Thermal Doppler broadening

The thermal motion of the atoms in the source volume leads to a broadening of the observed line, because the thermal velocity vth of the electron-emitting atom is added to the electron velocity v0. For an estimation of the resulting disturbance by this kinematical effect it is sufficient to select for the thermal velocity the two directions at which the Doppler effect becomes extreme, i.e., 

These velocity components vtb in a given direction follow from the Maxwell distribution 

Within these approximations, the broadening effect of an electron line expressed as the fwhm value A th on the energy scale follows from (note pth v0) 

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Doppler broadening arises from the random thermal agitation of the active systems, each of which, in its own test frame, sees the appHed light field at a different frequency. When averaged over a Maxwellian velocity distribution, ie, assuming noninteracting species in thermal equilibrium, this yields a line width (fwhm) in cm C... 

Doppler broadening, caused by the thermally induced movement of atoms relative to the spectrometer. (This is analogous to the apparent change in pitch of a train whistle as it approaches and passes an observer.)... 

Experimental details for the cross-section measurements were presented in the literature. Briefly, after the irradiation by electron beam pulse for a few nanoseconds, the time-dependent absorption for the atomic line transition Rg Rg -i-/zv was measured to observe the time-dependent population of the excited rare gas atoms Rg. The population of excited Rg was determined using an absorption law for the atomic lines, where the broadening of the absorption profile due to the thermal Doppler effect and due to the attractive interatomic potentials was reasonably taken into consideration. The time-dependent optical emission from energy transfer products, such as ... 

Doppler broadening arises from the random thermal motion of the atoms relative to the observer. The velocity V, of an atom in the line of sight will vary according to the Maxwell distribution, the atoms moving in all directions relative to the observer. The frequency will be displaced by... 

The positron-trap technique has been used by Surko and coworkers to measure the Doppler broadening of the 511 keV line for positrons in helium gas. This method does not have the drawback of the experiment described above, in which both positronium and free-positron events overlap on the angular distribution curves here the positrons are thermalized prior to the introduction of the gas and therefore cannot form positronium. A comparison of the theoretically predicted and experimentally measured Doppler spectra (Van Reeth et al., 1996) is shown in Figure 6.16. The theoretical results were obtained from the variational wave functions for low energy positron-helium scattering calculated by Van Reeth and Humberston (1995b) see equations (3.75) and (3.77). 

Doppler shifts due to the low thermal velocities of the highly-charged ions in the EBIT are not significant sources of uncertainty. On average the Doppler shifts will be zero as there is no preferred direction of motion. Doppler broadening is 1.8 eV for IkeV ions and we allow for a possible 1% asymmetry of velocity distribution resulting in a maximum Doppler shift of < 0.02 eV or < 4 ppm. 

As the line widths of diode lasers are considerably narrower than those of atomic spectral lines excited in a thermal atomizer, spectra can be recorded at very high resolution. When performing the atomization at reduced pressure (e.g. at 100-500 Pa), pressure broadening is low as compared with the Doppler broadening. As the... 

Doppler broadening, whereby the absorption of neutrons in the resonance region is increased with an increase in temperature. The increase in nonfissioning absorption by uranium-238 means that this effect reduces the reactivity of thermal reactors at higher temperatures ... 

Fundamentals. The energy of a gamma quantum emitted or absorbed by an atomic nucleus usually differs somewhat fi om the actual energy difference between the nuclear states because of recoil and Doppler-broadening induced by thermal movement. At room temperature, the value differs by about 10 to 10 eV from the true energy value vo of the nuclear transition. The recoil energy r and the corresponding momentum p are... 

The Doppler broadening effect is caused by the thermal motion of the emitting or absorbing atoms. Under typical conditions the speeds of atoms are about 1000 ms . Although this is considerably smaller than the speed of light (r = 3 X 10 ms ), it is fast enough for the Doppler effect to become noticeable. 

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