Polarization spectroscopy advantages

In the future, we can expect the development of novel experimental techniques in solid-state NMR spectroscopy for investigation of functioning catalysts. Important goals are (i) the enhancement of the sensitivity of solid-state NMR spectroscopy, for example, by a selective enhancement of the nuclear polarization taking advantage of laser-polarized xenon, (ii) increases in the temperature range accessible for the characterization of solid-catalyzed reactions, and (iii) the coupling of NMR spectroscopy with other techniques such as mass spectrometry. Furthermore, modern two-dimensional techniques of solid-state NMR spectroscopy such as MQMAS NMR spectroscopy will be applied to improve the resolution of the spectra. 

An extremely sensitive MODR scheme, microwave optical polarization spectroscopy (MOPS), was introduced by Ernst and Torring (1982). The most important features of MOPS are that it requires respectively 100 and 10 times lower laser and microwave intensities than MODR and results in 10 times narrower lines. This means that it will be possible to take full advantage of differential power broadening effects (Section 6.5.1) and to utilize low-power, frequency-doubled dye lasers and low-power, broadly tunable microwave sources (backward wave oscillators) in order to gain access to and systematically study perturbations. 

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. 

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

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]. 

In most detection schemes of saturation or polarization spectroscopy the intensity fluctuations of the probe laser represent the major contribution to the noise. Generally, the noise power spectrum PNoise(/) shows a frequency-dependence, where the spectral power density decreases with increasing frequency (e.g., l//-noise). It is therefore advantageous for a high S/N ratio to detect the signal S behind a lock-in amplifier at high frequencies /. 

Using the dispersion profiles of Doppler-free molecular lines in polarization spectroscopy (Sect. 7.4), it is possible to stabilize a laser to the line center without frequency modulation. An interesting alternative for stabilizing a dye laser on atomic or molecular transitions is based on Doppler-free two-photon transitions (Sect. 7.5) [5.77]. This method has the additional advantage that the lifetime of the upper state can be very long, and the natural linewidth may become extremely small. The narrow Is —2s two-photon transition in the hydrogen atom with a natural linewidth of 1.3 Hz provides the best known optical frequency reference to date [5.76]. 

Major advantages of microwave-optical polarization spectroscopy are narrower linewidths as compared to conventional laser-rf double resonance and smaller intensities required for the laser light field and the micro-waves, so that strongly saturating conditions can be avoided. Therefore, the sensitivity as well as the resolution can be greatly enhanced. 

Nanosecond fluorescence polarization spectroscopy is a technique well suited for the analysis of the conformation and dynamic properties of macromolecules (1,2). Fluorescence polarization measurements have until recently been mainly carried out under steady state conditions, but in recent years the time dependence of fluorescence polarization has been successfully applied to the study of size, shape and segmental flexibility of macromolecules (3,4) with the added advantage that complex rotational motions can often be explicitly resolved on the time axis (5). 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

Compared to the newer modulation techniques, polarization spectroscopy has the advantage of greater simplicity. And its reliance on atomic orientation or alignement can provide valuable information on angular momenta and disorienting collisions. However, the technique is limited to spectral regions where good polarizers are available, and considerable care is necessary to reach shot-noise limited sensitivity. 

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. 

---