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Difference between revisions of "Anomalous scattering"

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<Font color="blue">Diffusion anomale</Font> (''Fr''). <Font color="red">Anomale Streuung </Font>(''Ge''). <Font color="green">Difusión anómala (</Font>''Sp''). <Font color="black"> Diffusione anomala </Font>(''It''). <Font color="purple"> 異常散乱 </Font>(''Ja'')
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<font color="blue">Diffusion anomale</font> (''Fr''). <font color="red">Anomale Streuung</font> (''Ge''). <font color="black">Diffusione anomala</font> (''It''). <font color="purple">異常散乱</font> (''Ja''). <font color="green">Difusión anómala</font> (''Sp'').
  
 
== Definition ==
 
== Definition ==
  
The history of the description of the scattering of an atom when illuminated with X-rays is that initially wavelength dependencies were ignored. This was initially referred to as 'normal scattering'. The wavelength dependencies were then corrections to the normal scattering and also called anomalous. These had to describe changes in amplitude and phase, respectively initially given the symbols <math>\Delta f\,'</math>  and <math>\Delta f\,''</math>. Thus the X-ray scattering factor of an atom is described by the equation:-
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The history of the description of the scattering of an atom when illuminated with X-rays is that initially wavelength dependencies were ignored. This was initially referred to as 'normal scattering'. The wavelength dependencies were then corrections to the normal scattering and also called anomalous. These had to describe changes in amplitude and phase, respectively initially given the symbols <math>\Delta f\,'</math>  and <math>\Delta f\,''</math>. Thus the X-ray scattering factor of an atom is described by the equation
  
<math>f=f_o + \Delta f\,' + i\Delta f\,''</math>
+
<math>f=f_o + \Delta f\,' + i\Delta f\,''.</math>
  
The nomenclature changed when tunable synchrotron sources became available and whereby the <math>\Delta</math> prefixes were removed because changes between two wavlengths would then have required a double &#916; label, which is cumbersome. Thus the <math>\Delta f\,'</math> now means the change in <math>f\,'</math> between two wavelengths. The <math>\Delta</math> prefix to <math>f\,''</math> is dropped for consistency even though its use is based on its value at a single wavelength.  
+
The nomenclature changed when tunable synchrotron sources became available and the <math>\Delta</math> prefixes were removed because changes between two wavelengths would then have required a double <math>\Delta</math> label, which is cumbersome. Thus the <math>\Delta f\,'</math> now means the change in <math>f\,'</math> between two wavelengths. The <math>\Delta</math> prefix to <math>f\,''</math> is dropped for consistency even though its use is based on its value at a single wavelength.  
  
The values of <math>f\,'</math> and <math>f\,''</math> change most at the absorption edge of the element in question. Thus this resonance effect sometimes leads to the term being refererred to as 'resonant scattering'. However, since the off resonance <math>f\,''</math> effect is extensively used in crystal structure determination of the hand of a molecule (its chirality) 'anomalous scattering' is the best ''i.e.'' most widely embracing term. Another commonly used term is Multiple-wavelength Anomalous Dispersion ('MAD'), which involves measurements made at the resonance condition and at more than one wavelength obviously.  
+
The values of <math>f\,'</math> and <math>f\,''</math> change most at the absorption edge of the element in question. Thus this resonance effect sometimes leads to the term being referred to as 'resonant scattering'. However, since the off resonance <math>f\,''</math> effect is extensively used in crystal structure determination of the hand of a molecule (its chirality) 'anomalous scattering' is the best ''i.e.'' most widely embracing term. Another commonly used term is Multiple-wavelength Anomalous Dispersion ('MAD'), which involves measurements made at the resonance condition and at more than one wavelength obviously.  
  
 
=== See also ===
 
=== See also ===
  
[[anomalous dispersion]]<br>
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*[[Anomalous dispersion]]
[[resonant scattering]]<br>
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*[[Resonant scattering]]
[[multiwavelength anomalous diffraction (MAD)]]<br>
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*[[Multiwavelength anomalous diffraction (MAD)]]
  
 
[[Category:X-rays]]<br>
 
[[Category:X-rays]]<br>

Latest revision as of 17:58, 8 November 2017

Diffusion anomale (Fr). Anomale Streuung (Ge). Diffusione anomala (It). 異常散乱 (Ja). Difusión anómala (Sp).

Definition

The history of the description of the scattering of an atom when illuminated with X-rays is that initially wavelength dependencies were ignored. This was initially referred to as 'normal scattering'. The wavelength dependencies were then corrections to the normal scattering and also called anomalous. These had to describe changes in amplitude and phase, respectively initially given the symbols [math]\Delta f\,'[/math] and [math]\Delta f\,''[/math]. Thus the X-ray scattering factor of an atom is described by the equation

[math]f=f_o + \Delta f\,' + i\Delta f\,''.[/math]

The nomenclature changed when tunable synchrotron sources became available and the [math]\Delta[/math] prefixes were removed because changes between two wavelengths would then have required a double [math]\Delta[/math] label, which is cumbersome. Thus the [math]\Delta f\,'[/math] now means the change in [math]f\,'[/math] between two wavelengths. The [math]\Delta[/math] prefix to [math]f\,''[/math] is dropped for consistency even though its use is based on its value at a single wavelength.

The values of [math]f\,'[/math] and [math]f\,''[/math] change most at the absorption edge of the element in question. Thus this resonance effect sometimes leads to the term being referred to as 'resonant scattering'. However, since the off resonance [math]f\,''[/math] effect is extensively used in crystal structure determination of the hand of a molecule (its chirality) 'anomalous scattering' is the best i.e. most widely embracing term. Another commonly used term is Multiple-wavelength Anomalous Dispersion ('MAD'), which involves measurements made at the resonance condition and at more than one wavelength obviously.

See also