Line width pressure-broadened

Of the four types of broadening that have been discussed, that due to the natural line width is, under normal conditions, much the smallest and it is the removal, or the decrease, of the effects of only Doppler, pressure and power broadening that can be achieved. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

AxN, Axp, AxD, Axv line-profile half-widths in cm-1 for natural, pressure-broadened, Doppler-broadened, and combined Doppler-and pressure-broadened cases, respectively generally Ax = Av/c, where c is velocity of light... 

When pressure broadening dominates, the situation is more complicated because the resulting Lorentzian profile contributes significant area far from the line center. A further complication in this case is that the Lorentzian half-width cannot be accurately calculated and must be measured in other experiments. If both Doppler and pressure broadening are present, however, and if the Lorentzian to Doppler half-width ratio is small, the correction necessitated by pressure broadening is small. In this situation an accurate value of the Lorentzian half-width may not be needed. Line strength in the case of combined Doppler and pressure broadening may be obtained from the equivalent width by the use of tables (Jansson and Korb, 1968). 

Consider an example. Assume that we are looking at a spectral line and trying to decide how to observe the line for deconvolution giving width reduction by a factor of 3. Further assume that pressure broadening is... 

The sample must be in the gas phase. Microwave absorption lines are considerably broadened by molecular collisions at intermediate and high pressures at atmospheric pressure, microwave absorption lines are tens of thousands of megacycles wide. Hence the gas is kept at low pressure, typically 0.01 to 0.1 torr under these conditions, line widths run about MHz. The compound studied need not be a gas at room temperature, but it must have sufficient vapor pressure to give detectable absorption. To study involatile compounds such as the alkali halides, the waveguide must be heated to 500-1000°C high-temperature microwave spectroscopy presents great experimental difficulties, but it has been used to study most of the alkali halides. 

Pressure Broadening.—Increase in width of emission or absorption lines caused by increase in the total pressure of a system. 

The effect of atmospheric gas on the emission of an LIB plasma [146] exhibits selfabsorption in He compared to Ar as a result of increased free atom populations in the outer regions of the plasma. Spectral line widths do not correlate well with atmospheric gas. This rules out Doppler effects as a major source of broadening in the laser-induced plasma. The use of a low pressure (ca. 1 torr) to examine the influence of this variable on the shock wave or secondary plasma revealed an increased emission intensity, which confirmed the assumption that the secondary plasma was excited by the shock wave. 

Pressure broadening An effect that increases the width of an atomic spectral line caused by collisions among atoms that result in slight variations in their energy states. 

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... 

The accuracy with which the wavenumber of the center of a symmetrical isolated line can be measured is of the line width. For a gas at a sufficiently low pressure so that pressure broadening is unimportant, the width of the line is determined by the Doppler effect and is... 

Diffusion and electronic relaxations can lead to broadening, AT, of the line width, which would be difficult to resolve under pressure because of possible additional broadenings due to pressure gradients. 

In order to measure an absorption cross section, it is necessary to have a source of tunable radiation (or spectrometer) that has a spectral resolution narrower than the width of the molecular line (natural and/or Doppler) being measured. Inadequate spectral resolution will yield a too small (lower bound) value of ° i- An effective strategy is to pressure broaden the molecular line so that it becomes broader than the instrumental resolution (see Stark, et al., 1992 and Murray, et al., 1994). 

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