Saturation broadening

In Sect.2.8, we saw that a sufficiently strong radiation field can significantly change the population densities and N2 of an atomic system by induced absorption and emission. This saturation of the population densities also causes additional line broadening. The spectral line profiles of such partially saturated transitions are different for homogeneously and for in-homogeneously broadened lines [3.19]. We treat first the homogeneous case. 

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One effect of saturation, and the dependence of e on /, is to decrease the maximum absorption intensity of a spectral line. The central part of the line is flattened and the intensity of the wings is increased. The result is that the line is broadened, and the effect is known as power, or saturation, broadening. Typically, microwave power of the order of 1 mW cm may produce such broadening. Minimizing the power of the source and reducing the absorption path length t can limit the effects of power broadening. 

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.)... 

Saturation broadening, 134, 363 S branch, 188 Scalar, 187 Scalar product, 98 Scattering of radiation, 186,188 SCF orbitals, 65. See also Hartree-Fock method... 

Figures 5a, 5c, and 5e demonstrate the effect on the TNB spectrum of an increase in the concentration of adsorbed perylene cation radicals (Figure 5c, 4 X 1018 cation radicals per gram Figure 5e, 1 X 1019). The outer features of the anion-radical spectrum become much less evident as the perylene radical concentration is increased. This effect is more pronounced at low temperatures (Figures 5d and 5f). Although the spectrum for TNB adsorbed alone on decationated Y is less well resolved at the lower temperature (Figure 5b), the outer features are still clearly discernible with this system saturation broadening accounts for the loss of resolution.

Saturation Broadening in Flames and Plasmas As Obtained by Fluorescence Excitation Profiles... 

Direct Measure- ment Fluorescence "" Excitation Profile Saturation Broadening ... 

A high-resolution spectrum of the clock transition is shown in Fig. 2. The clock-laser power was reduced to 30 nW to avoid saturation broadening. The fit with a lorentzian curve results in a linewidth of 170 Hz (FWHM), corresponding to a fractional resolution bv/v of 1.3 10-13. A spectral window of 200 Hz width contains 50% of all excitations. According to our present experimental control of the ion temperature, electromagnetic fields and vacuum conditions, no significant Doppler, Zeeman, Stark or collisional broadening of the absorption spectrum of the ion is expected beyond the level of 1 Hz. The linewidth is determined by the frequency instability of the laser and the lineshape is not exactly lorentzian... 

In general, the intensity of an ESR spectrum increases with an increase in the microwave power R When the applied power level is sufficiently low, thermal relaxation processes can, to a good approximation, maintain the Boltzmann equilibrium between spin levels. When the power level exceeds that amount, the ESR spectrum broadens and its intensity begins to decrease and eventually disappears. This phenomenon is called the power saturation or saturation broadening effect and depends... 

Fig. 5.9 Saturation broadening and Rabi splitting of double-resonance signals with increasing RF power P...

With increasing RF intensity, however, saturation broadening is observed (Vol. 1, Sect. 3.6) and the double-resonance signal may even exhibit a minimum at the center frequency 0)23 (Fig. 5.9). This can readily be understood from the semiclassical model of Vol. 1, Sect. 2.7 for large RF field amplitudes Erf the Rabi flopping frequency Vol. 1, (2.90)... 

Lamb peaks (inverse Lamb dips) at the line centers of the absorbing transitions (Sect. 2.3). The line profiles of these peaks are determined by the pressure in the absorption cell, by saturation broadening, and by transit-time broadening (Vol. 1, Sect. 3.4). Center frequency coq, linewidth Aco, and line profile Pl(co) are measured as a function of the pressure p (Fig. 8.2). The slope of the straight line Aco p) yields the line-broadening coefficient [977], while the measurement of coo p) gives the collision-induced line shift. 

Note I [Ws/m2] is the spectral intensity.) Saturation broadening plays only a minor role. The absorption coefficient is determined only by those atoms within the natural linewidth. 

The halfwidth = ho) of the saturation-broadened line increases with the saturation parameter So at the line center coo If the induced transition rate at coo equals the total relaxation rate R, the saturation parameter 5o = [B 2p coo)VR becomes 5o = 1, which increases the linewidth by a factor /2, compared to the unsaturated linewidth Scuq for weak radiation fields (p 0). 

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