Spectral line#Spectral line broadening and shift

Line broadening ( also referred to as widening mechanism ) is usually in the physics undesirable increase of the line width of a spectral line of the radiation - emitting systems (for example laser). The minimum width that follows from the energy-time uncertainty relation, is the natural line width. For this reason, this width is not one of the line broadening.

Survey

We distinguish between homogeneous and inhomogeneous broadening mechanisms. For homogeneous broadening the emission likelihood of a particular frequency for all of the particles are equal. When inhomogeneous broadening this probability is not equal.

The following table gives an overview of the various broadening mechanisms.

Line broadening in X-ray, electron and neutron diffraction patterns may also be due to internal stresses of the sample or by the fact that only a very small range (< 10-5 cm) scatters coherently. This is applied in the X-ray stress measurement.

The expected also in a nearly defect-free crystal line width can be broadened by such effects. Also, stacking faults and other deviations from the ideal crystal structure have an influence on the line profile. The degree of widening is obtained by

Line broadening can be evaluated by various methods. This special functions are provided for the line profile, such as a Gaussian distribution or the Cauchy distribution. Using such methods, it is possible to break down the line broadening in a lattice distortion and a Teilchengrößenanteil.

In more complex mathematical Warren Averbach method is carried by a Fourier analysis of the line profile, which leads to a distribution function for the lattice distortion and the particle sizes.

Doppler broadening

If all possible directions of movement occur in the velocity distribution of the emitting particles relative to the receiver, there are positive and negative Doppler shifts of different sizes. Characterized the spectral line is broader. With increasing temperature, this effect is amplified.

Impact or pressure broadening

Radiation from hot gases or plasmas shows a line broadening which increases with pressure. This is due to the collisions of the issuers in which deform the electron shells more or less strong. Thus, the energy levels of initial and final states of the issuer will be moved to a. Second, the lifetime of the excited state is often ends prematurely by the shock. Both lead to the displacement of frequency or energy of the emitted photon.

Postgraduate

• Overview and explanation of the line broadening (PDF, 725 kB)
• Wolfgang Demtröder: laser spectroscopy. 5th edition. Springer, Berlin / Heidelberg / New York 2007, ISBN 978-3-540-33792-8.

• Laser Physics
• Spectroscopy