Debye–Waller factor

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The Debye–Waller factor (DWF), named after Peter Debye and Ivar Waller, is used in condensed matter physics to describe the attenuation of x-ray scattering or coherent neutron scattering caused by thermal motion.^[1]^[2] It has also been called the B factor or the temperature factor. Often, "Debye-Waller factor" is used as a generic term that comprises the Lamb-Mössbauer factor of incoherent neutron scattering and Mössbauer spectroscopy.

The DWF depends on the scattering vector q. For a given q, DWF(q) gives the fraction of elastic scattering; 1 - DWF(q) correspondingly gives the fraction of inelastic scattering. (Strictly speaking, this probability interpretation is not true in general.^[3]) In diffraction studies, only the elastic scattering is useful; in crystals, it gives rise to distinct Bragg peaks. Inelastic scattering events are undesirable as they cause a diffuse background — unless the energies of scattered particles are analysed, in which case they carry valuable information (inelastic neutron scattering).

The basic expression for the DWF is given by

${\text{DWF}}=\left\langle \exp \left(i{\mathbf {q}}\cdot {\mathbf {u}}\right)\right\rangle ^{2}$

where u is the displacement of a scattering center, and <...> denotes either thermal or time averaging.

Assuming harmonicity of the scattering centers in the material under study, the Boltzmann distribution implies that ${\mathbf {q}}\cdot {\mathbf {u}}$ is normally distributed with zero mean. Then, using for example the expression of the corresponding characteristic function, the DWF takes the form

${\text{DWF}}=\exp \left(-\langle [{\mathbf {q}}\cdot {\mathbf {u}}]^{2}\rangle \right)$

Note that although the above reasoning is classical, the same holds also quantum mechanically.

Assuming also isotropy of the harmonic potential, one may write

${\text{DWF}}=\exp \left(-q^{2}\langle u^{2}\rangle /3\right)$

where q, u are the magnitudes (or absolute values) of the vectors q, u respectively, and $\langle u^{2}\rangle$ is the mean squared displacement. Note that if the incident wave has wavelength $\lambda$ , and it is elastically scattered by an angle of $2\theta$ , then

$q={\frac {4\pi \sin(\theta )}{\lambda }}$

In the context of protein structures, the term B-factor is used. The B-factor is defined as:

$B=8\pi ^{2}\langle u^{2}\rangle$

It is measured in units of Å². The B-factors can be taken as indicating the relative vibrational motion of different parts of the structure. Atoms with low B-factors belong to a part of the structure that is well-ordered. Atoms with large B-factors generally belong to part of the structure that is very flexible. Each ATOM record (PDB file format) of a crystal structure deposited with the Protein Data Bank contains a B-factor for that atom.

References

↑ Debye, Peter (1913). "Interferenz von Röntgenstrahlen und Wärmebewegung". Ann. d. Phys. (in German) 348 (1): 49–92. Bibcode:1913AnP...348...49D. doi:10.1002/andp.19133480105.
↑ Waller, Ivar (1923). "Zur Frage der Einwirkung der Wärmebewegung auf die Interferenz von Röntgenstrahlen". Zeitschrift für Physik A (in German) (Berlin / Heidelberg: Springer) 17: 398–408. Bibcode:1923ZPhy...17..398W. doi:10.1007/BF01328696.
↑ Lipkin, Harry (2004). "Physics of Debye-Waller Factors". arXiv:cond-mat/0405023v1.

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