And pressure broadening

The spectral linewidths of fluorescence lines are determined in most spectral lamps by Doppler effect and pressure broadening and are therefore normally much broader than the natural linewidth, which is approached only by low-pressure hollow cathode lamps 23) operated at liquid helium temperatures. 

Multiplying this expression by n yields the Voigt function that occurs in the description of spectral-line shapes resulting from combined Doppler and pressure broadening. We elaborate on these phenomena in Section I of Chapter 2. 

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). 

Linewidth is also affected by pressure broadening from collisions between atoms. Collisions shorten the lifetime of the excited state. The uncertainty in the frequency of atomic absorption and emission lines is roughly numerically equal to the collision frequency between atoms and is proportional to pressure. The Doppler effect and pressure broadening are similar in magnitude and yield linewidths of 10-3 to I0-2 nm in atomic spectroscopy. 

Doppler and Pressure Broadening, 27 I 6.2 Line Profile in Resonance Emission Four Types of Resonance Lamps, 31... 

Heijmen TGA, Moszynski R, Wormer PES, Van der Avoird A, Rudert AD, Halpern JB, Martin J, Gao WB, Zacharias H (1999) Rotational state-to-state rate constants and pressure broadening coefficients for He— C2H2 collisions Theory and experiment. J Chem Phys 111 2519—2531... 

It has been found, however, in practice that a perfectly straight analytical working curve (— log T plotted against concentration) is seldom obtained in atomic absorption spectroscopy. The reasons for this are usually a combination of instrumental problems broadening of the emission line of the light source due to self-reversal, Doppler and pressure broadening of the absorption lines of the atoms in the flame, failure to exclude flame emission entirely, use of a focused instead of a parallel... 

Both Doppler broadening and pressure broadening are temperature dependent. 

J. Ward, J. Cooper, Correlation effects in the theory of combined Doppler and pressure broadening. J. Quant. Spectrosc. Radiat. Transf. 14, 555 (1974)... 

Figure 3.17 Dicke narrowing and pressure broadening of a rotational line of a vibrational transition in H2O at 1871 cm (A = 5.3 xm) as a function of Ar and Xe pressure [103]...

In the first part of this chapter the experimental techniques of linear and nonlinear Raman spectroscopy of gases are reviewed. The nonlinear techniques (Stimulated Raman Gain Spectroscopy, Inverse Raman Spectroscopy, Coherent Anti-Stokes Raman Spectroscopy, Photo-Acoustic Raman Spectroscopy, and Ionization-Detected Stimulated Raman Spectroscopy) have the capability of very high resolution, limited by the linewidths of the lasers used and pressure broadening effects. 

Whereas the resolution in linear Raman spectroscopy is limited in principle by the slit width of the spectrometer, a considerable improvement in the instrumental resolution was attained through the development of the techniques of nonlinear or coherent Raman spectroscopy, where the interaction of two laser beams with the third-order susceptibility of the sample creates the spectrum. In this case, the resolution is determined by the convoluted linewidth of the two lasers, the Doppler effect, and pressure broadening of the spectral lines. 

Barger, R. L., and Hall, J. L. (1969). Pressure shift and pressure broadening of methane line at 3.39 pm studied by laser-saturated molecular absorption. Physical Review Letters, 22, 4—8. 

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