Alkali Metal Atom Spectra

The experimental system for measuring the sonoluminescence spectrum of alkali-metal atom emission from an aqueous solution is similar to that for measuring the MBSL spectrum from water. Degassing the solution is an important procedure because the presence of dissolved air affects the emission intensity. In an air-saturated solution, no observation of alkali-metal atom emission has been reported, whereas continuum emission can be observed. A typical experimental apparatus using ultrasonic standing waves is shown in Fig. 13.3 [8]. The cylindrical sample container is made of stainless steel, and its size is 46 mm in diameter and 150 mm in... 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

For each of the alkali metals used the e.s.r. spectrum at 77°K consisted of a single narrow line (Fig. 12a, b). The relevant features of the e.s.r. spectra are summarized in Table 4. The absence of any effect of the cation on the line width or p-factors shows conclusively that the electron has been transferred completely from the alkali metal atom and is therefore not held in an expanded orbital around the cation, as suggested by Jortner and Sharf (1962). The difference in line width between the spectra in D2O (3-2 G) and in water (9-2 G) suggests that there is a hyperline interaction between the electron and the protons in water. This was shown conclusively by the observation of seven equally spaced hyperfine lines when a deposit prepared from water was warmed carefully (Fig. 12c), whereas no hyperfine structure was observed from a sample containing deuterium oxide. The hyperfine structure shows that the electron interacts primarily with six protons and that it is not delocalized over a large number of water molecules but is located in a well-defined trap surrounded by these protons. 

The reaction of sodium atoms with allyl alcohol is also somewhat unusual since the e.s.r. spectrum of the deposit showed that trapped electrons were not present and that the allyl radical was formed exclusively even at 77°K. Thus if electrons were trapped initially the trap must have decomposed rapidly to form the allyl radical and a hydroxyl ion. Such a reaction might occur in this case because of the much weaker CO-bond in the allyl alcohol. Alternatively the alkali metal atom might have reacted directly with the alcohol, the hydroxyl group behaving like a pseudo-halogen group. 

The species prepared by depositing alkali metal atoms and a stream of H2S gas upon a rotating cold-finger (14) gave a rather poorly resolved spectrum which was interpreted in terms of the data in Table VI. 

Trapped electrons are furthermore formed by the deposition of alkali-metal atoms on pure ice at 77°K. (3). The ice samples were microcrystalline or amorphous and from the ESR spectrum which exhibited hyperfine structure one could draw the conclusion that the electron was located in a well defined trap in which it was surrounded by six protons. The optical absorption band had a broad plateau ranging from about 600 to 680 n.m. 

Whereas the emission spectrum of the hydrogen atom shows only one series, the Balmer series (see Figure 1.1), in the visible region the alkali metals show at least three. The spectra can be excited in a discharge lamp containing a sample of the appropriate metal. One series was called the principal series because it could also be observed in absorption through a column of the vapour. The other two were called sharp and diffuse because of their general appearance. A part of a fourth series, called the fundamental series, can sometimes be observed. 

In the presence of an alkali salt, strong metal atom emission can be seen both in the emission spectrum and visually. This form of emission is described in detail in Chapter 13. Long-time exposure photographs comparing sonoluminescence and luminol and Na sonochemical luminescence are shown in Fig. 15.5a-c. 

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