Doppler broadening

The Doppler spread is caused by the Doppler effect of spectral broadening. In the optical spectral range is observed, or interfere with the small-molecule spectroscopy (including atoms) at a high temperature and a low gas pressure, and in the field of gamma radiation with nuclei.

In nuclear reactions occurs a similar Doppler broadening of the resonances.

Cause of the widening

Based on a specific spectral particles are oscillators which can vibrate at a characteristic frequency. An observer sees this frequency when it is compared with the particle at rest. Several particles have but by the thermal motion of no common rest frame, but they move relative to each other and to the observer. Sees different oscillation frequencies due to the Doppler effect. The Gaussian Maxwell -Boltzmann distribution for the velocity of the particles converts into a distribution for the frequencies (or wavelengths). In the center of mass system of particles, the mean of the distribution is unchanged, while the width (standard deviation) depends on the frequency distribution of the temperature and the particle mass:

With the Boltzmann constant. The line broadening is usually described by the half-width of the distribution. This is calculated by the Gaussian distribution. The line broadening is therefore

Considering, instead of the frequency, the wavelength scale, similarly applies:

Examples

The diagram shows the ratio of the standard deviation of the Doppler profile for the central wavelength as a function of temperature. At room temperature, it is only about 10-6. So is the Doppler width in the optical only about 0.001 nm in atmospheres hotter star, a relative width of up to about 10-4 is reached, which corresponds to the visible absolute width of about 0.1 nm. In fact, often much wider spectral lines, because of collisions with other particles during absorption or emission process, the so-called pressure broadening occurs.

Oxygen in comparison to hydrogen sixteen times heavier so that at the same temperature, the Doppler width is four times lower.

Effects

Since the Doppler broadening in atomic transitions is usually larger by several orders of magnitude than the natural line width, making it more difficult high-resolution spectroscopy. For example, to prevent the dissolution of the hyperfine structure. However, there are modern spectroscopic techniques such as Doppler-free saturation spectroscopy, which off by clever arrangements, the Doppler broadening.

Nuclear and neutron physics

In nuclear reactions occurs to a temperature -dependent expansion of the resonance, because the event of collision with a free particles, the impact energy also depends upon the thermal movement of the atoms or molecules of the material. This makes it more likely with increasing temperature that a collision with a particle of given energy is just crossing an energy level of that compound nucleus.

Particularly important is this extension for nuclear reactors. It leads with increasing temperature to a neutron loss through increased capture in uranium -238 nuclei. The effect fachsprachlich often simply called Doppler effect is described by the Doppler coefficient of reactivity. This indicates the Reaktivitätsbeitrag per degree of temperature increase and is always negative, ie to stabilize the reactor performance.

• Spectroscopy
• Optical effect
• Nuclear physics
• Reactor Technology