hodgkin in Biology for determinations by x-ray techniques of the structures of important biochemical substances including penicillin. 1979 A. McLeod Cormack & G. Newbold Hounsfield in Medicine for computed axial tomography. 1985 H. Hauptman and J. Karle in Chemistry for direct methods to determine crystal structures. 1988 J. Deisenhofer, R. Huber, and H. Michel in Chemistry for the structures of proteins that are crucial to photosynthesis. (by x-ray diffraction)
2nd order & 1st order diffraction both visible
For constructive interference, to form a bright spot at point P, the path length difference, between the rays r1 and r2 must equal an integer number of wavelengths of light, nλ.
Nobel Prizes relating to crystallography and diffraction
X-Rays 1901 Wilhelm Roentgen in Physics for the discovery of x-rays. (in 1895) 1914 Max von Laue in Physics for x-ray diffraction from crystals. 1915 W. H. Bragg and W. L. Bragg in Physics for crystal structure derived from x-ray diffraction. 1917 Charles Barkla in Physics for characteristic radiation of elements. 1924 K. M. G. Siegbahn in Physics for x-ray spectroscopy. 1927 Arthur Compton in Physics for scattering of x-rays by electrons. (Compton effect, inelastic) 1936 Peter Debye in Chemistry for diffraction of x-rays and electrons in gases. 1946 Hermann Muller in Medicine for the discovery of the production of mutations by means of X-ray irradiation. 1954 Linus Pauling in Chemistry for research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances. (x-ray studies of amino acids)
Diffraction can be easily demonstrated using a common laser pointer and a common factory-made compact digital audio disk (CD).
Scanning electron microscope photos of a CD, polycarbonate removed to show bumps.
X-Rays continued 1962 M. Perutz and J. Kendrew in Chemistry for the structure of hemoglobin (by x-ray diffraction) 1962 J. Watson, M. Wilkins, and F. Crick in Medicine for the structure of DNA. The key x-ray diffraction work was done by Rosalind Franklin between 1951 and 1953. See link
There are many diverse practical reasons why we study crystallography.
We will now discuss several of these.
(Elastic)
(Dieter, Mechanical Metallurgy, p. 61)
D
... or = nλ In the limit D >> d, the large right triangle with sides y & D is similar to the smaller one with sides d & in which sinθ = opp/hyp ≈ /d so ≈ d sinθ, nλ ≈ d sinθ Finally, for small angles, θ ≈ sinθ = y/D , so in simplest form nλ ≈ dy/D . Measure this yourself with a ruler and calculate the spacing d between tracks!