Doppler width of spectral line

One consequence of the high Q attained in these structures is that they become sharply tuned the system described above would show a FWHM of 1.5 MHz, comparable with the Doppler width of spectral lines in this region. Thus spectral lines viewed in a cavity may appear as an increased loss that lowers the Q at high pressures whereas at lower pressures their profile becomes distorted because the incident power density varies markedly with offset from the cavity resonant frequency. 

R.H. Dicke The effect of collisions upon the Doppler width of spectral lines. Phys. Rev. 89, 472 (1953)... 

From the variety of different techniques which recently have been developed to outwit the Doppler-width of spectral lines in gas phase spectroscopy only two examples are selected here. They shall illustrate the resolution achieved so far and the gain in information about molecular structures obtainable from resolved features in sub-Doppler spectra, which are completely masked in Doppler-limited spectroscopy. 

Homogeneous broadening. The Doppler width of spectral lines decreases as we go from the visible into the infrared region of the spectrum and eventually the line profile will be dominated by collision or natural broadening. 

The small scale of the hyperfine splittings and the large Doppler width of spectral lines makes accurate measurements by optical spectroscopy very difficult. Mag-... 

The second factor involves the theory that defines the natural width of the lines. Radiations emitted by atoms are not totally monochromatic. With plasmas in particular, where the collision frequency is high (this greatly reduces the lifetime of the excited states), Heisenberg s uncertainty principle is fully operational (see Fig. 15.4). Moreover, elevated temperatures increase the speed of the atoms, enlarging line widths by the Doppler effect. The natural width of spectral lines at 6000 K is in the order of several picometres. 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

Measuring Doppler widths of rotational lines by laser-probe techniques gives velocity distributions in just the same way as measuring Doppler widths of atomic lines by conventional means. In this method a laser beam with a very narrow band width is tuned over the spectral line to determine the profile of the Doppler broadened line. The line shape can be interpreted to give the average velocity of the product. As yet, this method has been applied only to rotational energy transfer studies however, with the availability of mode-locked lasers providing narrow band widths, this procedure may become more widely used. 

Lamb dip spectroscopy provides a very sensitive tool for studying small frequency shifts and broadening of spectral lines which normally would be undetectable because they may be small compared to the doppler width. These investigations yield information about collisions at low pressures, where the effect of far distant collisions is not suppressed by the more effective close collisions. This allows the potential between the collision partners at large intermolecular distances to be examined. 

TABLE 1 Doppler Widths of Lines (assuming T - 300 K and M = 50) and Typical Accuracies of Measurements for Different Spectral Regions... 

The Doppler width of a spectral line is given by the well-known relation AA = (7.162 x lO ) AT where wavelength units are in Angstroms, temperature is in degrees Kelvin and M is the atomic mass. This relation between four variables is amenable to representation by a nomograph, which does not appear to have been constructed but which would seem to be of practical value. Therefore such a nomograph is presented here. Its construction is briefly described so that the reader who has not made a plot of this type may follow the steps. 

The maximum signal is obtained when Av reaches the spectral width of the absorption lines which is generally the Doppler width with Av 1 GHz in the visible region. The length change Ad must be therefore sufficiently large to attain a modulation depth A V that reaches the Doppler width of the absorption line. The modulation frequency is limited by the mass of the mirror to a few kilohertz. 

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