Bandwidth atomic spectroscopy

An important difference between atomic and molecular spectroscopy is the width of absorption or emission bands. Spectra of liquids and solids typically have bandwidths of — 100 nm, as in Figures 18-7 and 18-14. In contrast, spectra of gaseous atoms consist of sharp lines with widths of —0.001 nm (Figure 21-3). Lines are so sharp that there is usu-... 

Photometers At a minimum, an instrument for atomic absorption spectroscopy must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for a measurement from other lines that may interfere with or diminish the sensitivity of the method. A photometer equipped with a hollow-cathode source and filters is satisfactory for measuring concentrations of the alkali metals, which have only a few widely spaced resonance lines in the visible region. A more versatile photometer is sold with readily interchangeable interference filters and lamps. A separate fdter and lamp are used for each element. Satisfactory results for the determination of 22 metals are claimed. 

The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties—from 10 (258) to several hundred atoms (259)—there seems to be a consensus that a cluster of about ISO atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (23). 

Figure 10.6. Atomic absorption with A) a sharp-line source and (B) a spectral-continuum source. AA = absorption line half-width AA, = source line half-width S = spectral bandwidth of monochromator. Adapted from G. D. Christian and F. J. Feldman, Atomic Absorption Spectroscopy Applications in Agriculture, Biology, and Medicine, New York Wiley-Interscience, 1970, p 58, by permission of John Wiley and Sons.

Lock-in amplifiers are ac amplifiers of very narrow bandwidth response (about 1 Hz). Use of the lock-in amplifier requires a reference signal modulated at the same frequency as the source. These amplifiers can provide very high gain, excellent stability, and an excellent signal-to-noise ratio. They are highly desirable for general application in atomic fluorescence spectroscopy. 

Our studies of the effect of velocity-changing collisions in an rf-laser double resonance experiment contribute to a new vista into the role of collisictis in laser spectroscopy of sub-level structures the limitation of the observation time of the active atoms due to narrow-bandwidth optical excitation and simultaneous velocity diffusion can be of importance for a variety of spectroscopic techniques that use a velocity-selective excitation and detection of either sublevel populations or sublevel coherence. On the other hand, the collisional velocity diffusion of sublevel coherence within an optical Doppler distribution can also give rise to new and surprising phenomena as will discussed in the next section. 

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. 

In the previous three chapters we have discussed the properties of gas lasers and have shown how they can be designed for single frequency oscillation, and also how the output frequency may be tuned continuously over the bandwidth of the Doppler-broadened gain curve. Unfortunately this tuning range is relatively narrow and the application of these gas lasers to atomic and molecular spectroscopy is restricted to studies of the laser transitions themselves, or to accidental coincidences with molecular absorption lines. It would therefore seem that the new and powerful technique of saturated absorption spectroscopy was also of relatively limited applicability. 

Fortunately, in the past decade, several different types of narrow-bandwidth widely tunable lasers have been developed and of these the organic dye laser has played the most prominent role in atomic and molecular spectroscopy. 

This new device has led to a rapid growth in the nonlinear spectroscopy of atoms and molecules. In addition the narrow spectral bandwidth and great intensity per unit spectral range of these dye lasers have made it possible to extend the range of classic spectroscopic techniques such as absorption and fluorescence spectroscopy. Even the more precise spectroscopic methods such as the Hanle effect, the optical double resonance, and the optical pumping techniques have all benefitted from the increasing availability of tunable dye lasers. Selective step-wise excitation using dye... 

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