Spectroscopy saturation

Saturation spectroscopy is based on the velocity-selective saturation of Doppler-broadened molecular transitions, treated in the previous section. Here the spectral resolution is no longer limited by the Doppler width but only by the much narrower width of the Lamb dip. The gain in spectral resolution is illustrated by the example of two transitions from a common lower level c) to two closely spaced levels a) and b) (Fig.7.8). Even when the Doppler profile of the two transitions completely overlap, their narrow Lamb dips can clearly be separated, as long as Aw = s- 

Saturation spectroscopy is therefore often called Lamb-dip spectros- copy. 

Saturation spectroscopy is based on the selective saturation of an inhomo-geneously broadened molecular transition by optical pumping with a monochromatic tunable laser. As has been outlined in Sect.3.6 the population density n. (v )dv of molecules in the absorbing state is selectively depleted of molecules with velocity components 

The monochromatic laser therefore burns a hole into the population distribution n. (Vz) of the absorbing state and produces simultaneously a peak at the same velocity component in the upper state distribution nj (Vz) (Fig.10.17). 

(a) Hole burning in the lower level population distribution ni(V2) of an absorbing transition and generation of a corresponding population peak in the upper level, (b) Increase of Bennet hole width with increasing saturating intensity 

Because of this population depletion the absorption coefficient a(o)) = decreases from its unsaturated value aQ(u)) 

Sq = 2S/(irY) is the saturation parameter at the line center (3.70) and S = B - p - (o))/R is the ratio of the depleting absorption rate B. j p(u)) to the sum R of all relaxation processes which refill the depleted level E. . Because the intensity absorbed over a path length dz 

Lamb dips for several values of the saturation parameter 5o 

Note The width of the Lamb dip in (7.29a) is 5cold = Ks- This corresponds, however, to the velocity interval Av = y /k because the opposite Doppler shifts Ao) — (cuo — oS) — of the two Bennet holes add when the laser frequency cd is tuned. 

If the intensity of the reflected wave in Fig. 7.5 is very small I2 /i), we obtain instead of (7.29) a formula similar to (7.26). However, we must replace by F = (y + ys)/2 since the pump and probe waves are simultaneously tuned. For So 1 the result is 

If the intensity of the reflected wave in Fig. 2.6 is very small (I2 h), we obtain 

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Yasuda A, Yoshizawa M, Kobayashi T (1993) Fluorescence spectrum of a blue-phase polydiacetylene obtained by probe saturation spectroscopy. Chem Phys Lett 209 281-286... 

The imaginary components of the third-order nonlinear susceptibility of palladium and iridium complexes of C6o and C70 were determined using saturation spectroscopy. In all cases, ImQ ) values are smaller than those of uncomplexed C60 (1.78 X 10 16 m2 V-2) or C70 (7.55 X 10 17 m2 V-2), a result explained by decreased conjugation in the molecule and consequent decreased electron delocalization, although differing photodynamics were not excluded. 

Historical Development. Saturation spectroscopy in flame systems dates back only to about 1972. The important early work... 

Figure 23. Schematic of the two-level model used to treat saturation spectroscopy data. See the text for details.

This Doppler width can be avoided by typical sub-Doppler laser spectroscopy techniques. Laser saturation spectroscopy with a resolution close to the natural line width was used for a test of Special Relativity at the ESR. For such sub-Doppler resolution one must also take into account the small additional broadening and shift arising from the angle 0 between laser beam and ion beam in the Doppler formula. At an interaction length of 10 meters and more, angles are easily controlled to be better than 1 mrad. This limits a possible shift, which enters by... 

T. W. Hansch, I. S. Shahin, and A. L. Schawlow, Optical Resolution of the Lamb Shift in Atomic Hydrogen by Laser Saturation Spectroscopy, Nature Physical Science 235, 63-65 (1972). 

T. W. Hansch, M. H. Nayfeh, S. A. Lee, S. M. Curry, and I. S. Shahin, Precision Measurement of the Rydberg Constant by Laser Saturation Spectroscopy of the Bahner a Line in Hydrogen and Deuterium, Physical Review Letters 32, 1336-1340 (1974). 

Oscillator strengths or absorption cross sections may be obtained by applying saturation spectroscopy techniques to multistep photoionization spectroscopy. A few transitions in uranium have been studied.One of the advantages of saturation spectroscopy is that it can be applied to any one of the steps in the schemes shown in Fig. 2. The disadvantages are that the experimental requirements are severe (laser-atomic beam interaction area,-frequency,-band width and-polarization) and interpertation of the data can be complex. A detailed discussion will not be given because little application has been made to the lanthanides and actinides. We will discuss in the Autoionization section the determination of photoionization cross sections by a saturation method. 

Beside the purely spectroscopic part there has also been important development of active optical components such as electro-optic and acousto-optic modulators. Such components allowed the use of modulation schemes earlier used in the microwave region to be used in optical spectroscopy. In the early 80 s the FM-sideband technique both for stabilisation to molecular lines as well as to passive cavities were demonstrated [45,46]. The developing field of non-linear optics and the wave-mixing picture established a framework in which line shapes of different modulation schemes in saturation spectroscopy could be understood [47, 45, 46, 48,49]. 

Raj R. K., Bloch D., Snyder J. J., Camy G. and Ducloy M., High-Frequency Optically Heterodyned Saturation Spectroscopy Via Resonant Degenerate Four-Wave Mixing , Phys. Rev. Lett. Vol. 44, 1980, pp. 1251-1254. 

Shirley J. H., Modulation transfer processes in optical heterodyne saturation spectroscopy . Opt. Lett., Vol. 7,1982, pp. 537-539. 

The dephasing time, T2, can be measured by the photon echo technique or determined from the homogeneous width of saturation spectroscopy, which is a Fourier transform of the former, as easily seen from Eq. (5.35). When a sample is irradiated with three consecutive laser pulses at times, 0, t2, and f3, an echo pulse is emitted at time, t2 + t. This is called stimulated photon echo. Several additional echo techniques have been proposed. 

Picosecond absorption spectroscopy has been employed to follow the time-dependence of the optical absorption during the formation of e, while pulsed-laser saturation spectroscopy has been used to examine the... 

Our present experimental arrangement for kinetic laser saturation spectroscopy with a g-switched ruby laser has been described in detail previously where it has been shown that the amplitude and temporal profile of the probe laser pulses contain the picosecond time history of the /i-level... 

Fig. 12. Schematic for laser saturation spectroscopy (LSS), showing depletion and recovery in a two-level system. Oscilloscope traces show four transmitted laser pulses at (a) low and...

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