Saturated absorption line

The intensity of a Mossbauer spectrum depends not only on the recoil-free fractions of the source and the absorber and on the number of absorbing nuclei, but also on the linewidth of the absorption lines and on whether or not saturation effects occur. The following approximate expression is valid for relatively thin absorbers [17] ... 

Saturation as described above can be observed either when all the molecules in the initial state have the same absorption probability, or when some of the state population is preferentially absorbed, depending on the spatial orientation of the molecules or on their transition frequency, selected from a certain frequency interval. The first case is called homogeneous line saturation, the second is due to inhomogeneous saturation or hole-burning. This different cases will be explained in more detail by discussing some relevant experiments. 

However, when the combination of concentration, N, and path length, /, is not small, the weak-absorber approximation is not valid, and the line-shape needs to be taken into account. The reason for this is that in the limit when absorption of light is already saturated at the center of the line, absorption due to added gas occurs only in the wings of the absorption line, which is sensitive to the lineshape. In the case of saturation at the center of the absorption line as shown in Fig. 14.8c, the absorption can be thought of as occurring in two regions, one from... 

A fourth cause of broadening is saturation broadening. In our semiclassi-cal treatment of radiation, we assumed the radiation intensity was sufficiently weak that a first-order perturbation theory could be used. If the radiation is very intense, this causes substantial depletion of the population of the lower level. A more accurate perturbation treatment is needed in this case, and the result is that the absorption line is appreciably broadened. (See Townes and Schawlow, Section 13-15 and references cited therein.)... 

Partial saturation of the HO absorption and use of excitation line widths wider than the HO absorption line width are in general undesirable, because they may produce spurious HO while exciting HO fluorescence with less than 100% efficiency (103). However, the issue is not as simple as matching the HO absorption profile exactly and avoiding all saturation. For instance, a matched laser excitation line width is more difficult to achieve and to keep tuned to the HO absorption, and the... 

For an idealized case (perfectly homogeneous magnet, low transverse fields, steady-state conditions, no saturation effects), an NMR absorption line in the frequency domain or the Fourier transform of a free induction decay has a Lorentzian profile ... 

Fig. 3.11. Is, 2s and 3s FE absorption lines in GaAs at 1.2 K (note the ordinate scale). The saturation of the Is and 2s lines is indicated by the dashed part. The exciton binding energy is 4.1 meV. The energy gap Es of GaAs is indicated (after [28]). Copyright 1985, with permission from Elsevier...

The result of this power broadening or saturation is to reduce the absorption in the line centre compared with the absorption in the wings of the line. This in turn leads to loss of analytical signal intensity and an apparent broadening of the absorption line profile. The resulting effect on the line shape function can be described by an equation due to Karplus and Schwinger, for low powers and incomplete saturation (ref 3, p. 50) ... 

As noted in Chapter 1, spectroscopic "absorption" and "dispersion" signals each provide equivalent information, provided that the system is "linear" (i.e., that the response amplitude is proportional to the excitation amplitude, so that no "saturation" occurs). Historically, the absorption spectrum has been preferred over dispersion, because the absorption line shape is narrower and more symmetrical. However, the various Fourier transform spectrometer si produce digitized absorption and dispersion spectra with equal vertical scaling, each containing Falf the available signal information.2 Moreover, even when only the absorption-mode signal is available, the dispersionmode spectrum may be generated by a Hilbert transform.3... 

In a later experiment the laser was replaced by a cw ring dye laser, the output of which was amplified in four stages, using a XeCl excimer laser as pump. In this way the cw output (30 60 mW) at 486 nm was amplified into 25 mJ, 10 ns pulses with a repetition rate of 50 s . In addition in this later experiment the frequency of the transition was determined with respect to the saturated spectrum of Tc2, a line of which lies within 50 MHz of half the 1 S- 2 S energy interval. Furthermore, an acousto-optic modulator was used to create a laser sideband at 50 MHz higher than the fundamental frequency so that each Tea absorption line consisted of 3 peaks, at vo, vo + 50 MHz and Vo + 25 MHz, the cross-over resonance. 

If the laser bandwidth 5i l matches the homogeneous width y Ilit of the absorption line, we find with y = A2 -1- 2/fx fi"om (2.19b) the saturation intensity... 

Fig. 2.7 Comparison of the saturation of a homogeneous absorption line profile (a) and an inhomogeneous profile (b) in a standing-wave field, (c) The traveling saturating wave is kept 2ita) = a)o and a weak probe wave is tuned across the line profile...

Figure 3 Spectral separation of two Doppler-broadened profiles by saturation spectroscopy (A) profiles of Doppler-broadened absorption lines with Lamb dips (B) observed Doppler-free profiles (cross-over signals omitted).

Esteva and Kamatak (1987) proposed a semi-quantitative estimate of line saturation effects usually encountered in absorption measurements by transmission and photoyield methods. The estimate of line saturation was obtained by conqiaring the 3d linewidth of La obtained by photo yield and by resonant photon scattering. Some quantitative information on the line saturation effect was obtained fi om resonant photon scattering experiments. As opposed to the election yield method, in the latter experiments, the scattered photon is detected as a function of incident photon energy. Spectra are scaimed around the 3ds/2 and 3d3/2 resonance absorption lines. The process involved in the scattering is complementary to photoexcitation, in which remission of a photon due to de-excitation of a 4f electron occurs. 

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