Advantages of Polarization Spectroscopy

Let us briefly summarize the advantages of polarization spectroscopy, discussed in the previous sections  

Meanwhile polarization spectroscopy has been applied to the measurement of many high-resolution atomic and molecular spectra. Examples can be found in [204, 230-236]. 

2) The sensitivity is 2-3 orders of magnitude larger than that of saturation spectroscopy. It is surpassed only at very low sample pressures by that of the intermodulated fluorescence technique (Sect.10.2.3). 

3) The possibility of distinguishing between P, R, and Q lines is a particular advantage for the assignment of complex molecular spectra. 

4) The dispersion profile of the polarization signals allows a stabilization of the laser frequency to the line center without any frequency modulation. 

The large achievable signal-to-noise ratio assures an excellent frequency stability. 

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Another advantage of polarization spectroscopy is the suppression of the broad signal back-ground observed in saturated absorption (Fig. 16a), when collisions redistribute the velocities of the pumped atoms over the Doppler profile, because these velocity-changing collisions drastically reduce the laser-induced anisotropy (the curves in Figure 16 are obtained in the same experimental cell of neon). A good review about Polarization Spectroscopy and related phenomena can be found in ref. I 25 I. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

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. 

The higher sensitivity of polarization spectroscopy compared with conventional saturation spectroscopy results from the detection of phase differences rather than amplitude differences. This advantage is also used in a method that monitors the interference between two probe beams where one of the beams suffers saturation-induced phase shifts. This saturated interference spectroscopy was independently developed in different laboratories [271, 272]. The basic principle can easily be understood from Fig. 2.43. We follow here the presentation in [271]. 

A rheoopticai method includes a mechanical test under static conditions (tensile test, stress relaxation, cyclic test) carried out simultaneously with an optical measurement. Among other optical methods such as x-ray scattering, nmr spectroscopy, polarized fluorescence or birefringence, the Fourier transform infrared (flir) spectroscopy has become one of the most frequently applied tools in rheooptics (22). Advantage of infrared spectroscopy is its sensitivity, rapidity, and the ability to investigate changes of molecular orientations not only in separate structural units but also in various phases of multiphase systems. From the ratio of intensities of beams polarized parallel and perpendicular to the deformation direction, orientation functions can be calculated. 

Other techniques to detennine the corrosion rate use instead of DC biasing, an AC approach (electrochemical impedance spectroscopy). From the impedance spectra, the polarization resistance (R ) of the system can be detennined. The polarization resistance is indirectly proportional to j. An advantage of an AC method is given by the fact that a small AC amplitude applied to a sample at the corrosion potential essentially does not remove the system from equilibrium. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

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