Saturation spectroscopy nonlinear

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. 

The experimental arrangement is shown in Fig. 2.48. The output of a tunable dye laser at X = 486 nm is frequency-doubled in a nonlinear crystal. While the fundamental wave at 486 nm is used for Doppler-free saturation spectroscopy [261] or polarization spectroscopy [278] of the Balmer transition 2Si/2- P /2 the second harmonics of the laser at X = 243 nm induce the Doppler-free two-photon transition 15 i/2 25 i/2. In the simple Bohr model [279], both transitions should be induced at the same frequency since in this model v(lS-2S) = 4v(2S-4P). The measured frequency difference Av = v(lS-2S) — 4v(2S-4P) yields the Lamb shift vlCI ) = Av — 8v] 2S) — Avfs(45 i/2 4Pi/2) <5vl(45 ). The Lamb shift (5vl(2/S) is known and Avfs(45i/2-4Pi/2) can be calculated within the Dirac theory. The frequency markers of the FPI allow the accurate determination of the hfs splitting of the 15 state and the isotope shift Avis( H- H) between the 1S-2S transitions of hydrogen and deuterium (Fig. 2.38). 

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). 

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. 

The experimental arrangement is shown in Fig.7.43. The output of a tunable dye laser at A = 486 nm is frequency doubled in a nonlinear crystal. While the fundamental wave at 486 nm is used for Doppler-free saturation spectroscopy [7.54] or polarization spectroscopy [7.70] of the Balmer transition 28 /2 second harmonics of the laser at A = 243 nm induces... 

This may significantly reduce noise problems caused by detector noise or background radiation. Furthermore, the large intensity allows new nonlinear spectroscopic techniques such as saturation spectroscopy (Sect.10.2), or multiphoton processes (Sect.10.5), which open new possibilities of studying molecular transitions not accessible to linear spectroscopy. 

The TM-EZ scan technique was applied to the Tryptophan-Ag colloid with different NPs concentration. The values obtained for the nonlinear susceptibilities are summarized in figure 18.9. One can also observe that, in a low filling fraction regime, the imaginary part of the third-order nonlinear susceptibility increase linearlv with the concentration of NPs, and a saturation behaviour is observed for/>3x10 , in accordance with the results obtained from the fluorescence spectroscopy (figure 18.8). 

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. 

The population density of molecules in the absorbing level is decreased by optical pumping. This results in a nonlinear dependence of the absorbed radiation power on the incident power. Such techniques are therefore summarized as nonlinear spectroscopy, which also includes methods that are based on the simultaneous absorption of two or more photons dining an atomic or molecular transition. In the following sections the basic physics and the experimental realization of some important methods of nonlinear spectroscopy are discussed. At first we shall, however, treat the saturation of population densities by intense incident radiation. 

In this section we will briefly discuss some variations of saturation, polarization, or multiphoton spectroscopy that either increase the sensitivity or are adapted to the solution of special spectroscopic problems. They are often based on combinations of several nonlinear techniques. 

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... 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

We will first describe spectroscopy on collimated atomic beams and on kinematically compressed ion beams. Two groups of nonlinear spectroscopic techniques will be discussed saturation techniques and two-photon absorp-... 

We will first describe spectroscopy on collimated atomic beams and on kinematically compressed ion beams. Two groups of nonlinear spectroscopic tecliniques will be discussed saturation techniques and two-photon absorption techniques. We will also deal with the optical analogy to the Ramsey fringe technique (Sect. 7.1.2). In a subsequent section (Sect. 9.8) laser cooling and atom- and ion-trap techniques will be discussed. Here, the particles are basically brought to rest, ehminating the Doppler as well as the transit broadening effects. 

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