Saturation spectroscopy technique

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. 

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... 

Infrared and Raman spectroscopy techniques have also been shown to be of value in determining oil authenticity, particularly when based on the degree of saturation (Aparicio and Baeten, 1998 Bertran et al., 2000), but these methods have not yet been substantially applied to cocoa butter and confectionery fats. 

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]. 

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. 

The line shapes of two-photon transitions are very simple as they are simply lorentzian curves, whereas the line shape in the saturation technique is quite complicated (its calculation involves the averaging of a non-linear effect which depends on the velocity component v ). In case of collisions, the two-photon line shape remains a lorentziai one and it is easy to measure the broadening and the shifts, whereas the velocity-changing collisions complicate still further the already complicated line shapes of saturation spectroscopy. 

This very sensitive Doppler-free spectroscopic technique has many advantages over conventional saturation spectroscopy and will certainly gain increasing attention [225, 226]. We therefore discuss the basic principle and some of its experimental modifications in more detail. 

The sensitivity is 2-3 orders of magnitude larger than that of saturation spectroscopy. It is surpassed only by that of the intermodulated fluorescence technique at very low sample pressures (Sect. 2.3.1). 

In the methods discussed in Sects. 2.3 and 2.4, the Doppler width had been reduced or even completely eliminated by proper selection of a velocity subgroup of molecules with the velocity components = 0 zb Ai , due to selective saturation. The technique of Doppler-free multiphoton spectroscopy does not need such a velocity selection because all molecules in the absorbing state, regardless of their velocities, can contribute to the Doppler-free transition. Therefore the sensitivity of Doppler-free multiphoton spectroscopy is comparable to that of saturation spectroscopy in spite of the smaller transition probabilities. 

The sensitivity of the saturated interference technique is comparable to that of polarization spectroscopy. While the latter can be applied only to transitions from levels with a rotational quantum number / > 1, the former works also for 7=0. An experimental drawback may be the critical alignment of the Jamin interferometer and its stability during the measurements. 

All these nonlinear techniques represent coherent third-order processes analogous to saturation spectroscopy, polarization spectroscopy, or two-photon absorption (Chap. 2), because the magnitude of the nonlinear signal is proportional to the third power of the involved fleld amplitudes (3.18a-3.18c). 

For the elimination of the residual Doppler width, the FIBLAS technique allows an elegant realization of saturation spectroscopy with a single fixed-frequency laser (Fig. 4.33). The ions are accelerated by the voltage U, which is tuned to a value... 

Fortunately, several methods have been developed that overcome these difficulties and that allow ultranarrow Ramsey resonances to be obtained. One of these methods is based on Doppler-free two-photon spectroscopy, while another technique uses saturation spectroscopy but introduces a third interaction zone at the distance z = 2L downstream from the first zone to recover the Ramsey fringes [1257-1259]. We briefly discuss both methods. 

II. The polarization detection scheme allows a sensitivity increase by three to four orders of magnitude compared to sophisticated mw spectroscopy techniques, at the same resolution. Application of saturation modulation which is a nonlinear mw spectroscopy technique developed by Tdrring, permits the detection of about 10"° absorption of the incident mw intensity. This allows the study of rotational transitions of alkaline earth monohalides in the 100 to 300 GHz range, i.e. transitions between levels with high rotational quantum numbers in the case of strontium and barium monohalides. Then the hfs is not resolved because... 

Really impressive progress toward higher spectral resolution has been achieved by the development of various Doppler-free techniques. They rely mainly on nonlinear spectroscopy, which is extensively discussed in Chap. 7. Besides the fundamentals of nonlinear absorption, the techniques of saturation spectroscopy, polarization spectroscopy, and multiphoton absorption are presented, together with various combinations of these methods. 

If the discharge cell has windows of optical quality, it can be placed inside the laser resonator to take advantage of the -fold laser intensity (Sect. 6.2.2). With such an intracavity arrangement. Doppler-free saturation spectroscopy can also be performed with the optogalvanic technique (Sect. 7.2 and [6.101]). An increased sensitivity can be achieved by optogalvanic spectroscopy in thermionic diodes under space-charge-limited conditions (Sect. 6.4.5). Here... 

Fig.7.41a-c. Comparison of different techniques for measuring the neon transition ls2 — 2p2 at X = 588.2 nm (a) intracavity saturation spectroscopy (Lamb peak of the laser output Il cd) with Doppler-broadened background) (b) laser-induced dichroism and (c) laser-induced birefringence [7.72]... 

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