Saturation spectroscopy, advantages

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 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 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]. 

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... 

All techniques of saturation spectroscopy discussed so far monitor directly some change in the probe beam. For this reason, they work best with samples of non-negligible absorption. For very weakly absorbing samples, it is often advantageous to detect the absorption of light in the sample indirectly, for instance by observing the laser-induced fluorescence. 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

Wall-coated flow tube reactors have been used to study the uptake coefficients onto liquid and solid surfaces. This method is sensitive over a wide range of y (10" to 10 1). For liquids this method has the advantage that the liquid surface is constantly renewed, however if the uptake rate is fast, the liquid phase becomes saturated with the species and the process is limited by diffusion within the liquid, so that corrections must be applied [70,72,74]. Many experiments were designed to investigate the interaction of atmospheric species on solid surfaces. In this case the walls of the flow tube were cooled and thin films of substrate material were frozen on the wall. Most of the reaction probabilities were obtained from studies on flow tubes coated with water-ice, NAT or frozen sulfate. Droplet train flow tube reactors have used where liquid droplets are generated by means of a vibrating orifice [75]. The uptake of gaseous species in contact with these droplets has been measured by tunable diode laser spectroscopy [41]. 

Experimental Setup. The instrumentation (both optics and electronics) for studying saturated laser induced fluorescence spectroscopy is much less conplicated than for CARS. The experimental setup shown in Figure 18, as used in our laboratory, is typical for these studies. In some experiments it is advantageous to use a monochromator rather than band pass filters to isolate the laser induced fluorescence signal. The lasers used are either flash lamp pumped systems or NdsYAG pumped dye lasers. 

Despite these problems of saturation of vibrational bands IR spectroscopy, described in the next subsection, has been recently shown to nevertheless remain an especially powerful method to observe H2O molecules. Special recently proposed set-ups can avoid saturation in the whole conventional IR region, thus taking full advantage of the power of IR to study H-bond networks. They are first described, before the contribution of recent time-resolved nonlinear IR spectroscopy is examined. Other methods such as NIR or Raman spectroscopy, which are intrinsically free of this saturation effects can also be used to study the HjO molecule. They are often limited to some specific problems, as they do not display the power of ordinary IR spectroscopy for the study of H-bonds or of H2O molecules and cannot consequently be considered as general methods. They are described in the last subsection of this section on vibrational spectroscopy. 

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