Doppler-Limited Techniques

In this section we discuss measurement techniques in which Doppler broadening, caused by the thermal movements of the atoms or molecules, limits the achievable resolution. Lasers with a comparatively large linewidth (0.01—0.001 nm) are therefore normally employed. Experiments within this category are aimed at the determination of primary energy-level structure without regard to finer details, such as hyperfine structure. Several important analytical applications are also discussed. 

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The sensitivity of the optothermal technique is illustrated by the comparison of the same sections of the overtone spectrum of C2H4 molecules, measured with Fourier, optoacoustic, and optothermal spectroscopy, respectively (Fig. 1.34). Note the increase of spectral resolution and signal-to-noise ratio in the optothermal spectrum compared to the two other Doppler-limited techniques [87]. More examples can be found in [88]. 

This is, first of all, due to the manifold of rotational and vibrational levels within each electronic state and furthermore to a larger variety of angular momentum coupling, such as spin-rotation interaction, A-type doubling, fine and hyperfine structure. In addition different kinds of perturbations may further increase the line density and the complexity of the spectrum. Even for small molecules, such as diatomic or triatomic molecules, the spacings between rotational lines of an electronic transition may become much smaller than the Doppler-width. This implies that single rotational lines often cannot be resolved with "classical" Doppler-limited techniques. 

Although most univalent ions are permeable in the gramicidin channel, the channel has some conductance properties that suggest that the permeation process may be more complicated than ion motion down a narrow tube . Channel block by divalent cations is mechanistically complicated (14). Also, when Tl+ ion is present as the sole permeant ion, it is an excellent permeant ion that retains linear behavior to very large transmembrane potentials (15). However, when this ion is present as the minority cation in variable mole fraction solutions of Tl+ and Na+, Tl+ ion severely limits set channel currents (16). Such anomalous behavior can be elucidated if the local velocity of a specific ion (e.g., Tl+ ion) can be determined experimentally. In fact, the laser Doppler scattering technique produces a detectable scatter only with the Tl+ so that ion motions within the gramicidin channels can be differentiated. 

P/S increases indefinitely, the two populations nQ and n tend toward the same limit N/2. the saturation of the interaction. The important point used in the Doppler-free technique is not the saturation itself but the nonlinearity the population ni of the excited state is less than doubled when the light intensity P/S is doubled from I to 21 this nonlinearity is stronger when one is approaching saturation. 

The different sensitive techniques of Doppler-limited laser spectroscopy discussed in the previous sections supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons, excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states E, the quantum efficiency tjk reaches... 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Besides these three examples, a large number of atoms and molecules have been studied in molecular beams with high spectral resolution. For atoms mainly hyperfine structure splittings, isotope shifts, and Zeeman splittings have been investigated by this technique, because these splittings are generally so small that they may be completely masked in Doppler-limited spectroscopy [9.6,9.7]. An impressive illustration of the sensitivity of this technique is the... 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

Figure 6.2 provides a representative example for a Doppler-limited rotational spectrum, while Figure 6.3 shows such a comparison for resolved hyperfine structure (Lamb-dip technique in conjunction with frequency modulation has been employed for recording the spectrum [88]). 

After the discussion of the "Doppler-limited spectroscopy". Chap.10 gives an extensive treatment of various techniques which allow sub-Doppler spectroscopy. ... 

No monochromator is needed since the absorption coefficient a(ca) and its frequency dependence can be directly measured from the difference Al(ca) = alp(o)) - Ij(w) between the intensities of reference beam and transmitted beam (Fig.8.lb). The spectral resolution is higher than in conventional spectroscopy. With tunable single-mode lasers it is only limited by the linewidths of the absorbing molecular transitions. Using Doppler-free techniques (see Chap.10) even sub-Doppler resolution can be achieved. 

The different sensitive techniques of Doppler-limited laser spectroscopy, discussed in the previous sections, supplement each other in an ideal way. 

While for most experiments in Doppler-limited spectroscopy-discussed in Chaps.8 and 9-wultimode lasers can be used (e.g., for optical pumping experiments, laser-induced fluorescence of atoms and simple molecules, or for Doppler-limited absorption spectroscopy) only some of the sub-Doppler methods, treated in this chapter, may be performed with pulsed or cw multimode lasers. Most of these techniques demand narrow-band tunable single ode lasers with a bandwidth which should be smaller than the desired spectral resolution. If the natural linewidth 6v has to be resolved, the laser frequency jitter should be smaller than 6v. This demands frequency stabilization techniques (see Sect.6.5) and there are many examples in this branch of high-resolution laser spectroscopy where the achieved resolution is- indeed limited by the stability of the laser. 

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